Psychophvsiotogy, 32 (1995), 436-443 . Cambr idge University Press. Printed in the USA.Copyright f^) 1995 Society for Psyehophysiological Research

Psychophysiological responses as indicesof affective dimensions

CHARLOTTE VANOYEN W I T V L I E T AND SCOTT R. VRANADepartment of Psychological Sciences, Purdue University, West Lafayette, IN, USA

Abstract

The startle reflex, facial electromyogram (EMG), and autonomic nervous sy.slem responses were exaniitied duringimagery varying in affective valence and arousal. Subjects (N = 48) imagined affective situations during lone-cucd8-s trials. Startle blink magnitudes were larger and latencies faster during negatively valent than during positivelyvalent conditions and during high-arousal than during low-arousal conditions. Greatest heart rate accelerationand fastest and largest skin conductance responses to startle probes occurred during high-arousal imagery. Zygo-matic and orbicularis oculi facial muscle activities were higher during high-arousal imagery, whereas corrugatormuscle activity was higher during low-arousal imagery. Zygomatic and corrugator activity also varied with emo-tional valence. The startle and facial EMG responses are most parsimoniously organized by the negative affect(NA) and positive affect (PA) dimensions, respectively. This NA/PA framework integrates previous research,dimensional theories of emotional behavior, and physiological assessment of pathological etnotion.

Descriptors: Startle, Reflex modulation. Emotion, Imagery, Skin conductance. Heart rate. Facial EMG

Most of the variance in emotional judgments and behavior canbe accounted for within a two-dimensional affective space(Mehrabian, 1970; Mehrabian & Russell, 1974; Russell, 1980).The space has often been defined by axes labeled valence (pos-itive-negative) and arousal (calm-arou.sing). These dimensionsare thought to represent basic strategic dispositions of organ-isms, broadly affecting the direction and intensity of all affec-tive behavior (Lang, 1994). Other emotion theorists describingthe same affective space have preferred to rotate the valence andarousal axes 45", labeling the resulting axes negative affect (NA)and positive affect (PA) (Watson & Clark, 1984; Wat.son & Telle-gen, 1985). In this conceptualization, the PA axis extends fromhighly arousing positive emotion on one end to low-arousal neg-ative emotion on the other; NA extends from highly arousingnegative emotion to low-arousal positive emotion (Larsen &Diener, 1992).

Recent work has demonstrated covariation between physi-ological variables and emotional dimensions (Greenwald, Cook,& Lang, 1989; Lang, Greenwald, Bradley, & Hamm, 1993b).One of the most important and consistently reported findings

This research was supported by National Institute of Mental Healthgrant MH4686I. This study was completed by C.V.W. under the super-vision of S.R.V. in partial fulfillment of the requirements for a Master'sdegree. We thank Janice Kelly and Stephen Tiffany for helpful com-ments on earlier drafts of this manuscript, Robert l.evenson for his con-structive comments while serving as action editor for this paper, andNatalie Milbrandt, Steve Branson, and Jennifer Beckham for assistancein data collection.

Address reprint requests to: Charlotte vanOyen Witvliet or .Scott K.Vrana, Department of Psychological Sciences, Purdue University, WestLafayette, IN 47907, USA.

has been the relationship between the startle reflex eyeblink andthe valence dimension. Eyeblink magnitudes to startle probesare larger in the context of negatively valent ernotion than in thecontext of neutral and positively valent emotion (Bradley, Cuth-bert, & Lang, 1990, 1991; Cook, Hawk, Davis, & Stevcn.son,1991; Hawk, Stevenson, & Cook, 1992; Vrana & Lang, 1990;Vrana, Spence, & Lang, 1988). However, the affective tnateri-als u.sed in many studies have confounded valence with arousal.Specifically, the negatively valetit materials used were also highlyarousing, not allowing for definitive statetnents about the rela-tive modulatory effects of these two dimensions. Recent stud-ies have attempted to disentangle the effects of valence atidarousal on startle modulation.

Both Cook et al. (1991) atid Hawk et al. (1992) studied tnod-ulation of the startle blink with affective materials not con-founded on valence and arousal. Startle blink magnitudes weresignificantly greater during itnagery of negative versus positiveemotions in both studies, with arousal having mixed effects ontnagnitude. In contrast, startle latencies were significantly facil-itated (i.e., of shorter duration) during imagery of high-arousalversus low-arousal emotions but were not affected by affectivevalence. These researchers concluded that startle blink tnagni-tude indexes affective valence, whereas blink latency indexesaffective arousal.

Cuthbert, Bradley, and Lang (1990) examined startle blinkmagnitudes under tnultiple levels of arousal and valence protnptedby slides. Startle magnitudes were largest for high-arousal neg-atively valent affect and smallest for high-arousal positivelyvalent affect slides. No differences were found between blinksduring negatively valent and positively valent slides at low levelsof arousal. Based on the.se data, Lang, Bradley, Cuthbert, and

436

Physiology of affective dimensions 437

Patrick (1993a) concluded that startle magnitude is facilitatedduring processing of negatively valent as compared with posi-tively valent material but only under highly arousing conditions.

Because of these seemingly different results, the primary aimof the ptesent study was to examine the effects of affectivedimensions on the tnodulation of startle blink magnitude andlatency usitig a design that balances valence and arousal withinsubjects. To streamline the design, we examined a single emo-tion category within each affective quadrant (Table 1); only thenegatively valent high-arousal quadrant unambiguously includesmore than one fundamental or basic emotion (e.g., fear, anger,disgust). Recent re.seareh directly comparing fear, anger, and dis-gust has found no difference among these emotions in modu-lating the staitle respon.se (Cook et al., 1991; Hawk et al., 1992;Vrana, 1994). Iti the alternatively defined affective space, fearand pleasant relaxation provide high and low etid poitits on theNA axis and joy and sadness provide the high and low end pointson the PA axis. The current study, therefore, allows examina-tion of the startle reflex response within both descriptions ofdimensional affectivity.

Table I. Emotion Categories and MaterialsUsed in This Study

Arousal

High1 OW

Negative

Fear"Sadness'

Valcnee

Positive

Joy"Pleasatit relaxation''

Fear sentences1 watch in horror as an oncoming car swerves into my lane and

realize I cannot avoid a head-on collision.A strange man is following me through a had part of (own; sweat

pours down tiiy lace as I listen to his footsteps getting closer.I'm babysitting a 2-ycar-old and gasp in horror to tind her stick-

itig a metal spoon in the liglit socket.Joy sentences

As 1 walk aeross camptts I catch the eye of a handsome/heautifulman/wom;ui from one of my classes walking lowarct me; he/shesmiles widely as he/she passes.

My professor stands in Iront of the lecture hall and rehashes howdisappointed he is reading our papers, but hetore I know ii he isreading my paper, the only A paper in the class!

As I walk otit of class the man/vvomati I have hccn watching all.semester stops and asks if we can study tor the exam together.Sadne.ss sentences

The streets of the city are alive with people having a wonderfultime, when 1 notice an older, shabby-looking getitlcmati rutntiiagingthrough a nearby dumpstcr tor something to cat.

I sit listening to the stranger next to me tell me about losing herjob and having to support her chikhen.

1 tearfully listen to the news on the radio tell about the impover-ished third world children whose severe tnedical needs go untreated.Pleasant rela\ation sentences

I am lying on the sand on a warm day, listening to childieti play-ing down the beach, their soft voices tningling with the sound of thewaves.

A wood tire dances iti the hearth, I feel snug and warm in thecabin, reading the book on my lap, enjoying a well-deserved rest.

A soft smile creeps over my tacc as I begin to cat my favorite fla-vor of iee cream while listening to tny favorite music on the stereo.

"High negative attcct (NA). ''High positive affect (PA). 'Low positiveaffeet (PA). ''I ow negative affect (NA).

A parallel issue in this study eoncerned skin conductanceresponses (SCRs) to startle probes. As with the startle reflexblink, Vrana (1995) found that SCRs were augmented by neg-atively valent high-arousal affect as compared with positive andneutral low-arousal conditions. Furthermore, affective modu-lation of the eyeblink and SCRs to the startle probes was cor-related both within and between subjects, suggesting that thesetwo effects were similarly tnediated. However, this conclusionis tempered because valence and arousal were confounded in thatstudy. SCRs tnay have been modulated by either affectiveditnension, and other research suggests that arousal is morelikely than valence to modulate SCRs (Cook et al., 1991; Langet al., 1993b). ln the present study, we examined the roles ofvalence and arousal in modulating startle-elicited SCRs.

In addition to the probe response measures, we also exam-ined the effects of affective imagery on heart rate and skin con-ductance. Many studies using facial configuration or relivedetnotion tasks have found similarly high heart rate increases forsadness and high-arousal emotions such as fear, anger (Ekman,Levenson, & Friesen, 1983; Levenson, Carstensen, Friesen, &Ekman, 1991; Schwartz, Weinberger, & Singer, 1981), and hap-piness (Levenson, Ekman, & Friesen, 1990). However, in thesestudies emotional contexts were not explicitly defined or con-trolled in tertns of valence and arousal levels. In contrast, stud-ies that have consistetitly defined emotional contexts with respect10 the affective dimensions, and in particular that defined sad-ness as low arousal, found that imagery of high-arousal emo-tions resulted in greater heart rate increa.se than did imagery oflow-arou.sal etnotions, including sadne.ss (Cook et al., 1991;Hawk et al., 1992). ln the present study, we similarly definedsadness as a negatively valent low-arousal emotion, so we pre-dicted that high-arousal imagery (fear, joy) would producegreater heart rate increase than would low-arousal imagery (sad-ne.ss, pleasant relaxation). The approach taken here was tochoose ettiotional materials for their location within the affec-tive space and to use the specific emotion label most represen-tative of the content of the tiiaterial (see Table 1). This does notimply that all possible situations within an emotion categorywould fit in that location in affective space. For example, sad-ness is an appropriate label for the negatively valent low-arousalmaterials used in this study, but other sad situations might behigh in arousal (e.g., grief) or tnay even involve positively val-ent material (e.g., a bittersweet memory).

The influence of affective dimensions was also measured viatacial electromyographic (EMG) data collected at the zygomatic,corrugator, and orbicularis oculi tnuscle regions. In previousstudies, zygomatic activity increased during positively valentemotion and corrugator activity increased during negativelyvalent etnotion (see Ditnberg, 1990; Fridlund & Izard, 1983),Increased orbicularis oculi activity has been found during high-arousal as compared with low-arousal affective imagery (Hawket al., 1992).

Method

SubjectsThe 48 (24 female, 24 tnale) subjects were introductory psychologystudents who received course credit in return for participation.

Stimulus MaterialsThe stimulus materials were 12 sentences describing three situ-ations of each of the following emotions; fear, sadness, joy, and

438 C.V. Witvliet and S.R. Vrana

pleasant relaxation. An independent sample of 55 (28 female,27 male) undergraduates rated 70 affective sentences along7-point Likert-type scales for the dimensions of valence, arousal,and dominance and the emotions of fear, joy, anger, sadness,surprise, grief, and pleasant relaxation. Sentences chosen for thisstudy were those that were tnost extremely located in their re-spective quadrants of the emotional space to meet the valenceand arousal qualifications specified (see Table 1) and that wererepresentative of their specific emotion category.

ApparatusAll digital data collection and timing of experimental events werecontrolled by a Samsung AT-compatible computer and on-linephysiological data collection software (Cook, Atkinson, & Lang,1987). Auditory stimuli (tones and white noise bursts) were pre-sented binaurally through headphones. The tones (500 ms long,73 dB[A]) were generated by a Coulbourn voltage-controlledoscillator with a selectable envelope rise/fall gate set for an80-ms rise/fall time. The acoustic startle-producing stimulus wasa 50-ms burst of 95 dB(A) white noise (10,000-20,000 Hz) withinstantaneous rise time produced by a white noise generator.

The startle reflex measures were obtained by recording EMGactivity at the orbicularis oculi. EMG was also recorded at thezygomatic and corrugator muscle regions. All facial EMG datawere collected using electrode plaeements suggested by Fridlundand Cacioppo (1986). Facial skin was cleaned with Sea Breezefacial soap and rubbing alcohol, and Med Associates electrodegel and miniature Ag-AgCl electrodes were applied. The signalswere amplified (x60,000) by a Hi Gain bioamplifier, using90-Hz high-pass and 250-Hz low-pass filters. Signals were thenrectified and integrated by a Coulbourn contour-following inte-grator (time constant = 80 ms). The orbicularis oculi was sam-pled at 1000 Hz for 250 ms after the onset of the startle probestimulus.

Skin conductance measures were gathered by a Coulbournskin conductance coupler that applied a constant 0.5 V acrosstwo standard electrodes. These electrodes were filled with a tnix-ture of physiological saline and Unibase (F-owles et al., 1981) andplaced on the hypothenar eminence of the left hand after sub-jects had rinsed their hands with tap water. The skin conduc-tance and facial EMG channels were sampled at 10 Hz with a12-bit analog-digital converter.

Electrocardiogram data were collected using two standardelectrodes placed on each of the subject's forearms. The signalwas amplified and filtered by a Hi Gain bioamplifier and sentinto a digital input on the computer, which detected R-wavesand measured interbeat intervals with millisecond resolution.

ProcedureAfter the subject arrived at the lab, the experimenter providedinformation about the experiment, applied electrodes, and readthe instructions. The experimenter then gave the subject twoindex cards, each with a typed sentence that described a situa-tion representing a different emotion. The subject was instructedto read each sentence and create a vivid image of personallyexperiencing and participating in the events described. Once thesubject reported having created vivid images to eaeh script, theexperimenter summarized the main points of the instructions,dimmed the lights, and exited the room.

The tone-cued imagery procedures closely followed those ofprevious studies (Vrana, 1993, 1994, 1995; Vrana & Lang, 1990).

With eyes closed, the subject listened lo a 10-inin series of tones,one every 8 s. The tones were differetitiated as high (1350 Hz),medium (985 Hz), and low (620 Hz) in frequency. The mediumtones were heard tnost frequently and signaled the subject torelax and think about the word one. The high and low tones eachcued the subject to itnagine the specified sentence content as avivid, personal experietice. The 8-s imagery trial continued untilthe next medium tone, when the subject returned to relaxation.The tones were presented in a quasi-random order, with threeto six relaxation periods (24-48 s) between each high- or low-tone-cued itnagery trial. Six trials of high-tone-cued itnagery andsix trials of low-tone-cued itnagery formed one block.

Within each block, the startle-provoking stitnulus was pre-sented in the latter half of four of the six high-tone-cued imag-ery trials and four of the six low-tone-cued trials. Four startlestimuli were also presented within the relaxation periods toincrease the subject's uncertainty about the occurrence of thestartle stimuli. The subject was instructed to ignore these stimuli.

Once the tone series was completed, the subject registered rat-ings of valence, arousal, dotninance, and vividness for the com-pleted imagery by using a ratings knob to manipulate a videomonitor display (see Hodes, Cook, & Lang, 1985). These rat-ings were converted by the computer to a 0-20 scale. The exper-itnenter then returned with the next pair of sentences describingdifferent affective situations and reinstated the tone series forthe second block. This procedure was repeated for a total of sixblocks. Upon completing all blocks, the subject had imaginedthree distinct joy, pleasant relaxation, fear, and sadness situa-tions. These affective situations were counterbalanced acrosssubjects such that all possible pairings of emotions were tnade,all emotions were imagined at the high and low tones equallyoften, and all materials appeared equally often at differentpoints of the experiment.

Data Reduction and AnatysisEach trial of reflex eyeblink was scored off line for peak ampli-ttide (in arbitrary analog-digital unit.s) and latency to blink onset(in milliseconds). SCR to the startle probe was scored as the firstresponse initiated within 4 s after probe onset. SCR atnplitudewas scored in microsiemens and then range corrected (Lykken& Venables, 1971). Skin conductance latency to respon.sc onsetwas scored to the nearest O.I s. When an eyeblink or SCR didnot occur to a probe, its amplittide was set to zero and latencywas defined as missing for that trial.

Cardiac interbeat intervals were converted off line to heartrate in beats per minute for each imagery period. The mean skinconductance level in microsiemens and muscle tension for thefacial EMG channels in tnicrovolts were calculated for the .satnetime periods. Becau.se skin conductance is responsive to startleprobes (Vrana, 1995), mean skin conductance levels were ana-lyzed only for imagery trials not containing probes. The startleprobe also protnpted increases at all facial EMG regions; how-ever, this effect was restricted to the half-second following probeonset. The data for the half-second immediately following thestartle probe were replaced by the tneans of the two surround-ing half-seconds for the facial EMG data, allowing use of all thetrials in the analysis. Data from the 1 s prior to the tone-signalingimagery were used as a baseline to create ehange seores for eaehphysiological measure.

The dependent measures were ratings of imagery (valence,arousal, dotninance, vividtiess), startle reflex blink tnagnitudeand latency, SCR magnitude and latency to the startle probe.

Phvsiohigy of affective ditncnsians 439

arul heart rate, skin conductance level, and facial EMG at rheorbicularis octrli, corrugator, and /ygomatic regions duringimagery. All available data across all observations within anemotion category were averaged. A 2 (valence) x 2 (arotrsal)analysis of variance (ANOVA) with repeated measures for eachvariable was carried out for each dependent measure. Whenevera Valence x Arousal irueraction was found, all emotion com-binations were compared, wiih /> values corrected irsing rhe Bon-

procedure (critical /; = ,05/6 = ,0083).

Kesiills

Rating.sMeans arul slarrdard deviations for all ratings are shown in Ta-ble 2, Subject ratings of valence differentiated joy and pleasantrelaxation inragery as positively valent and fear and sadness asrregatively valent, l\\,41) = 414,38, /) < ,O(K)1. Joy was ratedas more positive ihan pleasant rela,\ation. F(l.47) = 16,87, p<,0002, which was laled as more positi\c Ihan tear and sadness,holh /(1,47) > 200 (Valence x Arousal, / | 1,471 = 16,38. p <,0003),

High-arotrsal imagery was given higher arousal ratings thanwas low-arousal imagery, /Xl,47) = 221,38, /X ,0001, Higherarousal ratings were also given to negatively valent emoiion thanlo positively valent emotion, /"(1,47) = 67,60, p < ,tX)01, Theimpact of valence on the arousal ratings was much greater in ihelow-arousal imagery Ihan in Ihe high-arousal imagery. Valence xArousal / (1,47) = 17,27. /) < .(XK)1. Every emotion dilTeredfrom every other one when specific contrasts were performed,all /(1,47) > 15, / )< ,0001,

Sirbiecls raied feeling more dominant in their imagery ofpositively valent emotion than in negatively valent emotion,/ (1,47) = 124.26, /; < ,0001, and in imagery of low-arousal ver-sus high-arousal emotion, /"(1,47) = 70.07, /) < ,0001,

Strbjects rated positive imagery as more vivid than ncgaiiveimagery, / (1,47) = 21.81, / )< ,0001, and liigh-arousal imageryas more vivid than low-arousal imagery, F(I,47) = 14.93, / ' <.0005. Sadness was rated as less vivid than all other affectiveimagery (all contrasts /•'|1,471 > 33, /) < ,001), Valence xArousal, HI.47) = 18,60, p < .0001.

Response to Startle I'rohes

Startle reflex cvchtittk tuagnitudc. \s may be seen in I'igurc 1.probes prompted larger blink magniriides during negalively valentIhan dtrririg positively valent emotion, / (1,47) = 10,41,/X ,(X)3,

Talilc 2. I'ostimagery Ratittgs

Negalivc valence silivi' valence

High low Hrgh lowarousal" arousal'' arousal' arousaf'

Valence ,'̂ ,(12 (,V(M) ,"5,79(3,07) 17,69(2,01) 16,,̂ S (2,25)Arousal 17,17(3,69) 10,,'iO (,V(i7) 15,19 (,V76) ^,52 (,V8,'')noniiuaiKe 4,!'S (,VS9) 6,96 (,'(,SS) 11,02 (,1,,'O) 15,19 (,V34)Vividness 16,40(2,65) 14,06 (,V()5) 16,21(2,45) 16,52(2,45)

Note: Ratings were on a seafe oT 0 2t) atul are given as me;\iis (,S7)),''I ear, ''Siuliiess, \]o\. ''I'leasaiil rel;i\aiion.

700

650

600

550

500

MAGNITUDE

• I Positive Valence( Negative Valence

LowArousal

High

40

35

LATENCY

LowArousal

High

Figure 1. Mean srarlle reflex eyebfink magniuidcs and larcricies ro aeous-rie siarrfe probes during al'leeiivc imagery, Srandard errors are ,shownon bars.

and during high-arousal emotion than during lovv-arousal emo-tion, F( l ,47)= 18.40,/7< .0001.

Startle rcfle.x eyehlink latency. ,-\s also seen in Figure I, blinkonset lateticy was faster during negatively valent emotion thanduring positively valent emotion, F(l,47) = 11.97, p < .003.Faster blink latencies were also elicited during high-arousal emo-tion than during low-arousal conditions, F(l,47) = 6.59, p < ,03,

Covariance analyses of startle eyeblink magnitude andlatency. Because the analyses of \alenee, arousal, and \ividnessratings detnonstrated that materials were not matched on theseratings as completely as intended, further analyses of the datawere conducted by covarying these ratings. This was done toCNaminc the possibility that the effects of valence were due todiffercrrces in arousal, that the effecfs of arousal were due to dif-ferences in valence, or that valence and arousal effects were afunction of vi\idncss differences,

\\ hen valence ratings were covaried for all four affective con-ditions, a main effect of arousal still was demonstrated for bothblink magnitude, F(l,46) = 17,49, /? s .(XX)1, and latency,/•(1,46) = 8.41, /) < .01. and no effect of valence was found,W hen arotrsal ratings were covaried, a tnain effect of affective\alenee was found for both startle magnirude. /"(1.46) = 7,86,/) < .01, and latency, F(l,46) = 6.76, /' < ,02, but no arousaleffect was shown, Co\arying vividness ratings did not affect the

440 C.V. Witvliet and S.R. Vrana

results. Thus, the effects of both valence and arousal on bothblink magnitude and latency remained, even when differencesin subject ratings were covaried. This covariance strategy wasemployed for the remaining dependent variables. Whenever avalence effect was found, it was followed by an analysis usingarousal as a covariate; arou.sal effects were followed by an anal-ysis covarying valence. An analysis covarying vividne.ss followedany significant effects.

SCR magnitude. Subjects responded to startle probes withlarger range-corrected SCRs during high-arousal than duringlow-arousal emotion conditions, /"•"('.47) = 17,89,/?= .(XK)1. Theeffect of arou,sal remained after covarying valence and vividne.ssratings. Means and standard deviations for this and subsequentphysiological measures are presented in Table 3.

SCR latency. SCR latency data were unusable for 14 subjectswho did not respond often enough to provide data for every cellin the design. The SCR latency of the remaining subjects wasfaster during high-arousal than during low-arousal emotion con-ditions, F(I,33) = 4.23, p = .05. The same pattern of resultsremained after covarying valence, although the arousal effectwas no longer statistically significant, F(I,32) = 3.70,p= .0635.Covarying vividness did not alter the results.

Heart Rate and Skin Conductance LevelDuring ImageryHeart rate data were unusable for two subjects because of equip-ment problems. The remaining data demonstrated that heart ratewas higher during high-arousal than during low-arousal affec-tive imagery, F(l,45) = 12.01,/?< .003, an effect that remainedwhen valence and vividness were used as covariates. No signif-icant effects of valence or arousal were demonstrated for skinconductance levels, which is consistent with the results of otherstudies in which no relationship between skin conductance andIhe emotional content of imagery was found (May, 1977; Vrana,1993, 1994).

Facial EMU During Imagery

Zygomatic. Greater zygomatic activity occurred during pos-itive than during negative emotion imagery, F( 1,47) = 15.46, p <

.0005, and during high-arousal than during low-arousal imag-ery, F(l,47) = 13.95, p < .0005. There also was a Valence xArousal interaction, F(l,47) = 11.06, p < .003. Zygomatic activ-ity levels were higher during joy than during all other affectiveimagery conditions (all comparisons with joy, F(I,47] > 13.0,p < .0005). The results did not change when vividness was covar-ied. Covarying arousal maintained the Valence x Arousal inter-action, /-(1,46) = 5.11, /; < .03, although the main effect ofvalence just failed to reach significance, F(l,46) = 3.81, /? =.057. When valence was covaried, both the effect of arousal,F(l,46) = 12.0, p < .002, and its interaction with valence,F(l,46) = 5.42, p < .03, remained.

Corrugalor. Corrugator muscle tension was higher duringnegatively valent emotion conditions than during positivelyvalent conditions, F( 1,47) = 29.80, /; < .(X)01, even when arousalwas covaried, F(l,46) = 13.73, p < .0006. Corrugator muscletension was higher during low-arousal than during high-arousalconditions, F(l,47) = 4.79, /; < .05, even when valence wascovaried, F(l,46) = 4.88, /; < .04. When vividness was covar-ied the valence effect remained, F( 1,46) = 21.3, /; < .0001, butarousal was no longer statistically significant, F(l,46) = 3.20,p = .0803.

Orbicularis oculi. tor the orbicularis oculi, greater tensionoccurred in the context of high-arousal than in the context oflow-arousal imagery trials, F(l,47) = 19.42,/;< .0001, even aftervalence and vividness ratings were covaried.

l)i.scus.sion

Startle ReflexIn this study, we examined the effects of affective valence andarousal on multiple psychophysiological responses, with primaryemphasis placed on modulation of startle reflex eyeblink mag-nitude and latency. Previous research has indicated that startlemagnitude indexes valence and latency indexes arousal (Cooket al., 1991; Hawk et al., 1992) or that the startle retlex respon.seis sensitive to valence but only under high-arousal conditions(Lang et al., 1993a). The results of this study, however, dem-onstrated main effects for both affective valence and arousal onboth startle blink magnitude and latency. Blinks were larger and

Table 3. Physiological Measures

Measure

SCR 10 startle probeMagnitude (range-corrected)Latency (s)

Autonomic nervous syslemHeart rate (beats/min)Skin conductance level

F acial EMG (j<V)Orbicularis oculiZygomaticCorrugator

Negative valence

High arousal" Low arousal''

0.113(0.112)1.48 (0.34)

1.906 (2.492)0.007 (0.058)

1.497 (1.4.30)0.064(0.219)0.329(1.032)

0,059 (0.058)1.54 (0.47)

1.034 (L7O5)-0.0{J4 (0.041)

0.917 (0.940)0.044 (0.178)0.346 (0.563)

Positive valence

High arousal''

0.095 (0.092)1.36 (0.41)

1.507 (1.431)-0.009 (0.058)

1.528 (1.145)0.445 (0.750)

-0.303 (0.495)

Low arousal''

0.068 (0.070)1.42 (0.54)

1.062(1.578)-0.008 (0.035)

1.042(0.921)0.070 (0.297)

-0.043 (0.444)

Note: Values presented are means (SD)."Fear. *Sadness. 'Joy. ''Pleasant relaxation.

Physiology of affective dimensions 441

faster during negatively valent than during positively valentimagery, and blinks were larger and faster during high-arousalihan during low-arousal imagery. Although these results at firstappear to add only confusion to this issue, a closer examinationreveals striking consistency across studies.

For example, even though Cook et al. (1991) and Hawk et al.(1992) concluded that valence modulates blink magnitude andarousal modulates latency, examination of ihe dala from thesestudies reveals ihal they remarkably parallel the data of Ihepresent study, although the effects of valence on latency and ofarousal on magnitude generally failed to reach acceptable lev-els of statistical significance. The overall pattern of results sug-gests that with larger sample sizes these tesearchcrs would havereached the same conclusions reported here.

The discrepancy between the present data and the Langet al. (1993a) hypothesis-based on Cuthberi et al.'s (1990)dala —can be accounted for by attentional effects on the star-lie reflex. Cuihbert cl al. (1990) observed smaller startle mag-nitudes during high-arousal positively valeni slides Ihan duringlow-arousal positively valent slides, in contrast to Ihe presentfinding that startles were augmented during high-arousal imag-ery conditions. The startle response is attcnualcd when atlcn-lion is directed visually toward a slide during an acoustic probe(Anlhony, 1985). Interesting slides engage more attention thando dull slides and thus further attenuate the response to acous-tic probes (Simons & Zelson, 1985). Because arousal and inter-est are highly correlated in the Cuthbert ct al. (1990) slide set(Bradley, 1994), it is likely that Ihe stnaller startle during posi-tively valent high-arousal slides is due to the attention-engagingproperties of these slides. Imagery, however, does not engagesensory modality-specific atleniion (Vrana, 1994, 1995), allow-ing for observation of arousal-induced facilitation of the star-tle response.

greater during high-arousal than during low-arousal emotionconditions. Thus, facial EMG, like startle, is not uniquely mod-ulated by valence or arou.sal.

Negative/Positive Affect Organization of StartleReflex and Facial ExpressionMany emotion theorists have found it beneficial to rotate thevalence and arousal axes 45° and describe this same emotionalspace within the framework of negative affect (NA) and positiveaffect (PA). The pattern of startle and facial EMG resultsobtained, and especially the lack of correspondence betweenthese measures and either the valence or arousal dimensionalone, prompted an examination of these data from an NA/PAperspective. A sutnmary of the results from this point of view-can be seen in Figure 2. The startle reflex was modulated alongthe NA dimension. Blink magnitudes and latencies were larg-est and fastest during high-NA (fear) conditions, moderate dur-ing moderate levels of NA (joy, sadness), and smallest andslowest during low-NA (pleasant rela.xation) conditions. Toassess whether this organization of the data was supported sta-tistically, the linear effect of NA was examined using a one-wayANOVA with three equally weighted levels; high (fear), medium(joy, sadness), and low (pleasant relaxation). The linear effectof the NA dimension was significant for both magnitude,F(l,47) = 21.27, p < .0001, and latency, F(l,47) = 21.32, ;?<.0001. These Fvalues for the NA effect were larger than the Fsfor the effects of arousal or valence for each measure. Further-more, the linear effect of PA (with fear and relaxation compris-ing Ihe medium-PA level) was not significant for magnitude(F = 0.00) or latency (F = 1,05).

The NA dimension appears to map as well as or better thanvalence onto the appetitive-aversive dimension that Lang, Brad-ley, and Cuthbert (1990) proposed to modulate the startle reflex.

Autonomic Nervous System MeasuresIt) this study, we also examined the effects of valence and arousalin modulating SCRs elicited by startle probes. Larger and fasterSCRs to startle probes occurred only under high- rather thanlow-arousal conditions. This suggests that previous findings(Vrana, 1995), demonstrating facilitated SCR magnitudes andlatencies during fear as compared with pleasant and neutralimagery, reliccted arousal differences among the emotional con-texts. These results correspond with previous demonstrationsihat skin conductance responds to highly arousing stimuli thaiengages the sympathetic nervous system (Cook et al., 1991; Langet al., 1993b). Paralleling these results, heart rate was signifi-canlly faster during high-arousal than during low-arousal imag-ery, also consistent with previous findings (Cook el al., 1991;Hawk et al., 1992).

Facial EMGFacial E-.MG results showed valence effects consistent with pre-vious fitidings (see Dimberg, 1990; Fridlund & Izard, 1983) andalso demonstrated arousal effects for all muscle sites. Zygomaticactivity was significantly greater during positive than during neg-ative valence emotion and during high-arousal ihan during low-arousal contexts. In addition, a Valence x Arousal interactiondemonstrated that zygomatic activity was significantly greaterduring joy than during all other emotion conditions. Corrugatormuscle tension was significantly greater during negative thanduring positive valence emotion and in low-atousal Ihan in high-arousal contexts. Orbicularis oculi activity was significantly

LOWNA

(PLEASANT)

Smallest Startles

Slowest Startles

PositiveValence HIGH PA

(JOY)

Highest Orbicularis Oculi EMGHighest Zygomatic EMGLowest Corrugator EMG

High Arousal

Highest Heart RateLargest SCRsFastest SCRs

HIGH NA

(FEAR)

Lowest Orbicularis Oculi EMG

Lowest Zygomatic EMG

Highest Corruflator EMG NegativeValence

Largest StartlesFastest Startles

2, Organization of startle reflex eyeblink, facial EMG, and aulo-noniic nervous syslem data within the affective spaee defined by valeneeand arousal, and by negative affect (NA) and positive affect (PA). SCRsrefer to skin conductance respon.ses lo the startle probes.

442 C.V. Wilviiel and S.R. Vrana

It also fits well with Schnierla's behavioral model (1939; 1959),which differentiates between W-type (withdrawal/defen.se) andA-type (approach/consummatory) response behavior to high-intensity aversive and low-intensity appetitive stimuli, respec-tively. A high-NA stimulus would prompt a W-type response,whereas a low-NA stimulus would evoke an A-type response.High-NA situations require specific, automatic withdrawal orother protective behaviors. Graham (1979) characterized thestartle as a system that interrupts ongoing behavior to allow forstimulus identification. Thus, output in this retlex system wouldbe amplified in contexts demanding protective respon.ses. Otherhigh-NA emotional contexts such as anger and disgust, whichrequire rapid identification of stimuli and protective behavior,also amplify startle reflex responses (e.g.. Cook et al., 1991;Vrana, 1994). Further, the startle reflex has been a sensitive mea-sure in the assessment of high-NA psychopathology, such asposttraumatic stress disorder and specific phobia (Butler et al.,1990; de Jong, Merckelbach, & Arntz, 1991; Vrana, Constan-tine, & Westman, 1992).

The summary of the facial EMG data in Figure 2 suggeststhat these measures were organized along the PA dimension.For each of the muscle sites, the most extreme differences werebetween sadness and joy, the two extreme emotions on thePA dimension. This impression was examined by performingNA/PA analy.ses paralleling those conducted for the startle mea-sures. The linear effect of PA was significant for the zygomatic,F(l,47) = 15.81, orbicularis, F(l,47) = 21.92, and corrugator,F(l,47) = 39.21, muscle regions, allps< .0002. These f"valueswere all larger than the respective arousal or valence /"s. How-ever, the linear effect for NA was significant for the orbicularis,F(l,47) = 6.64, p < .02, and corrugator, F(l,47) = 9.41, p <.(X)5, regions. Therefore, the facial EMG results appear to benot as specifically tied to PA as the startle results are linked toNA. However, theoretical links between PA and facial expres-

sion suggest PA as the dimension along which to organize thesedata. The PA dimension has been consistently linked to extra-version and sociability (Laisen & Diener, 1992), constructs clo.selyrelated to facial expressiveness, an inherently social, communi-cative phenomenon (Fridlund, 1992). In addition, depression,which is often characterized by low PA, has been successfullyassessed by facial EMG (Schwartz, Fair, Salt, Mandel, & Kler-man, 1976a, 1976b; Schwartz et al., 1978).

ConclusionThe valence and arousal and the NA/PA perspectives are merelydifferent descriptions of the same affective space, and so thepreference for one set of dimensions over the other is dependenton the conceptual and practical advantages that are incurred.The NA/PA interpretation is valuable in providing a simplerorganization of the redexive and facial expression (though notthe autonomic) data in this and other studies. This approach alsosuggests new theoretical links and in particular is useful for inte-grating the startle-emotion literature with Graham's (1979) orig-inal characterization of the startle as an "interrupt" system.However, the value of this approach may be tempered by a lackof definitional clarity within the NA-PA literature. The labelspositive affect and negative affect are potentially misleading inthat they are unipolar valence labels applied to bipolar dimen-sions, each of which varies in both valence and arousal (Larsen& Diener, 1992). Further, the current study included only fouremotion categories to map the affective dimensions. Emotioncontexts have, in addition to their dimensional qualities, situa-tion-specific action sets that impact physiological responses(Vrana, 1993). The ultimate utility of the dimensional organi-zation of affective physiology will be determined by futureresearch assessing the modulation of these measures by a broadrange of emotional contexts that vary systematically in theirlocations within the affective space.

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(RKCEIVF.D October 25, 1993; ACCEPTED November 23, 1994)