Journal of Experimental Psychology:Animal Behavior Processes1980, Vol. 6, No. 2, 188-205

One-Trial Overshadowing: Evidenceof Distributive Processing

James H. James and Allan R. WagnerYale University

Evidence of "overshadowing" following a single training trial was obtainedin each of five experiments measuring stimulus-evoked suppression of lickingin rats. In three conditioned emotional response experiments, less conditionedsuppression was observed to either a light or a tone when the stimuli hadbeen paired with shock in compound than when either had been paired withshock in isolation. This difference occurred when the stimuli were diffuse andproduced differential orienting behaviors on the training trial (Experiment1). But, it was as demonstrable when the stimuli were designed to be localiza-ble from the same source and produce apparently compatible orienting be-haviors (Experiments 2A and 3A). The difference was also as substantialwhen the stimulus compound was composed of successive, nonoverlappingstimulus elements as when composed of simultaneous elements (ExperimentSA). In two companion habituation studies (Experiments 2n and 3fi) usingthe same stimulus arrangements but no pairing with shock, there was con-sistently less habituation of the unconditioned suppression to the visual andtonal stimuli when exposed in compound rather than in isolation. One-trial"overshadowing" is at odds with those theories that approach overshadowingonly as a multiple-trial phenomenon. The apparent generality of the effectprovides impetus for a theoretical account that supposes some manner ofdistributive processing.

When Pavlov (1927) described the phe- land & Mackintosh, 1971) are wont tonomenon of "overshadowing," his terminol- predict: If one emphasizes that the subjectogy suggested the obvious interpretation, has a limited attentional capacity, then it isHe indicated that when two isolable CSs congenial to suppose that attending to oneare trained in compound to signal a US, CS will decrease the attention to, and learn-one CS may "obscure" the other (Pavlov, ing about, another concurrent CS.1927, p. 142), so as to deprive it of being Recent accounts (e.g., Mackintosh, 1975;associated with the US to the same degree Rescorla & Wagner, 1972) of overshadow-as if it were trained in isolation. Overshad- ing and related Pavlovian phenomena haveowing is just the manner of effect that dis- argued against this interpretation. Rescorlatributive attentional theories (e.g., Suther- and Wagner (1972) acknowledged that the

organism "certainly does not have an un-limited capacity to process sensory informa-

This research was supported by National Sci- tion" but wondered "whether the organ-ence Foundation Grant BNS77-16886 to A. R. ism's capacity is so limited that it is neces-Wagner and by a postdoctoral fellowship to J. H. sar to assume tnat several highly distinctiveJames from the National Institute of Mental . .. . , . , . .Health (5 F32 MHOS760). stimuli, as employed in compound-stimulus

J. H. James is now at Bell Laboratories, In- Pavlovian training, cannot generally be si-dianapolis, Indiana. multaneously attended to" (p. 94). In the

Requests for reprints should be addressed to . -MI \ • <. \, /•tnic.\ t, jAllan R. Wagner, Department of Psychology, same veln' Mackintosh (1975) observedYale University, New Haven, Connecticut 06520. that "animals may have a limited capacity

Copyright 1980 by the American Psychological Association, Inc. 0097-7403/80/0602-0188$00.75

188

ONE-TRIAL OVERSHADOWING 189

for processing information, but it is hard tobelieve that this limitation prevents the si-multaneous analysis of the relevant salientstimuli typically used in studies of condi-tioning" (p. 296). As an alternative, Res-corla and Wagner (1972) and Mackintosh(1975) interpreted overshadowing as gen-erally being a special case of "blocking"(see Kamin, 1969) : To the degree that ini-tial compound training allows one CS com-ponent to acquire associative strength, itmay reduce the association between the re-maining CS component(s) and the US thatcan result from subsequent compound trials.The proposed mechanisms of this blockingeffect are either that the US is rendered lessreinforcing on any trial the better it hascome to be signaled by the totality of avail-able stimuli (Rescorla & Wagner, 1972)or that a potential CS is rendered less asso-ciable on any trial the more it has previouslybeen experienced as redundant to other ef-fective signals (Mackintosh, 1975).

Setting aside questions of the general via-bility of the mechanisms proposed by Res-corla and Wagner (1972) and Mackintosh(1975) in other circumstances, and thelikelihood that they may thus be implicatedin studies of Pavlovian overshadowing,there are reasons to suppose that processingantagonisms may also contribute to theeffect.

First, while the CSs that are employedin Pavlovian conditioning are typically ar-ranged to produce diffuse environmentalchange that can be appreciated without ne-cessity of special receptor-orienting acts,they are not therefore necessarily arrangedto produce homogeneous environmentalchange the experience of which is indiffer-ent to de facto orienting behaviors. For ex-ample, the changes in general illuminationand auditory surround that may be at-tempted in conventional, small-animal con-ditioning chambers result in complex spa-tial patterns depending on the reflecting sur-faces as well as the nature of the source.In response to such diffuse CSs, subjectshave been observed (e.g., Holland, 1977) tomake stimulus-specific orienting responsesthat are quite different in the cases of audi-

tory and of visual stimuli. In this circum-stance it is possible (for example) that asubject receiving a compound of a visualand an auditory CS during training may, asa result of the orienting responses evoked,experience a different pattern of visual stim-ulation than a subject receiving only thevisual CS during training and than eithersubject when receiving the visual CS inisolation during testing. Differences in testperformance indicative of overshadowingcould thus result from peripheral antago-nisms in the subject's attention.

Second, even if the CSs that are em-ployed in Pavlovian conditioning are ar-ranged to be obtrusive enough to ensureadequate reception, it is by no means clearthat any distribution of stimulus processingneed take place at the level of initial per-ception. It is as possible that the antago-nisms occur during the poststimulation pro-cessing that is believed to be important forassociative learning. It is known that post-stimulus "distractors" can interfere withshort-term memory (e.g., Whitlow, 1975)and associative consolidation (e.g., Wag-ner, Rudy, & Whitlow, 1973). And, muchof the recent evidence (e.g., Riley & Roit-blat, 1978) on distribution of attention insituations that more obviously attempt totax the animals' information processing ca-pacity could be attributed to competitionfor maintenance in short-term memory.

Finally, Mackintosh (1971) and Revusky(1971) have reported overshadowing inone-trial conditioning situations. In suchsituations overshadowing cannot be attrib-uted to the aforementioned alternatives ofRescorla and Wagner (1972) or Mackin-tosh (1975) that view the effect as beingthe consequence of repeated compound con-ditioning trials. By this default some dis-tributive stimulus processing appears themore likely to be involved.

The present investigations were moti-vated by the above facts. Of specific inter-est was the replicability of the one-trialovershadowing reported by Mackintosh(1971) and the possibilities that it eithermight be attributed to variations in receptor-orienting behaviors or might be a more

190 JAMES H. JAMES AND ALLAN R. WAGNER

general phenomenon. Experiment 1 was anessential repetition of the Mackintosh ex-periment, with the inclusion of behavioralobservations to document the variation inorientational activities that may occur insuch circumstances. Experiments 2 and 3departed from Experiment 1 in using CSsthat were calculated to reduce the possibili-ties of orientational antagonisms or concur-rent perceptual competition. They also in-cluded companion studies that employed aone-trial habituation, rather than a condi-tioning, paradigm with the same stimuli.In overview, the results attest to the re-producibility of one-trial overshadowingthroughout the range of procedural varia-tions involved.

Experiment 1

In the Mackintosh (1971) investigationof one-trial overshadowing, rats weretrained in a conditioned emotional response(CER) situation where conditioning wasevidenced by the suppression of drinkingduring the CS. The CSs that were employedwere common to this situation, a generalillumination of the subject's chamber byan overhead lamp and a train of tones ema-nating from a loudspeaker adjacent to thedrinking source. Comparison groups re-ceived either one pairing of a tone-lightcompound with a shock US, or a similarpairing of the tone alone with the US. Insubsequent test presentations of the tone,less conditioned suppression was observedin the compound group than in the tone-alone group.

Experiment 1 sought to replicate this over-shadowing effect, but it also included athird group trained with the light alone toevaluate the suppression to both membersof the compound CS versus that of appro-priate element-trained groups. In addition,systematic classifications were made of thesubjects' nondrinking behavior during theconditioning trial.

Method

Subjects. The subjects were 18 experimentallynaive male albino rats, obtained from the Holtz-

man Company, 195 days old at the start of train-ing. For approximately 1 mo prior to training,subjects were restricted to 10-min access to waterper day in their individual home cages where foodwas continuously available. During training, watercould be obtained only in experimental sessionsas described below.

Apparatus. Six identical operant chambers(24.5 X 19.5 X 18 cm) were used, each housedwithin a light- and sound-resistant chest. The iso-lation chests were equipped with an exhaust fan,which produced a background masking noise ofapproximately 70 dB (re 20 juN/m2; General RadioSound Pressure Meter, Model 1551, 20-kHz scale),and with a shielded, red-coated 7-W, 110-V lamp,which provided diffuse ambient lighting. Thechamber's longer side walls and ceiling were clearPlexiglas, and the shorter end walls were alumi-num. The floor consisted of thirteen .4-cm-thickstainless steel grids spaced 2 cm apart center tocenter and raised 9 cm above a drop pan filledwith wood shavings. A 1-cm-diam. hole, throughwhich the subjects had access to a glass drinkingtube connected to a water reservoir, was locatedoff-center in one end wall, 4.0 cm from the nearerside wall and 4.5 cm above the floor.

The visual CS was produced by delivering 90 Vto a 3-W, 110-V lamp mounted on top of butdirectly shielded from the chamber, so as to pro-vide diffuse reflected light from the sides andceiling of the isolation chest. The auditory CSconsisted of a 1800-Hz tone generated by a For-inger 116-4-M1 audio oscillator and deliveredthrough a 4-in. (10.16-cm) speaker mounted be-neath the floor level in the center of the end wallcontaining the drinking tube. The tone raised thesound pressure level to approximately 83 dB mea-sured at the drinking tube. The unconditionedstimulus was a ,5-sec, 1-mA shock deliveredthrough the grid floor from a locally fabricatedscrambler.

Contact of the rat's tongue with the drinkingtube was detected by a Hunter Model 3305 con-tact relay. The orifice of each drinking tube wasdrawn to .5 mm, which in steady drinking boutsdispensed 1 ml of water over approximately 400licks. Lick recording and stimulus programmingwere managed by electromechanical equipment inan adjacent room.

Procedure. Experimental training and testingwere conducted over 11 consecutive daily sessionsof 15-min duration, in which subjects were al-lowed free access to the drinking tube within theoperant chamber. On the first 3 days there was noother treatment, and all subjects adopted sustainedlicking.

On the following day the subjects were ran-domly assigned to one of three experimental groups(H = 6/group) and were given a single pairing ofthe light CS (Group L), the tone CS (Group T),or a simultaneous light-tone compound (GroupLT) with the shock US. In all cases the CS was30 sec in duration, and its presentation overlapped

ONE-TRIAL OVERSHADOWING 191

and terminated with the US. This conditioningsequence was initiated within the experimentalsession as follows: After the subject performed aninitial 100 licks, a 30-sec period was begun duringwhich a baseline record of licking was obtained;after this period, the subject was again allowed toperform 100 licks, at the termination of which the30-sec CS-US episode was introduced.

Following a single postconditioning session iden-tical to the preconditionig sessions, all subjectsreceived six test sessions, three with the light CSand three with the tone CS, in a simple alternationsequence. Half of the subjects in each group weretested first with the light and half with the tone.A test trial was introduced in each session in thesame manner as the conditioning trial had been,but it was never reinforced with the US and con-sistently involved an isolated CS.

Response measures. The tendency of the CSsto suppress licking on conditioning or test trialswas calculated according to a percentage suppres-sion measure, [(A-B)/A] X 100, where A wasthe number of licks in the baseline period and Bwas the number in the following equivalent-dura-tion CS period. With this measure, a score of 100indicates a complete suppression of licking duringthe CS.

The percentage suppression scores were supple-mented by behavioral judgments during the con-ditioning trial. Prior observations (James & Sharp,Note 1) suggested that differential orienting re-sponses to the specific visual and tonal stimuliemployed in this experiment could be readily dis-criminated. The most prominent behavior duringthe novel presentation of the diffuse light CS con-sisted of head contacts with the ceiling and sidewalls of the chamber. The category of behavior sodefined (henceforth referred to as W/C for "wall/ceiling investigation") was generally accompaniedby rearing and bears similiarity to the behaviordescribed by Holland (1977) to an overhead lamp.During the novel presentation of the tone CS, sub-jects were most commonly observed to projecttheir noses between the floor grids or to makeshort, rapid head wavings. The category of be-havior so defined (henceforth referred to as HW/G for "head waving/grid investigation") is simi-lar in part to the "head jerks" described by Hol-land (1977) to a tonal stimulus, but it also in-cluded apparent approaches to the speaker thatwas located below the floor level.

The reactions of each individual subject to theconditioning trial CS presentation were observedthrough an opening of the enclosure. The observermade 15 observations per CS presentation, at 2-secintervals beginning 1 sec following CS onset andpaced by an auditory signal delivered by head-phones. At each interval the subject's behavior wasjudged as falling into one of the mutually exclu-sive categories of wall/ceiling investigation, headwaving/grid investigation (each as defined above),or licking at the drinking tube (Li), or as other(O) than these behaviors. All observational data to

Table 1Percentages of Total Observations in EachBehavior Category on the Conditioning. Trialof Experiment 1

Behavior

Group

LTLT

Li

283420

W/C

502043

HW/G

22920

0

201717

Note. Li = licking; W/C = wall/ceiling investiga-tion (head contact); HW/G = head waving/gridinvestigation; O = other behavior. Groups weretrained with light (L), tone (T), or the compound(LT).

be reported were based on the judgments of asingle scorer. However, the reliability of the judg-ments was assessed by the simultaneous observa-tion of nine subjects by two judges. The percent-age of judgments in agreement was never lessthan 87% on individual trials.

Results and Disciission

On the conditioning day the three groupsshowed similar rates of licking prior to theCS and substantial suppression of lickingduring the CS. The mean numbers of licksduring the 30-sec baseline period for GroupsL, T, and LT, respectively, were 136, 126,and 111; the mean percentages suppressionduring the 30-sec CS were correspondingly74, 67, and 74. The differences in neithermeasure were statistically reliable (Fs <1.0).

Table 1 presents a summary of the ob-servational data during the conditioningtrial. As would be expected from the per-centage suppression measure, the majorityof the observations in each group involvednonlicking behaviors. In agreement withour preliminary observations, the responsesto the light and tone CSs in Groups L andT were clearly distinguishable. Group Lsubjects, presented with the light, showeda reliably greater frequency of wall- andceiling-directed behavior (50% of total be-havior) than did Group T subjects, pre-sented with the tone (20% of total behav-ior), t( 10) =2.88, p < .02. In contrast,Group T subjects showed reliably more

192 JAMES H. JAMES AND ALLAN R. WAGNER

LIGHT TESTS

(KQ.0.•

OGROUP L

• GROUP T

- » GROUP LT

3 1

TEST TRIALS

Figure 1. Mean percentage suppression to thelight and tone separately on individual test trialsin each of the three groups of Experiment 1.

grid-directed and head-waving behavior(29% of total) than did Group L subjects(2% of total), *(10) = 3.02, p < .02. Theorienting behavior of Group LT subjectspresented with the light and tone compoundsuggests that the light and tone were botheffective in compound: The frequency ofW/C behavior (43% of total) was some-what less than that observed to the lightalone in Group L (t < 1) but was reliablygreater than that observed in Group T,t( 10) =2.75, / > < . 0 5 ; the frequency ofHW/G behavior (20% of total) was some-what less than that observed to the tonealone in Group T (t < 1) but was reliablygreater than that observed in Group L,t(10) =2.91, p < .02. Thus, on first ex-posure to the CSs, prior to conditioning, allgroups substantially suppressed licking whileengaging in other behaviors that varied withthe stimuli involved.

Figure 1 presents the mean postcondition-ing percentage suppression to the light andtone separately on individual test trials ineach of the three groups. In view of theaforementioned unconditioned suppressionto either stimulus, it is important to seethat there was more postconditioning sup-pression to the light in Group L, which hadreceived a light-shock pairing, than inGroup T, which had not previously experi-enced the light, and, conversely, more sup-

pression to the tone in Group T, which hadreceived a tone-shock pairing, than inGroup L, which had not previously experi-enced the tone.

The central interest is in the comparativesuppression of Group LT to these samestimuli. As can be seen, Group LT sup-pressed less to the light than did Group L,and no more than did Group T. Group LTalso suppressed less to the tone than didGroup T, but more than did Group L. Over-shadowing is indicated by the lesser sup-pression to either test stimulus by GroupLT than by the element group that wastrained with the respective cue in isolation.The additional comparisons of Group LTwith the element groups that had not pre-viously experienced the test stimulus wouldsuggest that the compound trial producedsubstantial association of the tone but littleor none of the light.

Analyses of variance conducted on themean percentage suppression to each of thetest stimuli revealed a significant overalldifference among the three groups in re-sponse to the light, F(2, 15) = 4.66, p <.05, and to the tone, F(2, 15)• = 4.24, p <.05. Subsequent analyses indicated thatGroup L was overall more suppressed tothe light than either Group LT or' GroupT. t( 10)= 2.60, p < .05 and t(10) = 3.00,p < .02, respectively, while the latter twogroups did not reliably differ (t < 1.0). Onthe tone tests, the groups no longer dif-fered appreciably by the third test trial, butover Test Trials 1 and 2, Group T was moresuppressed than either Group LT, £(10) =2.31, p < .05, or Group L, t(10) = 3.72,p < .01, while the latter two groups werealso reliably different, t(10) =2A3, p<.05.

The conditioned suppression findings ofExperiment 1 are clear in replicating andextending the report of Mackintosh (1971)that overshadowing may be observed fol-lowing a single conditioning trial. There wasless suppression to the tonal stimulus inGroup LT than in Group T, as reported byMackintosh (1971). There was also lesssuppression to the visual stimulus in GroupLT than in Group L. That the two mem-

ONE-TRIAL OVERSHADOWING 193

bers of the LT compound each appeared todetract from the conditioning of the othermay be of theoretical significance beyond thefact that some overshadowing was observed.Mackintosh (1976) reported that such re-ciprocal overshadowing in multiple-trialCER studies is more detectable when thestimuli are of approximately equal salienceand of weak or moderate, as compared withhigh, intensities. Such favorable conditionsof stimulus intensities may apply to thepresent studies. However, the multiple-trialinterpretation offered by Mackintosh (1976)to rationalize the dependencies would not.

A question that was raised was whetherovershadowing in this situation might beattributable to the occurrence of differentialorienting responses in the compound andelement groups, which could have the con-sequence of pairing different patterns ofvisual and auditory stimulation with shockin the two cases. Simple observational datasuch as those reported could not be antici-pated to answer this question. But, they doserve to support its legitimacy. It is cer-tainly possible that the visual patterns ex-perienced by Group LT during the trainingCS were functionally different from thoseexperienced by Group L. Most notable isthe fact that Group LT spent 20% of thetime that the light was present with theirheads directed through the floor grids orin rapid wavings (HW/G behavior) where-as Group L spent only 2% of the time inthese same postures. Similarly, it is possiblethat the auditory patterns experienced byGroup LT during the training CS werefunctionally different from those experi-enced by Group T. Group LT rats spent43% of the time that the tone was presentwith their heads in contact with the ceilingsand walls (C/W behavior), while Group Tspent only 20% of the time in these samepostures.

Experiment 2A

If the overshadowing observed in Ex-periment 1 were the result of differentialorienting behaviors and differential conse-quent stimulation that are occasioned by a

tone-light compound versus those that areoccasioned by either stimulus in isolation,the phenomenon should be sensitive to ma-nipulations of the CSs that are employed.Clearly one should be able to increase themagnitude of overshadowing by the use ofCSs that would produce yet greater orien-tational antagonisms, and/or greater con-tingent variation in the pattern of stimula-tion that is received, than seem likely tohave been the case in Experiment 1. How-ever, successful manipulation in this direc-tion could be challenged, as making theexperimental situation the less representa-tive of Pavlovian conditioning. The alterna-tive is perhaps more germane to our theo-retical concern. One should be able to de-crease the magnitude of overshadowing bythe use of stimuli that would produce lesserorientational antagonisms than seem likelyto have been present in Experiment 1. Ex-periment 2A was designed to evaluate thispossibility.

In choosing the stimuli that were em-ployed in Experiment 2\, we were guidedby the impression that the unconditionedreactions to the CSs on the conditioningtrial of Experiment 1 were localization andapproach behaviors that were variable,within as well as between conditions, onlyinsomuch as the spatial stimulus patternswere complex and had different local max-ima. The light, although providing a gen-eral increase in level of illumination, pro-duced the most prominent changes at thePlexiglas ceiling and walls. The tone mayhave been more difficult to localize, produc-ing head wavings that would facilitateidentification of the source prior to attract-ing the subject's head toward the speakerbelow the floor.

On the basis of this reasoning, a stimu-lus module was developed from which ap-parently more localizable light and tonestimuli could be presented either separatelyor in combination. Preliminary research in-dicated that with placement of the modulebeneath the grid floor, subjects reacted toeither, an auditory or a visual output bymoving to a location above the module andfurther poking their heads through the grids

194 JAMES H. JAMES AND ALLAN R. WAGNER

in the direction of the source. It should benoted that the light and tone that were pro-duced still caused diffuse environmentalchanges in overall level of illumination andauditory surround. But the spatial patternswere more clearly graded in relation to thesource as compared with the stimuli of Ex-periment 1. To further favor stimulus lo-calization, the tonal CS was arranged to beintermittent, and the visual CS was madeless subject to mirroring by lining the Plexi-glas ceiling with flat black constructionpaper and by increasing the background levelof illumination emanating from outside thePlexiglas chamber walls. The effect of thiscomposite of changes, as observed in pre-liminary investigations, was to eliminatehead waving as a preferential response tothe tone as well as to eliminate ceiling- andwall-directed investigation as a preferentialresponse to the light, leaving the notablecommon response to the two stimuli asbeing approaches to the module location.

The design of Experiment 2A was essen-tially identical to that of Experiment 1 withthe exception of the aforementioned substi-tution of stimuli. Groups L, T, and LT re-ceived a single pairing of the designated CSwith shock prior to testing all subjects forsuppression produced by the individualstimuli.

Method

Subjects. The subjects were 24 male albinorats, 192 days old at the start of training, ob-tained from the Charles River Company. Foodand water exposure was the same as that de-scribed in Experiment 1.

Apparatus. The six experimental chambers em-ployed in Experiment 1 and two other identicalones were used. They were as previously de-scribed except for the following changes. Thedrinking tube was repositioned in the same wallas before so as to be centered left to right and4 cm above the grids. This location of the drinkingtube forced the subjects to have their heads cen-tered along the front wall while they were licking.The ceilings of the chambers were lined withblack construction paper, and a white-coated, 7-W,110-V lamp, mounted above the chambers, re-placed the previous source of ambient lighting.

Each chamber was equipped with two stimulusmodules under the grid floor, one to the right andthe other to the left of the drinking tube. Eachmodule contained two subminiature incandescentlamps (Chicago, Model 327) and a 1-in. (2.54-cm)

diameter miniature speaker (Speco, Model U101).The casing of the module consisted of the bottom3.8-cm section of a 3.2-cm-wide (OD) translucentpolypropylene centrifuge tube (Nalgene, Model3110), sealed by a tube closure (Nalgene No. 32)and mounted on a 1.5-cm-high piece of polysty-rene to face the grids. The end of the tube wasperforated with nine 1.5-mm holes behind whichthe lamps were directly located and then thespeaker, pressure fit in place. The assemblies werepositioned to be 2 cm below floor level and cen-tered 4.5 cm diagonally within the corners of thefront and side walls.

For any subject, only one of the two moduleswas ever activated. The auditory CS consisted ofa 1800-Hz tone interrupted 10 times a second,which when presented through the designatedmodule raised the sound pressure level in thechamber from approximately 70 to 86 dB as mea-sured at the drinking tube. The visual CS con-sisted of the parallel delivery of 28-V dc to thedesignated module lamps.

Procedure. The procedures of lick training, con-ditioning, and testing were identical to those inExperiment 1 except as noted. On the conditioningday, groups of eight subjects received a pairingof the US with a light (Group L), a tone (GroupT), or a light-tone compound (Group LT). Fol-lowing conditioning, all groups received 8 testdays, 4 with the visual CS and 4 with the tonalCS, in simple alternation but counterbalancedorders. For half of the subjects in each group, allCSs were consistently presented by the left mod-ule; for the remainder, they were consistentlypresented by the right module.

Behavioral judgments were made during theconditioning trial as in Experiment 1 with twomodifications. In addition to the categories of Ex-periment 1, the observations included notation ofthe position of the subject's head in relation tofour equal-sized floor quadrants. The quadrantswere separated left-right by the center position ofthe drinking tube and front-back by a center grid.Judgment was made of whether the head wastotally or predominantly in the quadrant contain-ing the active stimulus module (CS quadrant), thequadrant containing the inactive stimulus module(opposite front), or one of the quadrants contain-ing neither stimulus module (same rear or oppositerear). The subject's head position was noted on allobservations except when the subject was judged tobe licking and thus of necessity had its head posi-tioned on the border of the front quadrants. Inview of the expanded notations that were required,the number of observations in each CS period wasreduced to 10, beginning 1 sec after CS initiationand then paced at 3-sec intervals.

Results and Discussion

As in Experiment 1, there was substan-tial suppression of licking during the CSson the conditioning day which was not re-

ONE-TRIAL OVERSHADOWING 195

liably different for the three groups: Themean percentages suppression for GroupsL, T, and LT were 73, 50, and 78, respec-tively (F<1.0), from mean baselines of127, 102, and 135 licks during the pre-CSperiod (F < 1.0). However, as anticipatedfrom preliminary investigation with the stim-uli employed, the pattern of behavior that re-placed licking was markedly different fromthat observed in Experiment 1.

None of the observational classificationsdifferentiated among the three groups re-ceiving the different stimuli. This was duein part to the lack of appearance of some ofthe prominent responses of Experiment 1:The headwaving response preferentially oc-casioned by the tone in Experiment 1 wasnever observed in Experiment 2A. The ceil-ing-directed behavior preferentially occa-sioned by the light in Experiment 1 wasnever observed in Group L and occurredbut once in the 80 total observations of eachof Groups T and LT in Experiment 2A. But,among those behaviors of Experiment 1 thatdid occur in Experiment 2A, there were alsono recognizable, systematic differences amongthe groups. Notably, the behavior of direc-ting the head between the floor grids, prefer-entially occasioned by the tone in Experi-ment 1, was an equally prominent responsein each of Groups L, T, and LT, occurringon 40%, 46%, and 50%, respectively, ofthe total observations. The most salient ef-fect of the CSs was to cause the subjects toapproach the area of the stimulus module,where they behaved indistinguishably ac-cording to the categorizations that were at-tempted, notably, as has been indicated, bydirecting their heads through the grids atthe module.

Table 2 presents a summary of the posi-tional judgments that attest to this effect.Given for each group are the percentages oftotal observations in which the subjects werejudged to be licking. The remaining totalpercentage of other (i.e., nonlicking) ob-servations are then reported separately bythe location of the subject. As can be seen, allthree groups of subjects predominantly posi-tioned their heads in the CS quadrant whenthey were not drinking.

Table 2Percentages of Total Observations in EachBehavior Category on the Conditioning Trialof Experiment 2A

Behavior/location

Other

Group

LTLT

Li

172219

CSquad-rant

626759

Oppo-site

front

135

12

Samerear

525

Oppo-siterear

345

Note. Li = licking. Groups were trained with light(L), tone (T), or the compound (LT).

That this positioning was attributable tothe location of the CS source can best bejudged from comparison of the frequenciesof observations in the CS quadrant and thecorresponding frequencies in the oppositefront quadrant, as both of these chamberareas contained a stimulus module and wereotherwise counterbalanced for physical lo-cation, right or left of the drinking tube.The greater frequency in the CS quadrantwas statistically reliable in each individualgroup, /s(7) > 4.40, pa < .01, and not sig-nificantly different among the groups (F <1.0),

Further analysis of the frequencies of ob-servations in the CS quadrant increases con-fidence that the behavior noted reflects aninvestigation of the CS source. Figure 2 pre-sents separately for each group the percent-age of subjects judged in the CS quadrantand, for comparison, in the opposite frontquadrant, at each of the 10 within-CS ob-servation periods. As can be seen, in the pe-riods immediately following CS onset al-most all subjects were observed in the CSquadrant, but such positioning systematicallydecreased with continued exposure of theCS. By the time of CS termination (and USdelivery), if a subject had not returned todrinking, it was as likely to be in the oppo-site front quadrant as in the CS quadrant.It is again notable that the three groups didnot appear to differ in this temporal distri-bution of apparent investigative behavior.

196 JAMES H. JAMES AND ALLAN R. WAGNER

100

80

5? 60

K

< 20

-

-

ra

^\my

\

GROUP Ly

I1I-] 1 I~\ \ I[Jin< " 1 2 3 4 5 6 7 8 9 1 0-iO 100

DE

SIG

NA

TE

D

ro

-b

o>

CD3

O

O

O

O

-

-

pa

1 1

Fi

1GROUP T

1 IP

iiln100e

lij 80CO

CO

GROUP LT

DCS QUAD

OPP FRONT

1 8 9 10

SUCCESSIVE WITHIN-CS OBSERVATIONS

Figure 2. Percentage of subjects judged in theCS quadrant and opposite front quadrant in eachof the 10 successive within-CS observation pe-riods during the conditioning trial for the threeseparate groups of Experiment 2A.

Figure 3 presents the mean percentagepostconditioning suppression to the lightand tone separately over blocks of two testtrials in each of the three groups. The over-all pattern is similar to that observed inExperiment 1. Of major importance, GroupLT showed less mean suppression to thelight than did Group L, which was trainedwith the light in isolation, and also less meansuppression to the tone than did Group T,which was trained with the tone in isolation.

Analyses of variance of the mean suppres-sion over the four light tests indicated a re-liable difference among the three groups,F(2, 21) = 6.05, p < .01. Subsequent com-parisons confirmed that Group L was re-liably more suppressed than either GroupLT, /(14) =2.27, / > < . 0 5 , or Group T,f(14) = 3.56, p < .01, while the latter twogroups did not significantly differ, t(14) =1.55, p > .10. Similar analyses over the fourtone tests alone indicated a reliable differ-ence among the three groups, F(2, 21) =10.13, p < .01. However, in this instance

whereas Group L was reliably different fromboth Group LT, /(14) = 3.06, p < .01, andGroup T, f(14) = 4.52, p < .01, the differ-ence between the latter two groups did notreach conventional levels of significance,f(14) = 1.32, p > .10.

To all obvious appearances, the change inCSs between Experiment 1 and Experiment2A was successful in eliminating gross dif-ferential orienting behaviors occasioned bythe lights and tones employed and in pro-moting investigative behavior that was highlysimilar when either stimulus was presentedalone or the two were presented in com-pound. That overshadowing was still demon-strable in Experiment 2A must then dis-courage the possibility that such is peculiarto Pavlovian situations that may involveorientational antagonisms.

It is, of course, possible that importantdifferences in the orienting responses to thelight and tone of Experiment 2\ went unde-tected. And, it must be noted that whereasreliable overshadowing was observed of boththe tone and light in Experiment 1, the cor-responding difference was reliable only intests of the light in Experiment 2h. We canonly acknowledge that the former possibilitycan never be rejected while observing thatthe similarity of the overall pattern of con-ditioned suppression in the two experiments

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ONE-TRIAL OVERSHADOWING 197

(see Figures 1 and 3) is more striking thanthe differences in levels of statistical signifi-cance in the separate comparisons.

Experiment 2s

The data of Experiments 1 and ZA witnessthe fact that suppression of licking can re-sult from unconditioned orienting behaviorsto a stimulus (see preconditioning suppres-sion and Tables 1 and 2) as well as fromassociative tendencies to that stimulus fol-lowing pairing with shock (see postcondi-tioning suppression, Figures 1 and 3). Thefact that unconditioned suppression of lick-ing decreases with successive stimulus pre-sentations (e.g., Carlton & Vogel, 1967) andthat such habituation can be demonstratedafter a single exposure (e.g., Leaton, 1976)makes it possible to evaluate the possibilityof distributive stimulus processing in thepresent experimental context, independentof Pavlovian CS-US pairing. That is, it ispossible to ask whether the degree of habitu-ation that results from a single CS exposureis less when the stimulus is experienced incompound than when presented in isolation.

There is evidence that repeated exposuresto a stimulus in compound result in lesshabituation than equal numbers of exposuresto the same stimulus in isolation, using adecrease in associability (latent inhibition)index (e.g., Mackintosh, 1973; Rudy,Krauter, & Gaffuri, 1976). Experiment 2swas conducted to determine whether suchovershadowing-like effect is apparent (a)after a single stimulus exposure, (b) whenusing stimuli that do not appear to involvegross orientational antagonisms, and (c)as evidenced in differences in unconditionedsuppression. The stimuli and procedureswere essentially identical to those of Experi-ment 2A with the exception that no uncon-ditioned stimulus was administered on thesingle training trial, that is, the compoundCS or its separate elements were simplyexposed.

Method

As indicated, the method was essentially identi-cal to that of Experiment 2A. Groups L, T, and

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LT were each comprised of eight male albino rats,obtained from the Charles River Company, 158days old at the start of training. The apparatuswas, as before, without modification. The proce-dures were the same with the exception that thetraining session included only presentation of theCS (designated by the group labels). There fol-lowed two test sessions to each of the tone andlight stimuli in counterbalanced alternation se-quence for all groups.

Results and Discussion

With minor variations, the behavior of thethree groups on the differential stimulus ex-posure day was comparable with the be-havior of the three groups of Experiment2A on the corresponding conditioning day.From baselines of 151, 166, and 147 licks inthe 30-sec pre-CS period, Groups T, L, andLT showed mean percentages suppression of85, 50, and 89 during their respective CSs.When not licking, each group was judgedon the majority (70%, 60%, and 74%, re-spectively) of observations to be in the CSquadrant. Although Group L deviated some-what from Groups T and LT, there wereno reliable differences (Fs < 1.0) amongthe groups in these measures, nor were thereany other apparent systematic differencesworthy of note.

Figure 4 presents the mean percentagessuppression of the three groups on each ofthe two subsequent test trials to the light

198 JAMES H. JAMES AND ALLAN R. WAGNER

and the tone. If the single prior exposureto the designated stimulus produced habitu-ation of unconditioned suppression to thatstimulus, it would be expected that GroupL would show less suppression to the lightthan would Group T and that Group Twould show less suppression to the tonethan would Group L. This pattern canclearly be seen in Figure 4.

Of interest, then, is the comparative sup-pression in Group LT, which had a priorexposure to both stimuli in compound. Ascan be seen, Group LT suppressed more tothe light than did Group L and more to thetone than did Group T. That is, Group LTdid not appear to habituate as much to eitherstimulus as did the corresponding elementgroup.

In spite of the consistent pattern of meanson the light and tone tests, the only statisti-cally reliable differences were in responseto the light, and then only on the first testtrial. There was a reliable overall differenceamong the groups on the first light test,F(2, 21) = 5.95, p < .01, associated with alesser suppression of Group L than of eitherGroup T, f(14) = 2.48, p < .05, or GroupLT, f(14) = 3.00, p < .01, while the lattertwo groups were not significantly different(t < 1.0). The differences among the groupson the first tone test trial were not simi-larly reliable, F(2, 21) = 1.53, p > .10.

The pattern of results of Experiment 2sare analogous to those of Experiment 2\.The mean differences were indicative of re-ciprocal overshadowing in the conditioningof Experiment 2A and a correspondent inthe habituation of Experiment 2s. Althoughstatistical reliability was attained in eachcase only on the light tests, the similar pat-terns observed in the two experiments andin Experiment 1 further encourage the no-tion that during a single compound-CS ex-posure there may be less effective processingof a component than if that stimulus hadbeen presented in isolation.

Experiment 3A

The observation of overshadowing underthe stimulus conditions of Experiment 2A,and of an overshadowing-like habituation

difference under the similar conditions ofExperiment 2s, reduces the likelihood thatovershadowing depends upon variation instimulus experience resulting from differen-tial orienting acts during compound, as com-pared with element, presentation. An alter-native is that the effects are the result ofvariations in immediate stimulus encodingthat are more "centrally" determined, forexample, as suggested by Sutherland andMackintosh's (1971) notion of distributiveattention via competitive stimulus analyzers.But, it is as possible that the effects are theresult of postperceptual variations in process-ing following compound, as compared withelement, presentation, for example, as sug-gested by the commonly held (e.g., Atkin-son & Shiffrin/1968; Norman, 1968; Wag-ner, 1976) notion of distributive memorialprocessing due to the limited capacity ofshort-term memory.

It will be difficult to decide among thevarious theoretical alternatives that mightbe entertained. However, it would appearinstructive to know whether one-trial over-shadowing is favored by conditions of con-current presentation of the compound ele-ments (as existed in Experiments 1 and 2)in comparison with conditions of nonover-lapping presentation. It is known that over-shadowing can be seen in the latter case.Revusky (1971) demonstrated one-trialovershadowing in taste aversion learningwhen two flavors were successively ratherthan simultaneously administered prior tothe US. Revusky (1971) further presentedevidence that would discourage appeal to anysimultaneity of taste and "aftertaste" thatmight be considered plausible in this situa-tion. And, Mackintosh and Reese (1979)recently replicated the basic observation inone-trial, CER conditioning—a tone CS wasovershadowed by a nonoverlapping light CSthat was more proximal to the US. Experi-ment 3A was designed, using the stimuli andgeneral procedures of Experiment 2A, toprovide a direct comparison of the over-shadowing that may occur when the elementsof the compound are sequentially segregatedwith the overshadowing that may occurwhen they are simultaneously presented,

ONE-TRIAL OVERSHADOWING 199

without consistent differences in the relativeproximity to the US.

For Group L and Group T, the CS con-sisted of four 2-sec presentations of the desig-nated element distributed within the 30-secpre-US interval. For Group LT, the CS pe-riod included four 2-sec presentations of thelight and tone similarly distributed in simul-taneous compound. For Group L/T, the CSperiod included four 2-sec presentations ofthe light and four 2-sec presentations of thetone in nonoverlapping fashion (see Pro-cedure). All subjects were then tested forsuppression to the light-alone and tone-alonesequence.

Method

Groups L, T, LT, and L/T were each com-prised of eight male albino rats, obtained from theCamm Company, 125 days old at the start oftraining. The apparatus was as described in Ex-periment 2A, without modification.

Except as indicated, the procedures were iden-tical to those of Experiment 2\, The principalchange was in the intermittent nature of the CSsemployed. The difference among the treatments ofthe several groups can best be appreciated in re-lation to the conditions of Group L/T. For thisgroup, the 30-sec CS period on the conditioningday was a sequence of eight 2-sec stimulus pre-sentations, each separated by 2-sec intervals, withthe last terminating in the shock US. In this se-quence the light and tone were each presentedfour times in simple alternation, the CS beginningwith the tone and terminating with the light forhalf of the subjects and just the reverse for theremaining subjects. For Group L, the CS se-quences were designed to be the same as thosein Group L/T except that the tones were omitted.This meant that for half of the subjects the first2-sec light began 30 sec prior to the US and thelast light ended 6 sec before the US whereas forthe remaining subjects the first 2-sec light began24 sec prior to the US and the last light termi-nated with the US. For Group, T, the CS se-quences were similarly arranged to be the sameas those in Group L/T except that the lights wereomitted. For Group LT, the light and tone werepresented simultaneously rather than separately,in two subgroups that had temporal arrangementsthat were identical to the two subgroups of GroupL and Group T.

The US was a 1.0-mA shock as in the previousstudies. However, the duration was increased from.5 sec to 1.0 sec in anticipation of some decreasein suppression with the CS-US relations employed.In testing, all subjects were presented on alternatedays with either four 2-sec lights or four 2-sec

tones, separated by 6-sec periods as in the con-ditioning sequence of Groups L and T. Testing wasconducted over four sessions with each stimulus.Postconditioning suppression was measured overthe 30-sec period beginning with the first lightor tone. Similar suppression on the conditioningtrial was uniformly measured over the 30-sec pe-riod that preceded the US.

Behavioral judgments were made on the condi-tioning trial at 2-sec intervals starting 31 secprior to the US. On this schedule one judgmentwas made in each light and/or tone applicationand one judgment (Group L/T) or three (GroupsL, T, and LT) between successive stimuli.

Results and Discussion

On the conditioning day the mean per-centages suppression of licking during thetotal 30-sec pre-US period, including the CSpresentations, were 37, 48, 56, and 49, frommean baselines of 132, 131, 133, and 134licks, for Groups L, T, LT, and L/T,respectively. These percentages suppressiondid not differ reliably among the four groups(F < 1.0) but were less than the overalllevels of suppression characteristic of Ex-periments 1 and 2.

The intermittent stimuli produced a some-what different temporal distribution of orien-tational judgments on the conditioning trialthan did the continuous CSs of Experiments2A and 2s. Figure 5 illustrates this fact inpresentation of the percentages of subjectsjudged to be in the CS quadrant on suc-cessive observations, beginning prior to thefirst stimulus and continuing through thethe last stimulus in the CS period. As therewere no reliable differences in response tothe light and tone between Group L andGroup T or within Group L/T, and nonotable differences in the two subgroups ofGroup L/T receiving the two orders of thetwo stimuli, the data were collapsed similarlyto ignore stimulus identification. Summa-rized are the successive observations over thetwo subgroups of Group L/T (right panel),the successive observations over the two sub-groups of Group LT (middle panel), andthe successive observations over the twosubgroups of Group L and Group T com-bined (left panel). As can be seen, there wasa tendency, similar to that shown in Figure2, for location in the CS quadrant to decrease

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over the total observational sequence. How-ever, there was the further tendency forsubjects to be more frequently located in theCS quadrant during the periods in which astimulus was present than during the inter-stimulus periods. The latter tendency washighly reliable over all subjects, £(31) =3.02, p < .01.

In relation to the general concern with thepossibility that overshadowing could resultfrom orientational differences, Figure 5offers two facts. The alternating stimulussequence in Group L/T appeared to producepositional orientation to the separate stimuliwhen present that was not markedly (orreliably, t < 1) different from the orientationto the same stimuli when present in isolationin Groups L and T. At the same time, it isclear that the simultaneous compound inGroup LT did produce more frequent posi-tioning in the CS quadrant during theperiods of stimulus presentation than wasthe case for Groups L and T, and this dif-ference was reliable, t(22) — 2.63, p < .02.

Figure 6 presents the mean percentagessuppression of the four treatment groups onthe separate postconditioning tests to thelight and the tone CS. As can be seen, GroupL was more suppressed than Group T on the

light tests, whereas the reverse was the caseon the tone tests. In line with the findingsof Experiments 1 and 2A, Group LT wasintermediary in suppression to both cues.And, the same was true of Group L/T.Either simultaneous or successive com-pounds appeared to result in reciprocalovershadowing.

Separate analyses of variance revealedreliable differences among the four groupsover the four light tests, ^(3, 24) = 5.83,p < .01, and over the four tone tests, F(3,24) = 8.00, p < .01. Subsequent pairwisecomparisons on the light tests indicated thatGroup L/T was reliably less suppressedthan Group LT, ^(14) =2.41, p < .05.Furthermore, whereas Group L/T was sig-nificantly less suppressed than Group L,£(14) = 2.41, p < .05, and not reliably differ-ent from Group T, f(14) = 1.02, p > .10,Group LT did not reliably differ from GroupL, t(\4) = 1.12, p > .10, and was reliablydifferent from Group T, *(14) = 3.45, p <.01. (However, on the last two light testsalone, the apparent difference between GroupLT and Group L was also reliable, <(14) =2.35, p < .05.) Similar pairwise comparisonson the tone tests indicated that Groups LTand L/T did not significantly differ, £(14) =

ONE-TRIAL OVERSHADOWING 201

1.39, p > .10, but were both reliably lesssuppressed than Group T, £(14) = 2.64, p< .02 and f ( 14) =3.85, p < .01, respec-tively. Group LT was reliably more sup-pressed than Group L, ((14) =2.19, p <.05, whereas Group L/T was not (t > 1.0).

These results support the findings of Ex-periment 2A and the apparent generality ofthe phenomenon of one-trial overshadowing.The important finding was that overshadow-ing was observed as well in Group L/T, inwhich the elements were arranged to be non-overlapping, as in Group LT, in which theelements were simultaneously presented.Indeed, it may be noted that overshadowingwas more apparent in Group L/T than inGroup LT on the light tests, although theequivalence of the two groups on the tonetests and the similarity in their overall datapattern would caution against any strongconclusions of difference between the twotreatments. These results, along with thefindings of Revusky (1971) and Mackintoshand Reese (1979), make it clear that one-trial overshadowing is not dependent upon,or necessarily favored by, concurrent stim-ulation from the compound members.

Experiment SB

Experiment 3s was conducted, as wasExperiment 2fl, to offer an alternative eval-

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nation of the possibility of distributive pro-cessing in the present experimental context,independent of CS-US pairing. The stimuliand procedures were essentially identical tothose of Experiment 3A, in comparison ofGroups L, T, LT, and L/T, with the excep-tion that no unconditioned stimulus was ad-ministered on the training trial. The majorinterest was in whether Group L/T as wellas Group LT would evidence less habituationto the separately presented light and tonethan was apparent in Group L and Group T.

Method

The subjects were 32 male albino rats, obtainedfrom the Charles River Company, 160 days oldat the start of training. Groups of eight receivedthe same treatment as Groups L, T, LT, and L/Tof Experiment 3A except that no US was deliveredon the training day and only two test trials werescheduled with each stimulus. Group L/T wasdivided into two subgroups differing in the orderof the light and tone in simple alternation. In theabsence of the US, corresponding subgroupings ofGroups L, T, and LT had no significance for thesubjects but were constituted as in Experiment 3Afor purposes of equivalent measurement of suppres-sion and orientation on the training trial.

Results and Discussion

From mean baselines of 140, 160, 158, and149 licks in the pre-CS period, Groups T, L,LT, and L/T, respectively, had mean per-centages suppression of 74, 50, 71, and 98on the training day. The overall differencein percentage suppression was reliable, .F(3,28) = 4.85, p < .01, due to the greater sup-pression of Group L/T than each of theother groups, /s(14) > 2.50, />s < .05, whichdid not significantly differ. Associated withthe differences in percentage suppression wasa somewhat greater percentage of total ob-servations in which the subjects were judgedto be in the CS quadrant in Group L/T(66%) compared with Group T (54%),Group L (41%), and Group LT (51%).However, in contrast to Experiment 3A, inwhich Group LT showed more frequentpositioning in the CS quadrant during theperiods of stimulus presentation than didGroups T and L, neither of the compoundgroups in Experiment 3A differed reliably

202 JAMES H. JAMES AND ALLAN R. WAGNER

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from the element groups in any observationalmeasure.

Figure 7 presents the mean percentagessuppression for each of the four groups onthe separate postexposure test trials to thelight and tone stimuli. As was the case fol-lowing the similar habituation treatments ofExperiment ZB, Group L showed less sup-pression to the light than did Group T,whereas Group T showed less suppressionto the tone than did Group L. In comparisonwith this apparent habituation to the lightin Group L and to the tone in Group T.Groups LT and L/T evidenced an over-shadowing-like effect. There was in bothcompound groups more suppression to thelight than in Group L and more suppressionto the tone than in Group T.

Statistically, the apparent overshadowing-like effects were significantly only on thelight tests: Analysis of variance indicated areliable difference among the four groups,^(3, 28) = 3.09, p < .05, attributable to thelesser suppression of Group L than each ofthe other groups, /s(14) > 2.18, ps < .05,which did not reliably differ. The differencesamong the four groups on the tone tests didnot reach conventional levels of statisticalsignificance, F(3, 28) = 2.27, p > .10.

The results of this experiment complementthose of Experiment 3A, just as the resultsof Experiment 2s fit with the results ofExperiment 2A. Differences in stimulus pro-

cessing in compound and element trainingare apparent in habituation of unconditionedsuppression, as they are in acquisition ofconditioned suppression. In this case, theresults from Group L/T further deny thepossibility that an overshadowing-like phe-nomenon following a single exposure ispeculiar to simultaneous stimulus com-pounds.

General Discussion

The major thrust of the present researchis that one-trial overshadowing must be re-garded as a reproducible phenomenonobservable over a considerable range of pro-cedural variations. In each of the studiesthere was indication of reciprocal "over-shadowing" to suggest that less learning wasdemonstrable (although not always reliablyso) to both a tonal and a visual stimuluswhen trained in compound than whentrained in isolation. Pavlovian overshadow-ing was apparent when the componentstimuli were diffuse (Experiment 1) ormade more localizable (Experiments 2A and3A), and consequently with or without theelicitation of obviously antagonistic orientingresponses. Pavlovian overshadowing wasas apparent when the component stimuluselements were presented in successive,nonoverlapping fashion as when presentedconcurrently (Experiment SA). And, com-parable overshadowing-like effects wereobserved in the habituation of the uncondi-tioned suppression to stimuli exposed incompound rather than in isolation (Experi-ments 2s and 3B).

At the moment it is clearer that certaintheoretical interpretations of overshadowingare inadequate than just what alternativeinterpretations may eventually be required.Those interpretations (Mackintosh, 1975;Rescorla & Wagner, 1972) that have viewedPavlovian overshadowing solely as a specialcase of "blocking" requiring multiple com-pound trials are obviously contradicted byany of the present studies as well as theoriginal reports of Mackintosh (1971) andRevusky (1971). The weight of the presentevidence further discourages the view thata blocking analysis of overshadowing maysuffice except in certain very restricted cir-

ONE-TRIAL OVERSHADOWING 203

cumstances, for examples, those involvingobvious differences in de facto stimulusreception. Mackintosh and Reese (1979)have proposed that the phenomenon may beoverdetermined, resulting from some distri-butive processing (e.g., attentional) mech-anism on a single trial but further augmentedby a blocking-related mechanism over multi-ple trials. This is certainly reasonable, sothat studies of the type reported by Mack-intosh and Reese (1979), attempting todetermine conditions that may differentiallyfavor one-trial overshadowing as opposed tomultiple-trial overshadowing, are called for.However, in support of their reasoning,Mackintosh and Reese (1979) presenteddata to suggest that greater differences inthe intensity or saliences of the compoundstimuli may be necessary to produce one-trialovershadowing than multiple-trial over-shadowing. The present data, insomuch asthey demonstrate reciprocal overshadowingthat might imply relatively equivalent sali-ences of the two cues (Mackintosh, 1976),do not appear to support this particularempirical distinction.

One-trial overshadowing could be theresult of any number of distributive process-ing mechanisms. We can reject any mech-anisms that would restrict the presumedcompetition to the processing of concurrentstimuli (see Experiments 3A and 3s; Mack-intosh & Reese, 1979; Revusky, 1971). But,a selective attention theory in the spirit ofthat proposed by Sutherland and Mackin-tosh (1971) could still account for any ofthe presently reported data on the assump-tion that attention to one element sets thesubjects' analyzers appropriate to that ele-ment for some subsequent period of time sothat another element, requiring alternativeanalyzers, would be less likely to be fullydetected within that time.

Since it is commonplace to assume alimited capacity short-term memory system(e.g., Atkinson & Shiffrin, 1968; Norman,1968; Wagner, 1976), it is inviting to sup-pose that this is the essential locus of thecompetition that is responsible for one-trialovershadowing. A substantial recent liter-ature in Pavlovian conditioning and habitua-tion (e.g., Terry, 1976; Wagner, 1978;

Wagner et al., 1973; Whitlow, 1975) hassupported the usefulness of assuming thatextraneous "distractors" can interfere withthe transient, episodic memory representa-tions, the conjoint occurrence of which isthe presumed basis for associative learning.The findings of Mackintosh and Reese(1979) and Revusky (1971), that one CScan be overshadowed by a second CS thatis administered only after the termination ofthe first and more proximal to the US,would thus be naturally expected on theassumption that the second CS would inter-fere with the integrity of the memorial rep-resentation (trace) of the prior CS. Thisreasoning can, of course, be generalized toapply to the memory representations ofconcurrent stimuli as well as successivestimuli.

Revusky (1971) has proposed yet adifferent distributive processing possibility.According to Revusky, one CS does notinterfere with the processing of another CSper se. Rather, he assumes a principle of"concurrent interference" at the level ofassociation formation. That is, the formationof one CS-US association is assumed todiminish the possibility of formation of otherpotential CS-US associations involving thesame US. So stated, the principle soundssimilar to the Rescorla and Wagner (1972)model. However, the Revusky notion ismeant to apply to the results of a singletrial as well as the accumulated product ofmultiple trials and can thus account forone-trial overshadowing whereas the Res-corla and Wagner model does not. Onemight liken the Revusky (1971) notion toattribution theory (e.g., Heider, 1958;Kelley, 1967) in assuming that whereas themembers of a compound CS and a con-sequent US may each be faithfully processed,the US may be distributively attributed to(associated with) the alternative CS elements.

The results of Experiments 2B and 3B areof special interest in regard to the possibilitythat overshadowing does not result fromdistributive processing of the CS elementsper se but rather from an interference intheir associations with the US. There wasclear evidence of less habituation to the lightand tone when exposed in compound rather

204 JAMES H. JAMES AND ALLAN R. WAGNER

than in isolation. Thus, differences in learn-ing to the stimulus elements were observedin the absence of any apparent associationprocess with a consequent US. However,habituation can be conceived (Wagner,1976) as an associative product in which thehabituating stimulus stands in the place ofa "US" and comes to be anticipated bycontexual cues that then stand in the placeof a "CS." And, Revusky (1971) hasassumed that concurrent interference ope-rates also in limiting the associations thatmay be formed between the same CS anddifferent potential USs, which could makethe principle relevant to the case of presumedassociations between the common contextand different habituating stimuli.

We have reserved final comment toacknowledge that the available evidence con-cerning one-trial overshadowing does notdemand a distributive processing account. Amost common supposition (e.g., Revusky,1971; Sutherland & Mackintosh, 1971 ;Wagner, 1978) is that an alternative ornecessary adjunct to the Rescorla andWagner (1972) or Mackintosh (1975)approaches to overshadowing is a processinglimitation of one form or another wherebysame trade-off might be anticipated in thelearning that accrues to comtemporaneousstimuli within a training episode. One-trialovershadowing is simply consistent withsuch a supposition.

A different manner of approach has longbeen recognized but has been less attractiveto current theories, which for pragmaticreasons (see Rudy & Wagner, 1975) havean elementistic bias. The alternative is thatrather than appealing to some manner ofdistributive processing, one can assume thatthere is some manner of stimulus interactionsuch that the effective CS during compoundtraining is a relatively unique configuration.On this reasoning, learning may be equiv-alent in compound and element training butinvolve different stimulus representations.The fact of overshadowing is explained bynoting that common testing with the stimuluselements involves an effective stimuluschange following compound training but notfollowing element training so that there isa generalization decrement in the former

case but not the latter. Hull (1943), forexample, assumed a process of "afferentneural interaction" to accomplish the stim-ulus compounding referred to and thenfollowed the differential-generalization-decre-ment reasoning to account for Pavlovianovershadowing.

None of the available studies of one-trialovershadowing is necessarily incompatiblewith this approach. Mackintosh and Reese(1979) concluded that their finding of lesssuppression to a tone CS when it had beenfollowed in successive compound training bya light CS, compared with when it had beentrained alone, could not be so explained.They noted that the tone was isolated duringpresentation to both groups so that anychange in the stimulation from conditioningto testing was as great for one group as theother. Unfortunately, this reasoning ignoresthe assumptions that would likely be made(e.g., following Hull, 1943) concerning thedifferential stimulus representations at thetime of application of the US and thus in-volved in the consequent association. Theelement group would presumably experi-ence the US in the presence of a singulartrace of the tone; the compound group wouldpresumably experience the US in the pres-ence of a trace of the tone modified by theafferent neural interaction produced by thesubsequent light. The trace of the tone inisolation, as provoked by the test trials,would thus be expected to witness more of ageneralization decrement in the compoundthan in the element group.

It may be even more difficult to decide be-tween a stimulus interaction and a distribu-tive processing account of one-trial over-shadowing than it is to decide among variousavenues for distributive processing, if thelatter is assumed. The choice may largelyrest upon other considerations involving theoverall adequacy and tractability of the theo-retical schemata. On a more optimistic note,however, it was possible for Wagner (1971)and Rescorla (1970) to conclude that mul-tiple-trial overshadowing cannot be attrib-uted to stimulus-interaction and generaliza-tion-decrement phenomena. Following sub-stantial training of two CS elements on sepa-rate trials or in compound, there was greater

ONE-TRIAL OVERSHADOWING 205

responding of the elements group than of thecompound group, not just if they weretested with the separate elements but if theywere tested with the compound, which shouldhave occasioned a greater generalization dec-rement for the elements group.

Reference Note

1. James, J. H., & Sharp, P. E. Stimulus orientingbehavior in a conditioned-emotional-response(CBR) situation. Unpublished manuscript, 1978.

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Received August 20. 1979Revision received December 4, 1979 •