Qualitative Differences Between Conscious and Nonconscious Processing?On Inverse Priming Induced by Masked Arrows

Rolf VerlegerUniversity of Lubeck

Piotr JaskowskiUniversity of Lubeck and Kazimierz Wielki University

Aytac Aydemir, Rob H. J. van der Lubbe, and Margriet GroenUniversity of Lubeck

In general, both consciously and unconsciously perceived stimuli facilitate responses to following similarstimuli. However, masked arrows delay responses to following arrows. This inverse priming has beenascribed to inhibition of premature motor activation, more recently even to special processing ofnonconsciously perceived material. Here, inverse priming depended on particular masks, was insensitiveto contextual requirements for increased inhibition, and was constant across response speeds. Putativesigns of motor inhibition in the electroencephalogram may as well reflect activation of the oppositeresponse. Consequently, rather than profiting from inhibition of primed responses, the alternativeresponse is directly primed by perceptual interactions of primes and masks. Thus there is no need toassume separate pathways for nonconscious and conscious processing.

How do unconsciously perceived stimuli influence perceptionsand actions? For this question to be studied in the laboratory,briefly presented stimuli are made invisible by following masks.Particular interest has been devoted to stimulus designs in whichthe mask obscures the sensory impression of the stimulus, althoughit does not appear at the same location (metacontrast, e.g., Francis,2000; object substitution, Di Lollo, Enns, & Rensink, 2000). Thecase is less spectacular when masks cover the same location as thestimuli to be masked ( pattern masks, cf. the review by Enns & DiLollo, 2000). Yet the claim was recently made that when suchapparently simpler masks are used, consciously and unconsciouslyperceived stimuli undergo qualitatively different ways of process-ing (Eimer & Schlaghecken, 2002; Klapp & Hinkley, 2002). Wedispute this claim in the present article.

There are two complementary ways to assess the impact ofunconsciously perceived stimuli: Their effects on overt behaviormay be measured, and their traces left in brain activity may beexplored. An attractive feature of the work by Eimer,Schlaghecken, and associates, underlying the claim of qualitativelydifferent ways of processing, is that these approaches were com-bined. On the behavioral level, priming effects of the maskedstimuli were assessed by analyzing the changed response times tofollowing clearly visible stimuli. (Requiring overt responses, theselatter stimuli will be called targets in the following text.) Inversepriming was obtained as a rule; that is, participants’ responseswere slower when the targets required the response that wouldhave been appropriate for the masked stimuli (Aron et al., 2003;Eimer, 1999; Eimer & Schlaghecken, 1998, 2001, 2002; Eimer,Schubo, & Schlaghecken, 2002; Schlaghecken & Eimer, 1997,2000, 2001, 2002; Schlaghecken, Munchau, Bloem, Rothwell, &Eimer, 2003).1 This inverse priming turned into large noninverse(henceforth referred to as straight) priming when primes werevisible (Eimer & Schlaghecken, 2002; Klapp & Hinkley, 2002);that is, responses were faster when primes and following targetssignaled the same response. This change from inverse to straightpriming led to the claim that processing occurs along qualitativelydifferent lines for conscious and unconscious perception (Klapp &Hinkley, 2002). In addition to overt behavior, Eimer andSchlaghecken (1998), Eimer (1999), and, recently, Seiss andPraamstra (2004) measured differential electroencephalogram

1 We prefer the term inverse priming (and its counterpart straight prim-ing) to the recently proposed negative compatibility effect because thislatter term lacks the dimension of time, implies a specific mechanism thatwe dispute in this article (compatibility referring to stimulus–responserelations), and is lengthy, requiring the use of the acronym NCE.

Rolf Verleger, Aytac Aydemir, Rob H. J. van der Lubbe, and MargrietGroen, Department of Neurology, University of Lubeck, Lubeck, Ger-many; Piotr Jaskowski, Department of Neurology, University of Lubeck,Lubeck, Germany, and Department of Psychophysiology, KazimierzWielki University, Bydgoszcz, Poland.

Piotr Jaskowski is now at the Department of Cognitive Psychology,Warsaw University of Finance and Management, Warsaw, Poland.

Rob H. J. van der Lubbe is now at the Psychological Laboratory,Helmholtz Instituut, University of Utrecht, Utrecht, the Netherlands.

Margriet Groen is now at the Department of Experimental Psychology,University of Oxford, Oxford, England.

This research was supported by Grants Ve110/7-2 and Ve110/7-4 fromthe German Research Foundation to Rolf Verleger. The complete set ofresults was first reported in March 2002 at the annual meeting of Exper-imentally Working Psychologists (TeaP), Chemnitz, Germany.

Correspondence concerning this article should be addressed to RolfVerleger, Department of Neurology, University of Lubeck, RatzeburgerAllee 160, D 23538 Lubeck, Germany. E-mail: rolf.verleger@neuro.uni-luebeck.de

Journal of Experimental Psychology: General Copyright 2004 by the American Psychological Association2004, Vol. 133, No. 4, 494–515 0096-3445/04/$12.00 DOI: 10.1037/0096-3445.133.4.494

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(EEG) activation from scalp sites overlying the contra- and ipsi-lateral motor cortex. This differential activation formed threephases in the interval between the onset of the masked stimuli andthe overt response, interpreted as (a) activation by the maskedstimulus, (b) inhibition of this activation, and (c) activation by thetarget. This triphasic course of activation will be illustrated later onin our own results. Summarizing these results, Eimer andSchlaghecken (2002) and Klapp and Hinkley (2002) proposedwhat we call the inhibition hypothesis of priming: Inverse primingis the default case. This is because activation produced by theprimes automatically evokes its inhibitory counterresponse. Maskscause this effect to be seen in pure form, whereas unmasked primesovercome this effect by their continuing perceptual impact.

The present experiments were prompted by some discrepanciesof this view. Straight priming was obtained with these stimuli notonly when primes were insufficiently masked but also whenprimes were presented peripherally or in a degraded fashion(Schlaghecken & Eimer, 1997, 2000, 2002). To account for thisirregularity, Schlaghecken and Eimer (2002) concluded thatstraight priming will not only prevail when masking is insufficientbut also, on the contrary, when stimulation by the prime is weak:Then inhibition will not be evoked at all. By this corollary, thisapproach loses some of its elegance (and other corollaries arefound necessary in this article). Even more important is the dis-crepancy between this body of research and the bulk of resultsabout effects of masked stimuli. Priming was always straight ratherthan inverse (e.g., Cheesman & Merikle, 1984; Dehaene et al.,1998; Enns & Di Lollo, 2000; Klotz & Neumann, 1999; Merikle &Joordens, 1997a, 1997b), so the suspicion is that rather thanforming evidence for qualitative differences between consciousand unconscious processes, inverse priming depends on the par-ticular stimuli used in these recent studies, which were arrow headspointing left and right (see Figure 1).

The conception to be proposed as an alternative to the inhibitionhypothesis is the active-mask hypothesis. For the inhibition hy-pothesis, masks are absolutely neutral, only serving to leave timefor the developing automatic inhibition of prime-induced activa-tion. In contrast, we suggest that masks always play an active role.

Such an active role has been commonplace in research on effectsof masks on visibility of their preceding stimuli (e.g., Breitmeyer,1980; Enns & Di Lollo, 2000; Keysers & Perrett, 2002). It mightalso be true, although possibly by different mechanisms, for thepresent question of whether the motor-priming effects differ be-tween visible and masked stimuli. Specifically, we argue thatpattern masks serve two functions. First, as pattern masks proper(covering the same location as the stimuli to be masked), thesemasks cause a decay over time of the straight priming induced bythe masked stimuli, reducing this effect to zero (but not reversingit). Second, by covering more than just the same locations as themasked stimuli and by containing some structured patterns similarto the masked stimuli, such pattern masks by necessity interactwith processes evoked by the primes. It is by these interactions thatthe reversed priming effect might come to happen.

Behavioral and electrophysiological (EEG) evidence was col-lected in three experiments. For clarity of presentation, these twosets of evidence are separately presented and discussed and arethen followed by the GENERAL DISCUSSION section.

BEHAVIORAL EVIDENCE

The mask used by Eimer and Schlaghecken (1998) and in mostof their following studies was a compound of four arrowheads(henceforth called the arrows mask) formed by overlaying the twoprime stimuli (Figure 1). By covering the locations occupied byeither possible preceding prime, this mask appears particularlysuited to obscure the sensory impression of the primes. Whenlooked at in more detail, matters become more complex. Forexample, leftward arrows as primes will be exactly covered by theleftward arrows in the mask. If the mask would consist of theseleftward arrows only, the prime would not be perceived as astimulus of its own, because there is no temporal interval betweenprime and mask: Participants, and any ideal perceiver as well,would see one presentation of leftward arrows, called “prime” bythe experimenter in the first 17 ms and “mask” afterward. It is onlyby virtue of the complementary, rightward arrows of the mask thatthe prime comes into being: The prime can be defined as that partof the mask that starts 17 ms earlier than the complementary partof the mask. Thus, there are two sudden onsets, formed by theonset of the leftward arrows and by the onset of the rightwardarrows. If participants were able to perceive this rapid sequence,their attention might be drawn first leftward, then rightward.Likewise, independent of any attention priming, their motor sys-tem might be primed first leftward, then rightward.

These considerations led us to compare the priming effectsbetween different masks (in the section entitled Mask-ModeratedEffects on Mean Response Times and Error Rates). The mode ofpriming, straight or inverse, is shown to depend on the masks.Further, by randomly alternating these masks within one block, wechange the context for either effect (straight priming with onemask, inverse priming with the other), as compared with present-ing different masks in different blocks. This provides the oppor-tunity to study the automatic, encapsulated nature of either modeof priming. Further, we study variation of priming across time byanalyzing response-time distributions (in the section entitled De-pendence of Priming on Response Speed). Additionally, we reportresults obtained with masks formed by sequences of the two

Figure 1. Stimuli used in all three experiments. Except for the checksmask, these stimuli were replicates of the stimuli used by Eimer andSchlaghecken (1998).

495REVERSAL OF PRIMING BY MASKING

different masks (in the section entitled Compounds of the TwoMasks).

Mask-Moderated Effects on Mean Response Times andError Rates

Evidence in favor of attributing at least some part of the inversepriming reported by Eimer and Schlaghecken’s earlier studies tothe particular design of their mask is found in the results reportedin Schlaghecken and Eimer (2002) and Eimer and Schlaghecken(2002). In these studies, the priming effect on response times (thedifference between trials with identical and opposite primes)amounted to about 20 ms. This is less than, for example, the 53 msin Eimer and Schlaghecken (1998, Experiment 1). The mask usedin the more recent studies differed from those used in previousstudies, being an assembly of short lines of differing lengths andorientations, varying over trials. Whether these smaller effectswere related to the use of this new mask was tested in the presentstudy, in which we compared within experiments the arrows maskwith a mask similar to the one used by Schlaghecken and Eimer(2002), henceforth called the checks mask (see Figure 1). Thesemasks were presented in different blocks in Experiment 1 andalternated in random order in Experiment 2. In Experiment 3, wecompared both modes of presentation within one experiment,additionally using trials without any mask.

Experiment 1

Method

Participants. Participants were students of the University of Lubeck.None was informed about the hypotheses. They had normal or corrected-to-normal vision, were tested in a single session lasting about 11⁄4 hours (anadditional half hour was needed for preparing the EEG recording) andobtained 15 DM (approximately $9.00) per hour for their participation. Ofthe 13 participants originally tested, 2 had to be excluded because oftechnical failure during EEG recording. The 11 remaining participantswere 9 men and 2 women, mean age 26 � 4 years.

Stimuli. Stimuli were modeled after those used by Eimer andSchlaghecken (1998, Experiment 1). Primes and targets were pairs of blackopen triangles, symbolizing arrowheads, pointing left (��) or right (��).Primes could also be neutral, either �� or �� in random selection.Arrowheads were 1.25° high at their extended side and 0.9° wide. They had0.1° distance from the center, such that pairs of arrowheads were 2.0° wide.The lines forming the arrows were 0.15° wide. The fixation point was athin red ring, 0.2° wide, appearing 1 s before the primes until the partici-pant responded. Two different masks were used. The arrows mask wascomposed of four arrowheads, which were the two primes (or targets)overlaid on each other. The checks mask was a 2.25° � 1.65° rectangleformed by a 10 � 10 array of small checks (white, gray, and black)randomly selected in each trial, lest participants learn to filter out the mask(Schubo, Schlaghecken, & Meinecke, 2001). The screen background waswhite. Primes were presented for 17 ms, the immediately following maskfor 100 ms, and (in the response-time part) the immediately followingtarget for another 100 ms.

Procedure. Participants sat in a comfortable armchair in a darkenedchamber. Observation distance to the computer screen was 125 cm. Eacharm of the chair contained a response key.

The response-time part was followed by the prime-discrimination part ofthe experiment. Within each part were two blocks, one using the arrowsmask, the other the checks mask. The order of these two blocks alternated

between participants. Each response-time block contained 300 trials inrandom order: 100 trials with primes identical to the target, 100 withopposite primes, and 100 with neutral primes. Participants had to press theleft or right key in response to the leftward or rightward target arrows. Eachprime-discrimination block consisted of 100 trials with left-pointing primesand 100 trials with right-pointing primes in random order. No targets werepresented in these blocks (cf. Eimer & Schlaghecken, 1998). Participantshad to press the left or right key in response to the leftward or rightwardprime arrows. Data were retained only from those participants who couldnot identify the primes in these blocks, as measured by d�, estimated as0.86 ln[c%/(1 – c%)], where c% denotes the percentage of correct re-sponses (Smith, 1982).

Data analysis. Manual responses were measured as continuous signalsfrom force-sensitive keys. Responses were counted when force exceeded0.5 N (corresponding to 51 g) within 1,500 ms after prime onset. Percent-ages of wrong responses and mean latencies of correct responses, relativeto target onset, were calculated separately for the three types of primes andthe two masks and were evaluated statistically by analysis of variance(ANOVA) for repeated measurements with two within-subjects factors:prime (identical, neutral, opposite) and mask (arrows, checks). Effectsobtained from repeated-measurement factors with more than two levels(i.e., prime) were corrected by Huynh–Feldt’s �. The standardized measureof effect size, �2, was computed for significant effects in main analyses.

Results

Response times. Response times are reported in Figure 2, topleft panel. Mask type determined the prime effect: Mask � Prime,F(2, 20) � 19.1, � � .65, p � .001, �2 � .35. With the arrowsmask, priming was inverse: As confirmed by t tests, responseswere delayed after identical primes (by 26 ms; p � .001) andspeeded up after opposite primes (by 6 ms; p � .03). The 26-msdelay was larger than the 6-ms gain, t(10) � 4.4, p � .001. Withthe checks mask, in contrast, priming was straight: Responses weredelayed after opposite primes (by 10 ms; p � .03) and weaklytended to be speeded up after identical primes (by 8 ms; p � .26).Delay and gain did not differ with this mask, t(10) � 0.4, ns.Responses were generally slower with the arrows mask than withthe checks mask, F(1, 10) � 6.6, p � .03, �2 �.20, by 18 ms forneutral primes.

Errors. Error rates may be seen in Figure 2, bottom left panel.More errors were made with the arrows mask than with the checksmask, F(1, 10) � 8.4, p � .016, �2 � .25; for example, withneutral primes, the error rate was 6% versus 2%, respectively.Corresponding to the response-time results, these rates increased inthe arrows block when primes were identical and in the checksblock when primes were opposite: Prime � Mask, F(2, 20) �17.2, � � .87, p � .001, �2 � .33; confirmed by t tests, p � .01for both.

Prime discrimination. For the arrows mask, d� equaled 0.02(range �0.14 to �0.21); for the checks mask, d� equaled �0.02(range �0.21 to �0.12). Neither value differed from zero, t(10) �0.7, ns, and t(10) � �0.5, ns, respectively, nor did they differ fromeach other, t(10) � 0.8, ns. (The maximum individual value of d�,0.21, indicates 55% correct responses, 50% being guessingprobability.)

Expressing the difference of response times (abbreviated RT inthe equations) RT � RT(incongruent) – RT(congruent) by linearregression from d� across participants (Draine & Greenwald, 1998)yielded for the arrow mask RT � 10 � d� � 33. The slope (10)

496 VERLEGER ET AL.

was not significant, t(9) � 0.14, p � .89, implying that primingwas not affected by conscious prime visibility, and the intercept,�33 ms, was significant, t(9) � �4.9, p � .001, confirming theoccurrence of priming. For the checks mask, regression yieldedRT � �161 � d� � 15, implying both some tendency toward anincrease of priming with increasing conscious visibility—slope:t(9) � �1.9, p � .09—and only a tendency toward significantpriming—intercept: t(9) � 1.9, p � .09.

Discussion

Inverse priming with arrows as primes, masks, and targets wasreplicated. The average difference between identical and oppositeprimes amounted to 32 ms, somewhat less than in Eimer andSchlaghecken’s (1998) first report but clearly significant. Extend-ing previous findings, we found strong dependence of priming onthe mask. Further discussion of these effects is postponed untilafter Experiment 3.

Experiment 2

In Experiment 2, we investigated whether the different effects ofthe two masks would be replicated when masks were varied inrandom sequence rather than between blocks. Of particular interestwas whether the two modes of priming would exert mutual influ-ence. Modifying effects of context have indeed been shown onstraight priming exerted by masked stimuli (Bodner & Masson,2001; Jaskowski, Skalska, & Verleger, 2003). In a correspondingway, the presence of inverse priming might here affect the extentof straight priming and, of particular interest, the presence ofstraight priming might increase the pressure to implement theinhibitory process. After all, the purpose of inhibition is to preventresponses to the primes, only because these responses are notdesired according to the instructions and indeed might often beincorrect. So, if the inhibition hypothesis is correct, inverse prim-ing is expected to increase. Alternatively, the process might beentirely encapsulated, not modifiable by context.

Figure 2. Mean response times (top) and error rates (bottom) in Experiments 1, 2, and 3. The arrows mask isdenoted by crosses, the checks mask by triangles, and no mask (Experiment 3) by squares. For Experiment 3,lines are bold when masks are blocked and thin when masks occur in random order. There are 95% confidenceintervals (in one direction only) depicted for Experiments 1 and 2 (Bakeman & McArthur, 1996), but they wereomitted from the graphs of Experiment 3 for legibility.

497REVERSAL OF PRIMING BY MASKING

In addition to the “pure” checks and arrows masks, we usedcompounds of the two masks in series. Those results are presentedfurther below (in the Compounds of the Two Masks section).

Method

Of the 14 participants originally tested, the data of 1 had to be excludedbecause of technical failure. The 13 remaining participants were 10 menand 3 women, mean age 28 � 6 years. Three participants had also takenpart in the first experiment (1 year before the current experiment).

Stimuli were smaller than in Experiment 1 to be more similar to Eimerand Schlaghecken’s stimuli, which were generally smaller: for example,0.8° � 0.4° in Eimer and Schlaghecken (1998). Arrowheads were now0.55° high at their extended side, 0.5° wide, and at a 0.1° distance from thecenter, such that the pair of arrowheads was 1.2° wide. The lines formingthe arrows were 0.1° wide. The rectangle making up the checks mask was1.25° wide and 0.95° high and consisted of 11 � 11 checks.

Four masks were presented in random sequence across 960 trials, withshort breaks after every 320 trials: arrows (100 ms), checks (100 ms),arrows & checks (each presented for 50 ms, yielding 100 ms in total), andchecks & arrows (the same time sequence as arrows & checks). There were80 trials with identical, neutral, and opposite primes for each mask. Thefollowing prime-discrimination part consisted of 40 trials each for identicaland opposite primes for each mask. In this experiment, we included thetarget arrows in prime discrimination (because we reasoned that perceptionof the prime arrows might be easier when followed by the target arrows,providing an anchor by which to calibrate the perceptual impression of theprime) and instructed participants to try to respond to the primes. Unfor-tunately, stimulus presentation was unbalanced in prime discrimination(but not in response time) because of a programming error; therefore, d�could not be meaningfully determined.2 The ANOVA design was the sameas in Experiment 1, because results here are reported from the two puremasks only, checks and arrows.

Results

Response times. Response times may be seen in Figure 2, topmiddle panel. Again, the effect of primes depended on mask type:Mask � Prime, F(2, 24) � 78.7, � � .86, p � .001, �2 � .66.Responses were fast after opposite and slow after identical primeswith the arrows mask, F(2, 24) � 33.8, � � .93, p � .001, but slowafter opposite and fast after identical primes with the checks mask,F(2, 24) � 56.2, � � .69, p � .001. Different from Experiment 1,delay and gain did not differ with the arrows mask, t(12) � 1.21,p � .25, or with the checks mask, t(12) � 0.4, ns. Mean responsetimes did not differ between masks, F(1, 12) � 2.4, p � .12, withthe difference amounting to less than 1 ms for neutral primes.

Errors. The error rate may be seen in Figure 2, bottom middlepanel. Similar to response times, prime effects on error ratesdepended on the mask: Prime � Mask, F(2, 24) � 21.0, � � .67,p � .001, �2 � .34. Errors increased from opposite to identicalprimes with the arrows mask, F(2, 24) � 8.9, p � .002, anddecreased with the checks mask, F(2, 24) � 20.9, p � .001. Meanerror rates did not globally differ between masks, F(1, 12) � 2.2,p � .16, but tended to do so for neutral primes, where more errorswere committed with the arrows mask: breaking down the signif-icant Prime � Mask interaction to an effect of mask for neutralprimes, t(12) � 2.0, p � .07.

Prime effect on response times as a function of prime discrim-ination. Linear regression from d� on RT across those 10 par-ticipants with plausible prime-discrimination behavior (see Foot-note 2 and the right half of Figure 3) yielded for the arrow mask

RT � �20 � d� – 39 and for the checks mask RT � 6 � d�� 36. Slopes were not significant—arrows mask, t(8)� �1.01,p � .34; checks mask, t(8) � 0.43, p � .68—implying thatpriming was not affected by conscious prime visibility. Interceptswere significant—arrows mask, t(8) � �5.37, p � .001; checksmask, t(8) � 8.76, p � .001—confirming the occurrence ofpriming.

Comparison between Experiments 1 and 2. A between-subjects ANOVA on Experiments 1 and 2 showed that responseshad become equal for both masks in Experiment 2 because re-sponses were slower with the checks mask in Experiment 2 thanthey were in Experiment 1: Experiment � Mask, F(1, 22) � 5.8,p � .025, �2 � .09. Error rates were always larger with the arrowsmask than with the checks mask, F(1, 22) � 9.3, p � .006,irrespective of experiment: Experiment � Mask, F(1, 22) � 0.9,ns. The triple interaction of Prime � Mask � Experiment wassignificant for errors, F(2, 44) � 5.4, � � .70, p � .017, �2 � .06,and tended to be so for response times, F(2, 44) � 3.2, � � .78,p � .065, �2 � .03. This might indicate that prime effects weremore marked in Experiment 2 than in Experiment 1, either gen-erally or for one of the two masks only. This issue could not beunambiguously resolved, because Prime � Experiment effects foreither mask separately were not clearly significant, nor did theyclearly differ between masks. It was also interesting to compare thedelay by identical primes with the gain by opposite primes for thearrows mask across experiments (because the difference betweendelay vs. gain was significant in Experiment 1 but not in Experi-ment 2). This resulted in a marked effect of delay versus gain, F(1,22) � 10.9, p � .003, unmodified by experiment: Delay or Gain �Experiment, F(1, 22) � 1.9, p � .18.

Discussion

The central results of Experiment 1 were replicated also withrandom sequences of masks: Prime effects were inverse with thearrows mask and straight with the checks mask. Being largelyindependent of blockwise or random presentation of masks, thisbasic pattern proved to be stable against context and participants’strategic settings. Similarly, strategic settings induced by the pro-portion of go versus no-go trials or of left versus right responsesexerted no effect on inverse priming (Schlaghecken & Eimer,2001). However, the triple interaction in comparing Experiments 1and 2 might indicate some graded modulation of the primingeffects by context. This issue was further explored inExperiment 3.

2 In prime discrimination, primes were always to the left with the checksand the checks & arrows masks and always to the right with the arrows andthe arrows & checks masks. The following targets were always to the leftwith the checks and with the arrows masks and always to the right with themixed masks. Because trials with different masks appeared in randomsequence, neither the participants nor the experimenter noticed these reg-ularities. However, offline we could see by these regularities in separateanalyses of the four masks that 3 participants had quit any attempts todetect the prime and responded in the direction of the targets instead. (Notethat such behavior might easily be overlooked if the sequence is random-ized as it should be.) In the data from the other 10 participants, d� averaged�0.07 for the checks mask, �0.03 for the arrows & checks mask, 0.07 forthe checks & arrows mask, and �0.05 for the arrows mask, none signifi-cantly different from zero ( p � .48).

498 VERLEGER ET AL.

Experiment 3

In this experiment, masks differed both between and withinblocks. Further, to create a particularly powerful need to increaseinhibition, we also presented primes without any mask. Such trialswere expected to have a strong straight priming effect (Eimer &Schlaghecken, 2002; Klapp & Hinkley, 2002). The slight contexteffects in the previous Experiment 2 might be regarded as boostingof priming in the presence of the antagonistic mode of priming;that is, straight priming (with the checks mask) had become largerwhen inverse priming (with the arrows mask) occurred in the sameblock. Thus, it may now be expected that inverse priming willincrease and straight priming will decrease when particularlystrong straight priming occurs in the same block.

Method

Of the 20 participants originally tested, 2 had to be excluded because ofproblems at recording, 1 because of his misunderstanding of instructions,and 2 because of too many errors (48% and 92%) when primes wereunmasked and opposite. The 15 remaining participants were 4 men and 11women, mean age 25 � 3 years (20–31 years).

Stimuli were the same as in Experiment 2. But no mask compounds wereused, nor were neutral primes, to keep session length at a tolerable level.For the same reason, prime discrimination was not tested. There were fourblocks. In one block, primes were masked by the arrows mask, in another

block by checks, in a third block not at all (i.e., there was a 100-ms blankinterval between primes and targets), and in the fourth block all these threemask conditions varied randomly between trials. The constant-mask blocksconsisted of 240 trials each, the random block of 720 trials (with pausesafter every 240 trials); thus, there were 120 trials each with identical andopposite primes in each condition. Order of blocks varied over participantsaccording to a Latin square. ANOVA factors were mask (none, arrows,checks), prime (identical, opposite), and mode of mask presentation (con-stant, random).

Results

Response times. Response times for Experiment 3 are shownin the top right panel of Figure 2. Again, prime effects dependedon the mask: Mask � Prime, F(2, 28) � 164.4, � � .68, p � .001,�2 � .78. Marked straight priming occurred when there was nomask: separate effects of prime for the no-mask condition, F(1,14) � 180.0. As in Experiments 1 and 2, the arrows mask causedinverse priming, and the checks mask straight priming: Mask �Prime for these two masks only, F(1, 14) � 60.0, p � .001; effectsof prime for the arrows mask, F(1, 14) � 33.7, p � .001; effectsof prime for the checks mask, F(1, 14) � 19.3, p � .001.

Mode of mask presentation (constant vs. random) did matter:Mode � Prime � Mask, F(2, 28) � 5.0, � � 1.0, p � .01, �2 �.08. To understand this interaction, we analyzed each mask sepa-rately. Mode of presentation had no effect on no-mask trials, F �

Figure 3. Relationships across participants between performance in prime identification (d�) and the effect ofprimes on responses to targets (diff _rt) in Experiments 1 and 2.

499REVERSAL OF PRIMING BY MASKING

1.2 for mode and Mode � Prime. With the arrows mask, responseswere somewhat slower (by 13 ms) with random variation than withconstant masks, F(1, 14) � 5.8, p � .03, but the effects of primesdid not differ: Mode � Prime, F(1, 14) � 0.0, ns. With the checksmask, responses were also somewhat slower (by 22 ms) withrandom variation than with constant masks, F(1, 14) � 12.3, p �.003, and priming effects were smaller with random variation thanwith constant masks (7 ms vs. 28 ms): Mode � Prime, F(1, 14) �13.6, p � .002).

Errors. Error rates for Experiment 3 are shown in the bottomright panel of Figure 2. Results of errors by and large conformedto results of response times, but there were no effects of presen-tation mode. (All F values of mode effects were less than 0.8 in theoverall analysis.) Prime effects depended on the mask: Prime �Mask, F(2, 28) � 17.3, � � .63, p � .001, �2 � .27. In trialswithout a mask, responses were incorrect in 12% of opposite-prime trials, compared with 2.5% of identical-prime trials: theseparate effect of prime for the no-mask condition, F(1, 14) �13.6, p � .002. Replicating Experiments 1 and 2, more errorsoccurred after opposite than after identical primes with the checksmask (similar to the no-mask condition), with an opposite ten-dency for the arrows mask: Mask � Prime for these two masksonly, F(1, 14) � 15.1, p � .002; effects of prime for the checksmask, F(1, 14) � 4.7, p � .05; effects of prime for the arrowsmask, F(1, 14) � 2.0, p � .18.

Discussion

The lack of an effect of presentation mode on error ratessuggests that participants tried to keep their error rates at a constantlevel across blocks. This might have implied slight costs to re-sponse times when trials with different masks were presented inrandom order, indicated by the generally slower responses witharrows and checks masks in this block.

Of interest, the straight-priming effect with the checks mask wasdampened when presented in the same block as the unmaskedtrials. This is evidence that participants tried to inhibit the effectsof the primes in the presence of the large effect made by theunmasked primes (cf. for similar effects Bodner & Masson, 2001;Jaskowski et al., 2003). However, this presumed inhibition did nothave any effects on the amount of inverse priming that, accordingto the inhibition hypothesis, is caused by just this inhibition.

Discussion of Effects on Mean Response Timesand Error Rates

These three experiments provided reliable evidence for themoderating effect of the mask on priming effects exerted bymasked arrows. Priming was straight with the checks mask andinverse with the arrows mask. On the basis of the assumption thatinverse priming is the rule, the inhibition hypothesis has to assumethat the results obtained with the checks mask are irregular: Eitherthis mask is ineffective, leading to continuing impact of the primes(but see the next paragraph), or this mask is too effective, leadingto minimal activation from the primes and consequently to a lackof inhibition (Schlaghecken & Eimer, 2002). Alternatively, weassume that straight priming is the rule and that the results ob-tained with the arrows mask are irregular, as suggested above. Onehalf of the mask appears first, as prime, and the onset of the other

half occurs later; it is this rapid change of sensory impressions thatprimes the motor system. This appears as a rather unavoidableconsequence of the layout of these stimuli, so the lack of contexteffects on inverse priming in Experiment 3 may be seen as evi-dence for this assumption.

The straight priming effect with the checks mask is in contrastto Eimer and Schlaghecken (2002), Schlaghecken and Eimer(2002), and Aron et al. (2003): Although they used masks con-sisting of short lines instead of the arrows mask, these authors stillobtained inverse priming. This issue will be reconsidered in theGENERAL DISCUSSION section. Here, some specific features ofbehavior with the arrow mask will be highlighted.

Different Mechanisms of Masking

The straight priming effect with the checks mask cannot beascribed to insufficient masking, because d� was virtually 0.0. Onemight ascribe the lack of a difference of d� from zero to a Type IIerror, due to the finite sample size of participants, but this objec-tion would hold equally for both masks. Nevertheless, on the basisof the regression from prime discrimination to priming (Figure 3),it may be argued that the arrows mask worked better than thechecks mask, because priming and discrimination were less relatedto each other with the arrows mask. However, note that accordingto our above description of how the arrows mask works, primediscrimination is fundamentally different with the two masks:With the checks mask, primes may be discriminated as far as they“shine through” the mask (Herzog & Koch, 2001). With the arrowsmask, discriminating primes means making a temporal-order judg-ment about whether the left- or right-pointing part of the maskappeared first (separated by 17 ms). Outside the field of maskedperception, dissociations between the effect of several task param-eters on temporal-order judgment and response times have beenfrequently found, with larger effects on response times than onjudgment (Jaskowski, 1996). Therefore, the present dissociationwith the arrows mask between participants’ failure to discriminatethe primes and the effects of these primes on their responses fitsthat general regularity.

Interactions Between Arrows Mask and Target Arrows

Responses were generally slower and errors were more frequentwith the arrows mask than with the checks mask. Thus, the task ofresponding to the target arrows seemed to be more difficult withthe arrows mask. Occurring even with neutral primes, this effect isnot likely to be due to prime-induced interference but rather due tointerference of the mask on target processing. Similar to the aboveargument on prime–mask relationship, there is a relationship be-tween mask and target such that the target (e.g., a leftward arrow)directly continues the same (e.g., leftward) elements of the mask.So its onset can only be discriminated by virtue of the offset of thecomplementary (e.g., rightward) element of the mask. This rela-tionship might lead to confusion on some trials and make the taskmore difficult.

Asymmetrical Priming With the Arrows Mask

Quite remarkable is the asymmetry of prime effects with thearrows mask: Compared with the delay after neutral primes, the

500 VERLEGER ET AL.

delay after identical primes was distinctly larger than the benefitafter opposite primes. Although not reliably replicated in Experi-ment 2, this asymmetry may be found in results of those previousstudies that used neutral primes (Eimer, 1999; Eimer &Schlaghecken, 1998; Schlaghecken & Eimer, 2001). It is unlikelythat this asymmetry is due to a floor effect, because in Experiment1, even the fastest condition with the arrows mask was, on average,still as slow as the slowest condition with the checks mask. In fact,this asymmetry follows from the assumptions of the inhibitionhypothesis. One of the two motor cortices is first activated by theprime, then automatically inhibited. This motor cortex must firstbe released from inhibition before it can be activated by a follow-ing target that is identical to the prime. However, if the followingtarget is opposite to the prime, therefore activating the other motorcortex, why should this other motor cortex be in a more favorablestate than it would be with neutral primes? Why should responsesbe faster after opposite primes than after neutral primes? Inhibitionof one motor cortex simply need not imply increased activation ofthe other motor cortex.

The active-mask hypothesis can likewise account for this asym-metry by the mechanism described in the previous paragraph: If,indeed, by directly continuing the same elements of the mask,onset of a target (e.g., leftward arrows) can only be discriminatedby virtue of the offset of the complementary (e.g., rightward)element of the mask, then prime-incongruent trials get affectedabove all, because of the preceding prime–mask relationship. Forexample, after leftward primes, attention will be biased toward therightward mask elements. Therefore, a following rightward target(prime-incongruent) will be at some disadvantage for being iden-tified, because identification entails perceiving the offset of thecomplementary leftward mask element. This disadvantage mightbe slight only, relative to the advantage that motor priming is onthe correct side, thus reducing but not eliminating the differencefrom neutral-prime trials.

Errors

More errors were made with the arrows mask when primes werecongruent. This seems to be trivial because more errors are ex-pected to occur in any condition where responses are delayed,otherwise participants would trade speed for accuracy. This hasalso been the regular finding in previous work by Schlagheckenand Eimer (e.g., Eimer & Schlaghecken, 1998). Yet this mecha-nism cannot easily be accounted for by the inhibition hypothesis:Why should participants press the wrong key with congruentprimes? Response errors to targets are expected to occur in re-sponse to primes, and this is what happens with incongruentprimes with the checks mask and, drastically, without mask: Forexample, the target demands right but the incongruent prime hadcued left so the left key might be erroneously pressed. However,with the arrows mask, these errors occur after congruent primes,where no response of the other hand was cued, according to theinhibition hypothesis! One might be inclined to attribute theseerrors to an excess of inhibition. But inhibition of the right-handresponse should lead to participants refraining from responding,that is, delayed responding, rather than to more left-hand re-sponses. If this is a consequence of the inhibitory mechanism, thenthis mechanism works as stop–change rather than as stop-only (DeJong, Coles, & Logan, 1995), which is dysfunctional. In contrast,

the increased error rate with congruent primes poses no problem tothe active-mask hypothesis because this hypothesis indeed pro-poses that the alternative response is cued by the mask. Well awareof this difficulty, Eimer and Schlaghecken (1998) suggested that“both possible responses will be latently activated at the start ofeach trial. If one of these alternative responses is inhibited, thisshould give the other response a competitive advantage” (p. 1746).This notion of a balanced competition between both alternatives,intuitively plausible as it may be, tends to undermine the simpleand elegant account for the asymmetry between delays and gains,as given above: If evidence against one response is actually evi-dence for the other response, then the gain induced by oppositeprimes should not be smaller than the delay induced by identicalprimes. Taking this idea one step further, it can even be argued thatthis notion of a balanced competition between both motor corticestends to undermine the entire notion of inhibition: If evidenceagainst one response is actually evidence for the other response,then the process can be theoretically modeled by a simple racewhere nothing but the difference between activations is critical(see Vorberg, Mattler, Heinecke, Schmidt, & Schwarzbach, 2003),making the notion of extra inhibition redundant. For the moment,a reasonable solution for this difficulty can be given by a two-process model, emphasizing the asymmetry between delays andgains of response times with the arrows mask. The small gainswith opposite primes (6 ms in Experiment 1) and a correspondingsmall proportion (6 ms) of the total delay (26 ms) with identicalprimes (as well as response errors with identical primes) would bedue to the mechanism of balanced competition, whereas the extradelay with identical primes (26 ms – 6 ms � 20 ms) would be dueto inhibition.

Dependence of Priming on Response Speed

A convenient way to learn more about some effect is to study itstemporal dynamics. Therefore, responses were ordered from fast toslow and these response-time distributions were analyzed by de-ciles. Using this approach for the arrows mask, Eimer (1999)found inverse priming for the slower half of responses only, witheven some straight priming for the very fastest responses. How-ever, mask duration was shortened to 50 ms in that study, so resultsmight not be typical of results obtained with the usual 100-msmask duration.

Method

Because basic prime effects ran opposite between masks, analysis wasdone separately for each mask to avoid trivial interactions. Separately foreach participant and each prime–target relation, correct response timeswere ordered across trials by response speed and averaged for each decileof this distribution. ANOVA factors were prime (identical, neutral, oppo-site), decile (9 levels, leaving out the 10th decile to avoid single outlyingresponse times), and experiment (1 vs. 2) or mode of presentation (blockedvs. random; for analysis of Experiment 3). Only interactions of the decilefactor will be reported. When significant, these analyses were followed bycomparisons of the time courses of identical and opposite priming (inanalyses of Experiments 1 and 2, where neutral primes formed a basis forcomparison). In these comparisons, the differences opposite � neutral andneutral � identical were entered into a Prime Difference � Decile �Experiment ANOVA, then separate tests of decile effects for each differ-ence and t tests of prime-difference effects against zero for each decilewere run.

501REVERSAL OF PRIMING BY MASKING

Results

Experiments 1 and 2

With the arrows mask (Figure 4, upper panels), inverse primingincreased from the 1st to the 4th decile and remained constantafterward: Prime � Decile, F(16, 352) � 5.9, � � .28, p � .001,�2 � .25. This increase was entirely due to opposite primes: PrimeDifference � Decile, F(8, 176) � 4.2, � � .42, p � .007; decilefor the neutral–identical difference, F(8, 176) � 1.8, ns; for theopposite–neutral difference, F(8, 176) � 8.7, � � .35, p � .001.Response times did not differ before the third decile betweenopposite and neutral primes ( p � .92 in the first decile, p � .12 inthe second, p � .001 afterward) but differed throughout betweenidentical and neutral primes ( p � .001 or p � .001). These effectsof decile tended to be more marked in Experiment 2: Prime �Decile � Experiment, F(16, 352) � 1.9, � � .28, p � .11,�2 � .02.

With the checks mask (Figure 4, lower panels), straight primingdecreased from the first decile onward, Prime � Decile, F(16,352) � 18.2, � � .50, p � .001, �2 � .30, almost symmetricallyfor identical and opposite primes, Prime Difference � Decile, F(8,176) � 2.1, � � .56, p � .08 only. Decile was significant both forthe neutral–identical difference, F(8, 176) � 14.2, � � .56, p �.001, and for the opposite–neutral difference, F(8, 176) � 6.3, � �.47, p � .001. Identical and neutral primes ceased differing in thesixth decile ( p � .009 or better before) and opposite and neutralprimes in the ninth decile ( p � .003 or better before). Like with thearrows mask, these effects of decile tended to be more marked inExperiment 2, F(16, 352) � 1.9, � � .50, p � .065, �2 � .02.

Experiment 3

Results are displayed in Figure 5. With the arrows mask, inversepriming was smallest in the first decile with the blocked sequence:

Figure 4. Mean response times, shown separately for deciles of response speed in Experiments 1 and 2.Diamonds denote trials with identical primes, circles denote trials with opposite primes, and squares denote trialswith neutral primes. Exp. � Experiment; RT Bin � deciles of the response-time distribution.

502 VERLEGER ET AL.

Prime � Decile � Mode, F(8, 112) � 2.5, � � .53, p � .05, �2 �.02. The Prime � Decile interaction was calculated separately forrandom presentation, F(8, 112) � 0.5, ns, and for blocked presen-tation, F(8, 112) � 3.3, � � .31, p � .04.

With the checks mask, straight priming decreased withresponse speed: Prime � Decile, F(8, 112) � 11.1, � � .36,p � .001, �2 � .23. Figure 5 suggests that, similar to the effectwith the arrows mask, this decrease was more marked withblocked than with random presentation. Separate analyses forblocked and random presentation confirmed this impression:Prime � Decile, F(8, 112) � 13.2, � � .42, p � .001, versusF(8, 112) � 3.3, � � .33, p � .038. However, Prime �Decile � Mode was not significant in the overall analysis, F(8,112) � 0.8, ns.

For the no-mask condition, prime effects did not reliably changeacross deciles: Prime � Decile, F(8, 112) � 0.7, ns; Prime �Decile � Mode, F(8, 112) � 0.4, ns.

Discussion

With the arrows mask, inverse priming was smaller in the firstdeciles (Experiments 1 and 2 and blocked presentation of Exper-iment 3). A similar but stronger effect was reported by Eimer(1999) with even some straight priming for the fastest responses.That particularly strong effect might be related to the shorterduration of the mask in that study; this possibility is discussedbelow in the Compounds of the Two Masks section. Here, thedecrease of inverse priming was entirely due to opposite primes.(Eimer’s, 1999, data do not allow the distinction between effects ofidentical and opposite primes because neutral primes were notused.) Therefore, inverse priming with identical stimuli appears asa stable phenomenon, independent of response speed. What isdependent on response speed is the relative disadvantage of oppo-site primes, making the benefit after these opposite primes lessmarked than the delay after identical primes. Above it was as-

sumed within the framework of the active-mask hypothesis thatthis disadvantage is due to mask–target interactions: Onset of thetarget (e.g., leftward arrows) can only be discriminated by virtue ofthe offset of the complementary (e.g., rightward) element of themask. Now this disadvantage was found to disappear with slowerresponses. This makes sense, because the longer the target isanalyzed (which might be the case for slow responses), the lessrelevant the problem of discriminating its onset is. The inhibitionhypothesis can likewise account for the different time courses withidentical and opposite primes, because two different factors had tobe introduced anyway within this hypothesis (cf. with the Discus-sion section after the three experiments, above): The effect ofidentical primes being constant across response speed may indicatethat inhibition is a fast-acting process, effective already for fastestresponses. The effect of opposite times increasing across responsespeed may indicate that the consequence of this inhibition to thealternative response, enhancing its activation by balanced compe-tition between the two motor cortices, is a slower acting process,not yet available when responses are fast.

Findings similar to the response-distribution analyses of thepresent study and of Eimer (1999) were obtained by Schlagheckenand Eimer (1997, 2000), Aron et al. (2003), and Schlaghecken etal. (2003) when prime-target stimulus onset asynchrony (SOA)was explicitly manipulated: Inverse priming became larger whenSOA increased from 0 to 96 ms, not occurring at brief SOAs at all(Schlaghecken & Eimer, 1997, 2000), and remained stable after-ward (SOAs � 96 to 192 ms; Schlaghecken & Eimer, 2000),similar to the stable effect from the fourth decile onward in thepresent data. Unfortunately, neutral primes were not used in thosestudies, so it is not clear whether these SOA effects were mainlydue to responses to opposite primes, as in the present study.

In intriguing contrast to the stability of inverse priming with thearrows mask is the continuous decrease of straight priming withthe checks mask. This is also in marked contrast to priming

Figure 5. Mean response times, shown separately for deciles of response speed in Experiment 3. Diamonds denotetrials with identical primes, circles denote trials with opposite primes. Blocked order is denoted by empty symbols,random order by filled symbols. RT Bin � deciles of the response-time distribution; RT � response time.

503REVERSAL OF PRIMING BY MASKING

without any mask (Experiment 3) where straight priming did notdecrease across response speed. This decrease of priming mightmean that the checks mask needed some time to have its fullmasking effect. Thus, with fast responses, there were still someeffects of the primes on behavior, and it was only with slowresponses that the mask had come to reach its full masking effect,abolishing any effects of the primes. Alternatively, the mask effectmight be constant over time but, similar to the short-lived activa-tion relevant to the Simon effect (De Jong, Liang, & Lauber, 1994;Wascher, Schatz, Kuder, & Verleger, 2001), sensory impressionand motor activation triggered by the primes might decay overtime in case of masking and thus might affect fast responses only.

The inhibition hypothesis would interpret the decay of straightpriming with the checks mask as reflecting the end of the activa-tion phase, better visible with the checks mask than with thearrows mask because the checks mask is less efficient. However,it is not clear in this view why this decrease of straight priming isnot followed by a reversed, inhibitory effect on slow responses.After all, automatic inhibition of preceding activation is the basicassumption of the model. The lack of any effect on the time coursewith unmasked primes also does not accord too well with themodel. The model would have been confirmed very elegantly by atriphasic effect: first a straight effect, reflecting activation by theprimes, followed by a reversed effect, or at least some reduction ofthe straight effect, reflecting automatic inhibition, in turn followedby a large straight effect, reflecting continuing impact of theunmasked primes.

Compounds of the Two Masks

In addition to the pure masks reported so far, compounds of thetwo masks were presented in Experiment 2. The arrows & checkscompound consisted of the arrows mask followed by the checksmask, and the opposite construction made up the checks & arrowscompound. All four masks (arrows, checks, arrows & checks,checks & arrows) were presented in random sequence, as de-scribed in the Method section of Experiment 2.

Effects of the primes could be expected to be revealed byanalysis of the response-time distributions, reflecting the fast se-quential presentation of the two mask components. On the basis ofits basic assumption that inhibition is the system’s response topremature activation, the inhibition hypothesis predicts that primeswill have straight effects on fast responses and inverse effects onslow responses, in principle independent of the mask. Within theactive-mask hypothesis, one main question is how the interactionof arrow primes and arrow masks will be altered by the interveningchecks component of the checks & arrows compound. There willprobably remain some shine through of primes with the 50-mschecks-mask component. If so, then this remaining trace of primesmight still interact with the following arrows-mask component.Therefore, fast responses, mainly determined by the former com-ponent, will be affected by straight priming, slow responses byinverse priming. For the arrows & checks mask, the active-maskhypothesis predicts a radically different pattern: The reversingmechanism active with the pure arrows mask also applies here butis weakened by the checks mask intervening between the arrowsmask and target. Thus, negative priming should prevail for fastresponses and should level off with slow responses.

Mean Response Times

Mean response times may be seen in the upper left panel ofFigure 6. Primes did not have any effects on mean response timeswith the two compounds: arrows & checks, F(2, 24) � 0.1, ns;checks & arrows, F(2, 24) � 0.8, ns. Responses were generallyfaster with the checks & arrows mask than with the other masks:main effect of mask, F(3, 36) � 4.0, � � .87, p � .019, �2 � .15.A tentative account for this effect is that the arrows component ofthe checks & arrows mask, because of its onset after the interven-ing 50-ms checks component, might have been erroneously clas-sified by participants as target, thereby unspecifically speedingtheir response initiation.

Errors

Error rates may be seen in the upper right panel of Figure 6.Primes did not differentially induce errors with the arrows &checks mask, F(2, 24) � 0.6, ns. But with the checks & arrowsmask, most errors were made with opposite primes and fewesterrors with identical primes, F(2, 24) � 4.4, p � .02, similar tochecks-mask trials. Again, tentatively, this may be interpreted tobe in line with the preceding account: If indeed the second maskcomponent is erroneously taken as the target, then the relevantinformation on direction (at least for fast responses; see below) isprovided by the prime, which may lead to erroneous keypressingon the side indicated by the prime.

Dependence on Response Speed

ANOVAs were performed within each mask compound (withthe factors prime and decile) or between two masks (adding maskas a factor). With both compounds, primes had straight effects onfast responses, no effect in the middle deciles, and inverse effectson slow responses (see the lower part of Figure 6): Prime � Decilewith arrows & checks, F(16, 192) � 15.4, � � .82, p � .001, �2 �.40; with checks & arrows, F(16, 192) � 16.3, � � .34, p � .001,�2 � .41. These effects were larger for the checks & arrows maskthan for the arrows & checks mask: Prime � Decile � Mask,F(16, 192) � 3.2, � � .63, p � .001, �2 � .05. Separate analysisof the straight priming part (the first three deciles) showed thatstraight priming was less marked with both mask compounds thanwith the pure checks mask from the very beginning—Mask �Prime in the first three deciles, for checks & arrows versus purechecks, F(2, 24) � 10.8, � � .61, p � .001; for arrows & checksversus pure checks, F(2, 24) � 16.4, � � .77, p � .001—andtended to be smaller for the arrows & checks compound than forthe checks & arrows compound, F(2, 24) � 3.2, � � .90, p � .07.Similarly, inverse priming for slow responses was less markedwith both mask compounds than with the pure arrows mask—Mask � Prime in the last three deciles, for checks & arrows versuspure arrows, F(2, 24) � 10.2, � � 1.0, p � .001; for arrows &checks versus pure arrows, F(2, 24) � 21.2, � � 1.0, p �.001—with no difference between the two mask compounds, F(2,24) � 0.7, ns. It might be argued that inverse priming, which doesnot develop before the final deciles of the mask compounds,should be compared with the first rather than the final deciles ofthe pure arrows mask. Doing so showed still larger inverse primingfor the pure arrows mask than for the arrows & checks mask,

504 VERLEGER ET AL.

Mask � Prime, F(2, 24) � 4.9, � � .82, p � .02, but only atendency to this direction between the arrows mask and the checks& arrows mask, Mask � Prime, F(2, 24) � 2.3, � � .79, p � .13.Irrespective of primes, the faster speed of responses with thechecks & arrows mask, noted above in analysis of mean responsetimes, was restricted to fast responses until the fourth decile:Mask � Decile, F(8, 96) � 7.6, � � .28, p � .002, �2 � .18.

Discussion

Prime effects were similar with the two mask compounds:Straight priming acted on fast responses, inverse priming onslow responses. Prime effects did reliably differ between bothcompounds, though, reflecting less straight priming of fastresponses with the arrows & checks compound than with thechecks & arrows compound, as well as somewhat less stableinverse priming of slow responses with the arrows & checkscompound. So these differences went in the direction expected

by the active-mask hypothesis, although they admittedly can beseen as weak reflections, at best, of the reversal of effects thatwas expected by this hypothesis for the arrows & checkscompound.

The inhibition hypothesis predicted straight priming for fastresponses followed by inverse priming for slow responses. Resultsconfirmed this prediction. Yet how do these results relate to theresults obtained with pure masks, where this biphasic pattern wasnot obtained? Consider in particular the arrows & checks mask:Why is there straight priming at first, in contrast to the resultsobtained with the pure arrows mask? The inhibition hypothesismust assume that the late checks component of the arrows &checks compound somehow, by acting backward in time, prolongsthe activation phase, which is not plausible.

Similar response-time distributions to the present mask com-pounds were reported by Eimer (1999), with straight priming forfast responses and inverse priming for slow ones. Common to both

Figure 6. Results of the mask compounds in Experiment 2. Upper panels: Mean response times (left) and errorrates (right). The data for the pure arrows and checks masks, already presented in Figure 2, are included forcomparison (dotted lines, crosses, and triangles). The mask compounds are denoted with solid lines. Plus signsdenote the checks & arrows mask, asterisks denote the arrows & checks mask. Lower panels: Mean responsetimes, shown separately for deciles of response speed. Diamonds denote trials with identical primes, circlesdenote trials with opposite primes, and squares denote trials with neutral primes. RT � response time; RT bin �deciles of response-time distribution.

505REVERSAL OF PRIMING BY MASKING

studies is the reduced duration of the arrows mask, from 100 ms to50 ms. Within the active-mask hypothesis, the 50-ms arrows maskmay be seen as too weak to exert its specific, reversing effect inevery trial; thus, primes may have their straight priming effect inthose trials when processing is fast enough to win the race againstthe reversing effect of the arrows mask. The probability that thiswill happen will increase when the arrows mask is delayed, as isthe case with the checks & arrows compound.

Summary of Response-Time and Error Results

Results suggested the following problems and modifications tothe inhibition hypothesis.

1. Prime effects strongly depended on the particular mask used.Specific problems include (a) concepts do not cover how to dealwith prime–mask and mask–target interactions, except for purelyquantitative statements saying that masking is weak or strong, and(b) a biphasic prime effect on response times (activation followedby inhibition) only occurs with mask compounds but not with thearrows mask, not with the checks mask, and not without a mask.

2. Inverse priming, assumed to reflect inhibition of unwantedprime activation, did not increase when (unmasked) primes indeedinterfered with correct behavior in the same block. Thus, thepresumed inhibition was not applied for inhibitory purposes.

3. The inhibition hypothesis proper only applies to delays due toidentical primes. To account for faster responding with oppositeprimes and for errors committed with identical primes, the assump-tion of balanced competition between alternative responses has tobe added. To account for the variation by response speed, it has tobe assumed that this process is slower acting than inhibition is.

EEG POTENTIALS

Evidence from event-related EEG potentials presented by Eimerand Schlaghecken (1998) and Eimer (1999) has been taken as acornerstone for the inhibition hypothesis (Eimer & Schlaghecken,2002; Klapp & Hinkley, 2002). The triphasic waveshape of thelateralized readiness potential (LRP), that is, the contralateral �ipsilateral difference (hereafter referred to as the contra–ipsilateraldifference) between two electrodes overlying the left and rightmotor cortex, beautifully illustrated the assumed sequence of ac-tivation, inhibition, and final activation. We aimed at replicatingthese results, asking the question of whether the second phase ofthis triphasic waveshape indeed reflects inhibition or rather indi-cates activation of the other motor cortex.

Method

Recording and Data Processing

EEG was recorded in all three experiments from 19 scalp sites (F3, Fz,F4, FC3, FC4, C3, C1, Cz, C2, C4, P7, P3, Pz, P4, P8, PO7, PO8, O1, andO2) with Ag/AgCl electrodes (FMS, Munich, Germany). An electrode atthe nose served as common reference and one at the forehead served asground. Electrooculogram (EOG) was recorded to control the intrusion ofocular potentials into the EEG: vertical EOG from above versus below theleft eye, horizontal EOG from the outer canthi of both eyes. The EEG andEOG were amplified within 0.032 Hz to 70 Hz by a Nihon-KohdenNeurotop (Tokyo, Japan) and were digitized at 200 Hz from 100 ms beforethe prime to 900 ms afterward. Offline, trials were excluded when therewere flat lines, slow drifts larger than 60 �V, or fast shifts larger than 100

�V/500 ms. The transmission of vertical and horizontal EOGs into theEEG, as ocular artifacts, was estimated in areas of maximum EOG varianceand was subtracted from the EEG data (Verleger, Gasser, & Mocks, 1982).To obtain the stimulus-evoked time course of activation differences be-tween the two motor cortices (LRP, Coles, 1989; viz., corrected motorasymmetry, De Jong, Wierda, Mulder, & Mulder, 1988) like in Eimer andSchlaghecken (1998) and Eimer (1999), trials with right- and left-pointingtarget arrows were separately averaged. The difference left � right record-ing (C3 � C4) was formed in the average of right targets, the differenceright � left recording (C4 � C3) was formed in the average of left targets,and these two differences were averaged to yield the LRP as the generaldifference contra- � ipsilateral, |C3 � C4|, relative to target-arrow direc-tion (thus also relative to the responding hand). This was done separatelyfor each condition, excluding trials with incorrect keypress responses. Thesame procedure was applied to symmetrical pairs of sites neighboring C3and C4 (|C1 � C2| and |FC3 � FC4|) to increase the signal/noise ratio.

Statistical Analysis

Mean amplitudes were measured during epochs of interest from the threeanterior recording pairs |FC3 � FC4|, |C1 � C2|, and |C3 � C4| oversuccessive 25-ms windows (5 data points each, sampled every 5 ms). Eachwindow was submitted to ANOVAs as described for response times, withpair of recording sites as an additional factor.

Results

Replicating the Basic Effect

Figure 7 displays the LRP waveshapes from |C3 � C4|. Unfor-tunately, the signal/noise ratio of these differences was too low inExperiment 3, so only data of Experiments 1 and 2 will beanalyzed in detail.

The time course of contra–ipsilateral differences obtained withthe arrows mask (see the upper left part of Figure 7) replicatedprevious results (Eimer, 1999; Eimer & Schlaghecken, 1998).First, there was activation contralateral to prime direction (easilyidentified in Figure 7 by divergence between identical and oppo-site primes). Second, of most interest, this activation reversed tothe other side of baseline. Third, negativity (plotted upward)prevailed contralateral to the side of the responding hand (thus tothe direction of the target arrow). This third phase was not ana-lyzed further because contralateral negativity preceding and ac-companying manual responses is a trivial finding. Effects of primewere significant: First phase 205–250 ms in Experiment 1, F(2,20) � 5.6 and 5.2, p � .02, �2 � .20; 255–275 ms in Experiment2, F(2, 24) � 4.4, p � .02, �2 � .21; second phase 330–400 msin Experiment 1, F(2, 20) � 4.9, 6.2, 3.5, p � .02, .01, .05, .13 ��2 � .24; 330–425 ms in Experiment 2, F(2, 24) � 3.7, 6.0, 8.1,4.9, p � .02, .01, .007, .019, .12 � �2 � .27.

Effects With Other Masks

LRPs were also recorded with the checks mask and the maskcompounds. As Figure 7 shows, the first phase (prime-evokedactivation) was larger with the checks mask than with the arrowsmask in Experiment 1. For Prime � Mask in ANOVAs on contra–ipsilateral differences from the three anterior recording pairs, 230–250 ms, F(2, 20) � 4.5, p � .025, �2 � .10; 255–275 ms, F(2,20) � 6.0, p � .009, �2 � .13. This large activation returned tobaseline but was not followed by a correspondingly large

506 VERLEGER ET AL.

reversal, as would be predicted by the inhibition hypothesis, noteven in later phases of the waveshape. Rather, not crossing base-line at all, activation generally stayed more contralaterally negativewith the checks mask than with the arrows mask during the periodwhen activation reversed with the arrows mask. ANOVAs yieldedeffects of mask, reflecting more negativity with the checks maskthan with the arrows mask: 330–350 ms, F(1, 10) � 5.8, p � .04,�2 � .18; 355–375 ms, F(1, 10) � 17.1, p � .002, �2 � .42;380–400 ms, F(1, 10) � 31.3, p � .001, �2 � .58; 405–425 ms,F(1, 10) � 14.9, p � .003, �2 � .39.

Regarding the mask compounds, of particular interest is thereversal of waveshapes with the checks and arrows mask asindicated by an effect of primes with this mask at 405– 425 ms,F(2, 24) � 3.9, p � .048, �2 � .14. This reversal is similar tothat found with the pure arrows mask, and its late latencycorresponds nicely with the finding that inverse priming oc-curred with this mask in the late portion only of the response-time distribution. In contrast, no reversal could be found in thewaveshapes of the arrows & checks mask— effects of primeswith this mask at 405– 425 ms and 430 – 450 ms, F(2, 20) � 0.4,ns—although the reversing effect on response times was onlyslightly smaller than with the checks & arrows mask.

Attempt at Measuring Activity of Either HemisphereSeparately

Returning to the LRP reversal with the arrows mask, theinteresting question is whether this reversal indeed reflectsinhibition of preceding activation. This interpretation implicitlyassumes that the measured contra–ipsilateral difference reflectschanging activation of one motor cortex (contralateral to primedirection) relative to a silent counterpart (ipsilateral to primedirection). Alternatively, according to the active-mask hypoth-esis, this latter motor cortex might get activated rather thanremain silent, thereby reversing the difference. Third, as acompromise between these opposing alternatives, balancedcompetition may be assumed: Activation of one hemisphereimplies and leads to inhibition of the other hemisphere. Toarrive at a decision between these interpretations, we tried tosplit the LRP from trials with identical primes into its contra-and ipsilateral components by separately referring the contra-and ipsilateral recordings to a common reference (the nose)rather than to each other and then comparing these waveshapesto potentials evoked by neutral primes (these latter recordingsaveraged across right and left recordings, because there is no

Figure 7. Contra–ipsilateral difference electroencephalogram potentials (lateralized readiness potentials) re-corded from C3 and C4, the pair of sites overlying the left and right motor cortex. Grand averages are presentedseparately for trials with identical primes (bold black), neutral primes (thin), and opposite primes (bold gray).Voltage negative contralateral to direction of the imperative arrows (on the side of the responding hand) isplotted upward. Scale units are in �V. Time 0 denotes onsets of the primes, thus the masks have their onset at17 ms, the target arrows at 117 ms. Exp. � Experiment.

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contra- and ipsilateral side relative to neutral primes), askingwhether the contralateral waveshape evoked by identical primeswas more positive than the neutral one was (suggesting inhibi-tion), the ipsilateral waveshape evoked by identical primes wasmore negative than the neutral one was (suggesting activation),or both alternatives were true (suggesting balanced competi-tion). These potentials, displayed in Figure 8, unfortunatelycontain large, overlapping stimulus-evoked potentials (conve-niently subtracted out in the contra–ipsilateral difference con-sidered so far; cf. Coles, 1989) differing between neutral andidentical primes. Therefore, potentials evoked by neutral primesare not a neutral reference, and thus the results were ambiguous.Although there was increased ipsilateral activation with iden-tical relative to neutral primes in Experiment 1, confirming theactive-mask hypothesis, neutral primes evoked generally morenegativity than did identical primes in Experiment 2, makingany test impossible (different from what could be accomplishedwith other data by using similar methods to separate contra- andipsilateral contributions, e.g., Verleger et al., 2003; Vidal,Grapperon, Bonnet, & Hasbroucq, 2003).

Effects Of Speed

Similar to the above analyses of response times by response-time deciles, analyses of the LRPs obtained in Experiment 1were conducted, wherein the LRPs were averaged separately forresponse-time quartiles. (More fine-grained separations, e.g.,into deciles, did not make sense because of the low signal/noiseratio.)

This analysis proved useful for understanding the func-tional hallmark of the reversal: Is its absolute level decisive,

that is, the fact that the activation difference sinks belowbaseline, or is its relative level decisive, that is, the fact that thewaveshape is less negative than with the other primes? Theanalyses and considerations presented by Eimer andSchlaghecken (1998) and Eimer (1999) leave few doubts thatthese authors regarded the absolute level as decisive. In botharticles, the significance of the reversal was tested againstbaseline rather than against the waveshape evoked by neutralprimes, even for the price of failing to obtain a significant effect(Eimer, 1999). The latter article did not even discuss the pos-sibility that this absolute criterion might be too conservative,presumably because the absolute level below baseline is intu-itively much more plausible in terms of the inhibition hypoth-esis; compare Figure 4 in Eimer and Coles’ (2003) reviewwhere the first and second phases of the waveshape from Eimer(1999) were marked by conspicuous labels on different sides ofthe x-axis.

Grand means, split by quartiles, are displayed in Figure 9.Because there were three relevant components with the arrowsmask but only two with the checks mask, results will be firstdescribed for the checks mask.

With the checks mask, response speed affected the overlap oftwo components:

1. Negativity contralateral to the primes always occurred about250 ms after prime onset, as confirmed by effects of primes inseparate analyses for each quartile: first quartile, 205–300 ms, F(2,20) � 8.0, p � .009; second quartile, 230–275 ms, F(2, 20) �10.9, p � .002; third quartile, 255–275 ms, F(2, 20) � 7.1, p �.007; fourth quartile, 230–275 ms, F(2, 20) � 5.1, p � .03. Itappears from these tests that the effect was larger when responses

Figure 8. Attempt at attributing the reversal of the contra–ipsilateral difference |C3 � C4| with identical primesand arrows masks (top row, identical to the bold black lines in the two upper left panels of Figure 7, critical epochmarked by vertical lines) to either contralateral inhibition or ipsilateral activation. To decide between thesehypotheses, we resolved the contra–ipsilateral difference to its constituent contralateral and ipsilateral elements(lower panels) by referring both the contralateral and the ipsilateral recording sites to a third recording site (thenose) rather than to each other. Bold black depicts recordings from the site contralateral to the target, bold grayfrom the ipsilateral site. These potentials were compared with potentials evoked with neutral primes (thin line).All potentials were averaged across C3 and C4. We conclude that the neutral waveshape might serve as pointof reference to define whether contralateral is positive or ipsilateral negative in the data of Experiment 1 butdefinitely not in the data of Experiment 2.

508 VERLEGER ET AL.

were faster. Indeed, in an omnibus ANOVA, response speed(response-time quartile) and prime effect interacted in the 230–250-ms window, F(6, 60) � 2.9, p � .02. But this interaction wasfar from significance in the other 25-ms windows.

2. Negativity contralateral to the target arrows, leading to thefinal response, obviously varied with response speed, as confirmedby effects of response-time quartile from 280 ms to 400 ms, F(3,30) � 4.5, 7.7, 9.2, 4.3, 4.0; p � .03, .004, .002, .03, .03.

Of interest, this latter effect affected the overlap of the twocomponents. For fast responses, the prime-evoked component rodeon the rising response-related negativity, whereas from the secondresponse-time quartile onward, response-related negativity startedlate enough to leave the prime effect symmetrical to baseline(identical primes above baseline, opposite primes below).

Finally, in no response-time quartile did the prime effect reverse. Theapparent hint for such a reversal in the fourth quartile between 300 msand 400 ms, with identical primes being least negative, was far fromsignificance, F(2, 20) � 1.1 at best, in the 355–375 ms window.

With the arrows mask, LRPs consisted of three components:prime effect, reversal of the prime effect (in addition to the checksmask), and target effect.

1. Negativity contralateral to the primes, again about 250 ms afterprime onset, was somewhat weaker than with the checks mask, so p �.10 tendencies will be noted as well: first quartile, 205–250 ms, F(2,

20) � 5.4, 3.4; p � .01, .07; second quartile, 205–275 ms, F(2, 20) �3.8, 3.0, 3.3; p � .04, .07, .06; third quartile, 255–275 ms, F(2, 20) �2.7, p � .09; fourth quartile, 230–300 ms, F(2, 20) � 3.0, 3.4, 5.8;p � .08, .09, .04. Reaction-time quartile and prime effect did notinteract in the omnibus ANOVA, that is, the prime effect did notreliably differ between quartiles.

2. From about 320 ms onward, the prime effect reversed: firstquartile, 330–375 ms, F(2, 20) � 4.4, 2.5; p � .046, .10; secondquartile, 355–375 ms, F(2, 20) � 9.3, p � .001; third quartile,380–425 ms, F(2, 20) � 2.9, 3.1; p � .09, .07; fourth quartile,405–450 ms, F(2, 20) � 3.7, 2.9; p � .04, .08. These effects raisethe impression that the reversed prime effect was shifted in timewhen responses were slow. Indeed, response-time quartile andprime effect interacted during the 405–450-ms epochs, F(6, 60) �3.1 in both windows, p � .01, suggesting that the reversed primeeffect sustained longer when responses were slow.

3. Negativity contralateral to the target arrows, leading to thefinal response, again varied with response speed, statistically con-firmed by effects of response-time quartile from 305 ms to 425 ms,F(3, 30) � 4.6, 6.8, 5.8, 4.7, 6.4; p � .02, .003, .02, .02, .003.

Of interest again is the overlap of this latter negativity with theprime effects. Different from the checks mask, there was onlyslight, if any, overlap with the first prime-related component in thefirst quartile. This lack of overlap is in line with the generally

Figure 9. Contra–ipsilateral difference electoencephalogram potentials (lateralized readiness potentials) sep-arately averaged for quartiles of response speed in Experiment 1. Upper four panels: Grand averages, shownseparately for trials with identical primes (bold black), neutral primes (thin), and opposite primes (bold gray).Lower panel: Grand averages across all primes, separately for first (bold black), second (bold gray), third (thin),and fourth (hair-thin) quartiles. Potentials were recorded from C1 and C2 rather than C3 and C4 because of somenoise intrusions to the latter sites. Voltage negative contralateral to direction of the imperative arrows (on the sideof the responding hand) is plotted upward. Scale units are in �V. Time 0 denotes onsets of the primes, thus themasks have their onset at 17 ms, the target arrows at 117 ms. The arrows mark mean response times withidentical (black), neutral (open triangles), and opposite (gray) primes.

509REVERSAL OF PRIMING BY MASKING

slower responses with the arrows mask, such that target-relatedactivation generally was too late to raise the early prime effect tothe negative range. However, there was considerable overlap withthe following inverse prime effect: Riding on the rising flank oftarget-related negativity, this reversed prime effect was visible inthe first and second quartile as modulation of this flank, much thesame way the earlier (straight) prime effect rode on the risingnegativity in the first quartile with the checks mask. In contrast, inthe third and fourth quartiles, target-related negativity started solate that it left the reversed prime effect symmetrical to baseline oreven (in the fourth quartile) slightly shifted below baseline.

Discussion

We replicated the triphasic waveshape of the LRP first reportedby Eimer and Schlaghecken (1998;3 cf. Eimer, 1999; Seiss &Praamstra, 2004). Those authors interpreted the three phases asbeing a reflection of a sequence of events occurring within thecontralateral motor cortex: primary activation, automatic inhibi-tion, and final correct activation. Critical is their interpretation ofthe second phase, where baseline was crossed to yield a reversal ofpolarity. Rather than inhibition of the previously activated side,this polarity reversal might reflect activation of the other motorcortex.

Our attempt to arrive at a decision between these alternatives byresolving the contra–ipsilateral difference into its contralateral andipsilateral constituents did not yield clear results.

Relative Rather Than Absolute Positivity

However, separately averaging trials by response-time quartilesprovided some relevant information. Most important, the secondphase (reversed prime effect) occurred in different combinationswith the third phase (target-related negativity): With fast re-sponses, the reversed effect rode on this negativity and thus re-mained on the negative side of baseline. With slow responses, thereversed effect occurred before the onset of target-related negativ-ity and thus lay on the positive side of baseline. In spite of thesedifferences in absolute polarity, the response-time delays inducedby identical primes were of similar size across response-timequartiles. Therefore, evidently, the absolute level of positivityreached by the LRP is not relevant to the behavioral effect. Rather,the relative degree of positivity seems important, relative to thelevels reached in trials with neutral or opposite primes in a givenresponse-time quartile: This relative positivity might delay re-sponding after identical primes by preventing the target-relatednegativity from reaching the response threshold. This conclusion isin good agreement with data recently published by Eimer andSchlaghecken (2003) presenting LRPs recorded for differentprime-target SOAs. In the one SOA condition (96 ms) that pro-duced reliable inverse priming, a relative reversal was obtained,just touching baseline but not crossing it.

If, then, relative positivity is relevant rather than polarity rever-sal below baseline, the inhibition hypothesis loses some of itsintuitive plausibility. Rather than being prevented from gettingactivated by the assumed inhibition, the motor cortex may getinhibited at the same time it gets activated for the final response.Although not impossible, this appears somewhat paradoxical. Sucha paradox does not show up for the alternative hypothesis, which

assumes that the relative positivity is created by activation of theother motor cortex. Activation of both motor cortices may easilyoccur simultaneously.

Relative Positivity in Other Tasks

Relative positivity of the LRP waveshape is not at all a newphenomenon peculiar to masked arrows, but it has been describedin several choice–response paradigms as an effect of incongruousinformation, for example, of response-incompatible stimuli flank-ing the target (the Eriksen effect; first reported by Gratton, Coles,Sirevaag, Eriksen, & Donchin, 1988), of response-incompatiblelocation of the target (the Simon effect; De Jong et al., 1994;Sommer, Leuthold, & Hermanutz, 1993), and of response-incompatible masked primes other than arrows that produce re-sponse delays (Dehaene et al., 1998; Jaskowski et al., 2003;Jaskowski, van der Lubbe, Schlotterbeck, & Verleger, 2002; Leut-hold & Kopp, 1998). Studies on the Eriksen effect that varied theSOA between flankers and target (Mattler, 2003; Wascher, Rein-hard, Wauschkuhn, & Verleger, 1999) and split trials by responsespeed (Heil, Osman, Wiegelmann, Rolke, & Hennighausen, 2000;Osman et al., 2000) demonstrated that, like in the present response-time quartile analysis, the relative positivity occurred as baselinecrossing when target-related activation had not begun yet and rodeon the target-related negativity otherwise. Most important, no-where in this literature has the assumption been made that therelative positivity associated with incompatibility is due to inhibi-tion of the correct motor cortex. Rather, this relative positivity hasbeen generally ascribed to incorrect activation of the other side,either of the motor cortex or (cf. below) of attention-related centersin the premotor cortex.

Source of Prime-Related Activity

There can be little doubt that target-related contralateral nega-tivity (the third phase of the triphasic sequence) is generated in themotor cortex. Good evidence for this claim has been provided byintracranial recordings from the human motor cortex (Clarke,Halgren, & Chauvel, 1999). However, this is not a matter of coursefor prime-related activity. Doubts come from two sets of evidence.First, when (unmasked) arrows were used as cuing stimuli toindicate the side of the target stimuli or of the response appropriateto the target stimuli, these arrow cues evoked contralateral nega-tivity. This negativity was proposed to reflect motor-cortex acti-vation (Eimer, 1995) but actually turned out to be independent ofhand movements (Eimer, Forster, & van Velzen, 2003; Verleger,Vollmer, Wauschkuhn, Van der Lubbe, & Wascher, 2000) and tobe distributed slightly more anteriorly than hand-movement activ-ity (Verleger et al., 2000); therefore, it has been interpreted aspremotor-cortex activity, related to encoding the action-relevantparameters of the cue (Verleger et al., 2000) in anatomical coor-dinates (Eimer et al., 2003). A problem in identifying that com-ponent with the present prime-related activity is its latency of 400

3 These authors computed the difference potential as ipsilateral � con-tralateral, rather than contralateral � ipsilateral, and formed the sum, ratherthan the average, over the averages of left-hand trials and right-hand trials.Therefore, compared with the present data, their data are inverted and twiceas large.

510 VERLEGER ET AL.

ms postcue, whereas the first peak of prime activation is at 250 mspostprime in the present task. The second set of evidence comesfrom laterally presented target stimuli that induce a Simon effect.Good arguments have been put forward by Praamstra and Oost-enveld (2003) that what has often been interpreted as subthresholdmotor activity in response to stimulus location (e.g., by Valle-Inclan, 1996; Wascher et al., 2001) rather is attention-relatedactivity of the premotor cortex. This activity peaks at 250 mspoststimulus, like the present prime-related activity. From theseinstances, it may also be suggested that the present first peak ofprime-related LRP indicates premotor activity of attending toaction-relevant parameters rather than activity of the motor cortex.According to the inhibition hypothesis, its reversal would thenreflect inhibition of the preceding premotor activity. Thus, there isno paradox anymore that inhibition would occur simultaneouslywith final activation: It is not the motor cortex that is inhibited butafferent activity from the premotor cortex. This notion would alsobe consistent with the null effect on priming obtained by applyingrepetitive transcranial magnetic stimulation before the task toinduce exhaustion of the motor cortex (Schlaghecken et al., 2003):Stimulation did affect the motor cortex, as seen by the generaldelay of responses, but left priming unaffected. This might haveoccurred because priming is not a process within the motor cortex.

Yet, by this notion of premotor cortex activation, the inhibitionhypothesis further loses some of its intuitive plausibility, becausewhat is inhibition for if it is not the motor cortex that getsinhibited? In contrast, the active-mask hypothesis does not haveany problem in interpreting this variant. The positive reversalwould reflect activation of the other premotor cortex, due to thelaterally biased impression evoked by mask onset.

Timing of Activation and of Its Reversal

Prime-induced activation and its reversal had their peaks con-sistently at about 250 ms and 360 ms and thus were at least 100 msapart. The inhibition hypothesis may simply assume that this is thefixed time needed by the inhibitory process. To the active-maskhypothesis, however, this interval of 100 ms poses a problem: Whynot 17 ms, that is, the interval between prime onset and maskonset? Some inertia is apparently built in, preventing a fast changeof activation, but why is this so? Should it not be possible toactivate one (pre)motor system immediately after the other? Oneway out of this problem is to assume that there is some form ofbalanced competition: In this task, where only one response can becorrect, activation of one response excludes activation of the other;therefore, the alternative response does not get activated before theactivation of the original response has decayed. If conceived as apremotor attention-related process, this notion of balanced com-petition would be akin to the controversial notion of attentionaldwell time (Ward, Duncan, & Shapiro, 1996; Woodman & Luck,1999).

Thus, both the inhibition hypothesis and the active-mask hy-pothesis need the concept of balanced competition, although forentirely different reasons. Without it, the inhibition hypothesiscould not explain the increased rate of errors with identical primesand the active-mask hypothesis could not explain the basic phys-iological result of two activations separated by 100 ms.

Summary of LRP Results

The LRP waveshape consists of prime-evoked activity andtarget-related activity. These components may overlap. The formeris biphasic with the arrows mask but monophasic in all otherknown cases (checks mask, irrelevant flankers, irrelevant location,metacontrast-masked primes). The prime-evoked component mayreflect activity of the premotor cortex rather than of the motorcortex proper.

The inhibition hypothesis faces three problems: Why is thebiphasic pattern so highly specific to arrows masked by the arrowsmask? In cases of overlap, can the second phase be adequatelydescribed as inhibition, with activation taking place at the verysame time? Why should interpretation of this phase be differentfrom the interpretation given in other tasks? The active-maskhypothesis, in turn, has to find an answer to the question of whythe two peaks of prime-related activity (evoked by prime andmask) are separated by such a relatively long interval.

GENERAL DISCUSSION

Summary of Issues Critical to the Inhibition Hypothesis

1. Inverse priming was abolished by replacing the arrows maskwith the checks mask. Further, priming has always been straight inthe literature on masked priming when stimuli different fromarrows were used (e.g., words, letters, shapes). Therefore, theinhibition hypothesis is in danger of overgeneralizing a phenom-enon that is specific to arrows as primes, masks, and targets.

2. A relative rather than an absolute amount of LRP positivitywas associated with inverse priming. Such relative LRP reversalshave been consistently interpreted in the literature as activation ofthe other response rather than as inhibition of the preparedresponse.

3. The putative inhibition mechanism was not applied for inhib-itory purposes: Inverse priming did not increase in the presence ofmassive, interfering straight priming.

4. The inhibition hypothesis needs the additional assumption ofbalanced competition to account for the basic effects of primes onerrors and of opposite primes on response times.

Summary of Issues Critical to theActive-Mask Hypothesis

The active-mask hypothesis also met with problems, though.1. Mask compounds of arrows & checks did not produce inverse

priming followed by straight priming, as would be expected by thishypothesis.

2. The temporal interval between the peaks of prime activationand prime reversal in the LRP waveshape is much larger thanwould be expected by this hypothesis.

In view of these inconsistencies, further research is needed, andit is certainly an important merit of the work by Eimer andSchlaghecken (Eimer, 1999; Eimer & Schlaghecken, 1998, 2001,2002; Eimer, Schubo, & Schlaghecken, 2002; Schlaghecken &Eimer, 1997, 2000, 2001, 2002; Schlaghecken et al., 2003) thatthese new questions may be asked. In the remainder of thisdiscussion, we outline some relevant lines along which researchshould proceed.

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Inverse Priming With Other Masks and Stimuli

In recent studies, Schlaghecken and Eimer (2002), Eimer andSchlaghecken (2002), and Aron et al. (2003) had abandoned thearrows mask in favor of a lines mask (likewise Seiss & Praamstra,2004). This mask was similar to our checks mask, although notidentical (unfortunately; our experiments were carried out in 2000and 2001), consisting of a rougher “macrostructure” (i.e., a matrixof 6 � 5 elements vs. 10 � 10 or 11 � 11 in the present study) anda finer “microstructure” (i.e., small lines in the elements vs.uniform squares). Even with this new mask, primes had inverseeffects. Further, the mask used by Klapp and Hinkley (2002),consisting of the letters W and X, produced inverse priming, likethe arrows mask. These recent results appear to form a strongargument that the inhibitory phenomenon is less stimulus specificthan summarized above.

We do not have a ready-made answer to these questions at thismoment. But we make the following points on Klapp and Hin-kley’s (2002) study:

1. Participants were able to identify the primes at a rate betterthan chance, in contrast to participants of our study.

2. Targets were presented for 16 ms only, as those authors’explicit intention was for the participants to be influenced more bythe preceding stimuli. That is, although those authors’ theoreticalfocus was on response selection, reduced stimulus identificationmight have played a decisive role in obtaining the effects.

3. Perceptual learning occurred in a counterintuitive way. Usu-ally with repeated presentations, observers learn to ignore the maskand to identify the masked stimuli. Conditions that promote suchperceptual learning are constant masks (Schubo et al., 2001) andovernight sleep (Gais, Plihal, Wagner, & Born, 2000). Usingprecisely these conditions (constant masks and testing observerson a second day), Klapp and Hinkley (2002) obtained decreases ofprime identification performance together with drastic increases ofinverse priming. Did observers unlearn to see the prime or, moreprobably, did they learn to see something else? Did they perhaps,by constant practice with the arrows, learn to see the maskingletters X and W as compounds of these arrow elements? This is avery interesting question, absolutely not trivial, and should beexplored in future experiments. Therefore, we believe that Klappand Hinkley’s (2002) results are important in offering a new,interesting perspective on perceptual learning rather than on re-sponse selection.

Returning to Schlaghecken and Eimer’s (2002) lines mask, wetherefore believe that the perceptual hypothesis should not bedismissed. Our hypothesis on the perceptual mechanism inducedby the arrows mask is obviously not general enough. Furtherexperiments should explore the difference between that lines maskand the present checks mask. Critical might be that the short linesin the lines mask had the same width as the lines used for the arrowprimes and masks and that many lines in the lines mask had thesame 45° angle as lines in the arrowheads, that is, lines masks werescrambled versions of the primes, whereas the checks mask is an“energy mask” that works by interrupting perception of the primesaltogether. In other words, one prerequisite for inverse primingmight be that the mask contains identifiable elements of therelevant stimuli (Jaskowski & Przekoracka, 2004; Lleras & Enns,2004). Still, the regularities stated in the introduction for thearrows mask might hold for such scrambled masks: Arrow ele-

ments in the scrambled mask that point to the same direction as thepreceding prime might be less salient. Indeed, there is evidencethat arrows are not identified as well when they point in thedirection at which a response is prepared (Musseler & Hommel,1997; Musseler, Steininger, & Wuhr, 2001; Wuhr & Musseler,2001). It seems plausible that such an effect is less drastic thanwith the original arrows mask where the mask elements com-pletely cover the corresponding prime element, and this wouldaccount for the smaller amounts of inverse priming with thescrambled mask than with the arrows mask that was noted in theintroduction. It may be objected that participants should neglectthese mask elements in signal discrimination and therefore shouldbe biased to respond to the opposite-pointing elements also insignal discrimination, but note that, as demonstrated by Musselerand colleagues, identification of response-compatible elements isworse only when a speeded response is prepared for the precedingstimulus at the same time. This applies to identification of maskelements in the response-time task but not in the prime identifica-tion task. Yet this account is tentative and, clearly, more workneeds to be done on this issue.

Relationship of Masked Inverse Priming toOther Paradigms

There are obvious parallels between Eriksen’s flankers task(Gratton et al., 1988) and the present masked-priming task, inparticular in the version introduced by Schlaghecken and Eimer(2002). In both tasks, accessory stimuli (called primes in one case,flankers in the other) are presented at locations flanking fixation,and a target is presented at fixation. These stimuli may be arrowsin both tasks (e.g., in the flankers task, Kopp, Mattler, Goertz, &Rist, 1996; Mattler, 2003; Wascher et al., 1999). In both tasks,SOAs of 0 ms and 100 ms may be used between accessory stimuliand targets. In spite of these similarities, results differ betweentasks. In Schlaghecken and Eimer’s (2002) task, priming wasstraight with an SOA of 0 ms and inverse with an SOA of 100 ms(likewise in Aron et al., 2003, and Seiss & Praamstra, 2004). Incontrast, in the flankers task, flankers have straight effects withboth SOAs, and the effect is much larger with a 100-ms SOA thanwith a 0-ms SOA (e.g., Mattler, 2003; Wascher et al., 1999).Furthermore, flanker-induced activity in the LRP did not reverse,in contrast to the reversal found with masked primes, not evenwhen SOAs were prolonged to 500 ms to leave room for suchactivity to become apparent (Mattler, 2003; Wascher et al., 1999).One might argue that these differences are simply due to thedifference between masked and unmasked stimuli. But priming inthe Eriksen task was also positive with masked flankers (Schwarz& Mecklinger, 1995). Letters were used as stimuli in the latterstudy, though, so this again points to the possibility that inversepriming of masked stimuli might be a stimulus-specific phenom-enon. Furthermore, on a more theoretical level, it is hard to seewhy inhibition, as conceived by the inhibition hypothesis, shouldnot also be successfully exerted on the effect of flankers. But theavailable evidence suggests that this is virtually impossible(Miller, 1991), even in the extreme case when the flankers couldnot be consciously identified (in neglect patients; Danckert, Ma-ruff, Kinsella, de Graaff, & Currie, 1999). Obviously, this contra-diction between closely related paradigms needs to be resolved byfurther studies.

512 VERLEGER ET AL.

Parallels may also be drawn between inverse priming and thephenomenon known as negative priming: Responses tend to bedelayed to stimuli that served as distracting stimuli in precedingtrials (May, Kane, & Hasher, 1995; Milliken, Joordens, Merikle, &Seiffert, 1998). Similar to the present inhibition hypothesis, thateffect has been related to inhibition, in that case because of aninhibitory tag placed on the response to the distracting stimuluselement that was still effective in the following trial. And indeed,some phenomena reported in this research context, in whichmasked, to-be-ignored words were used as stimuli (Ortells, Fox,Noguera, & Abad, 2003), bear a striking similarity to the presentparadigm.

However, and this might also be fruitful to the present researchcontext, an alternative account of negative priming has been pro-posed in terms of episodic retrieval: Rather than indicating inhi-bition, response delays reflect interference at the moment of se-lecting the appropriate response in the present trial (e.g., Verleger,1991, in a Stroop color-naming task). Application of the episodic-retrieval account to masked priming has indeed been proposed foreffects of masked words on responses to following words byBodner and Masson (2001). Participants recruit resources providedby masked stimuli only if this is of advantage for selecting theresponse to the present stimulus: Context matters. However, con-text effects did not occur for inverse priming in the present data, soinverse priming might be more distant from classical effects ofnegative priming than what would seem at first sight. But clearly,these questions merit more research.

Identifying Straight Priming With Conscious Processing

Varying parameters of prime visibility (prime duration or maskdensity), Eimer and Schlaghecken (2002) obtained inverse primingwhen primes could not be discriminated and straight priming whenthey could. This relationship could also be demonstrated by cor-relation across individuals and was interpreted in terms of adichotomy between straight conscious and inverse nonconsciouspriming. We are moved to comment: As a matter of course,straight priming is larger when the primes are more visible. Thismay be seen by the difference between unmasked and maskedpriming (i.e., in the present Experiment 3; Klapp & Hinkley, 2002,Experiment 1) or between incomplete and complete metacontrastmasking (Jaskowski et al., 2002). Eimer and Schlaghecken’s(2002) critical claim is that the point of transition from straight toinverse priming corresponds to the point of transition from con-scious to nonconscious processing. This may be true for the datareported in that article. But in Klapp and Hinkley (2002), inversepriming was obtained also in those participants who could identifythe primes with more than 55% probability. Therefore, inversepriming can evidently not be identified with nonconscious pro-cessing. Correspondingly, stable straight priming was obtained bySchlaghecken and Eimer (1997, 2000, 2002) when primes werepresented peripherally or in a degraded way, which was interpretedby these authors to mean that the primes were too weak to elicitinverse priming. Therefore, even in this very paradigm, straightpriming can evidently not be identified with conscious processing.Moreover, Vorberg et al. (2003) demonstrated large increases ofstraight priming with increases in prime-target SOA in the absenceof any visibility of the primes (masked by metacontrast), confirm-ing that priming effects on response times may be entirely inde-

pendent of conscious perception. Thus, postulating a dichotomybetween inverse nonconscious and straight conscious priming doesnot seem justified at present, and we prefer the alternative inter-pretation that by varying parameters of prime duration and maskdensity, perceptual relationships between primes and masks werechanged that are essential for inverse priming.

On a most general level, it may be frustrating that the importantquestion about differences between conscious and unconsciousprocessing boils down to the question about how exactly masks areconstructed. Admittedly, this does not have much to do anymorewith implications of the general question about unconscious influ-ences, such as whether mankind is inadvertently driven by rationaland irrational forces, whether id and ego are mutually exclusiveforces or may be reconciled. We have provided evidence that theresults on masked priming cannot necessarily be interpreted infavor of a dichotomy between conscious and unconscious process-ing. By inference, there is no qualitative dichotomy betweenrational and irrational behavior and between id and ego. But sayingthis at the same time means admitting that we cannot draw anysound inferences to these big questions from the results on inversepriming.

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Received March 3, 2003Revision received April 19, 2004

Accepted April 21, 2004 �

New Editor Appointed for Journal of Occupational Health Psychology

The American Psychological Association announces the appointment of Lois E. Tetrick, PhD, aseditor of Journal of Occupational Health Psychology for a 5-year term (2006–2010).

As of January 1, 2005, manuscripts should be submitted electronically via the journal’s ManuscriptSubmission Portal (www.apa.org/journals/ocp.html). Authors who are unable to do so shouldcorrespond with the editor’s office about alternatives:

Lois E. Tetrick, PhDIncoming Editor, JOHPGeorge Mason UniversityDepartment of Psychology, MSN, 3F54400 University Drive, Fairfax, VA 22030

Manuscript submission patterns make the precise date of completion of the 2005 volume uncertain.The current editor, Julian Barling, PhD, will receive and consider manuscripts through December31, 2004. Should the 2005 volume be completed before that date, manuscripts will be redirected tothe new editor for consideration in the 2006 volume.

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