Ž .International Journal of Psychophysiology 42 2001 11�34

Event-related brain potentials differentiate positive andnegative mood adjectives during both supraliminal and

subliminal visual processing

Edward Bernat a,�, Scott Bunceb, Howard Shevrinc

aDepartment of Psychology, Uni�ersity of Minnesota, Elliot Hall, 75 East Ri�er Road, Minneapolis, MN 55455, USAbHahnemann Uni�ersity School of Medicine, Philadelphia, PA, USA

cDepartment of Psychiatry, Uni�ersity of Michigan Medical Center, Ann Arbor, MI, USA

Received 21 October 2000; received in revised form 29 January 2001; accepted 29 January 2001

Abstract

Ž .This experiment provides brain event-related potential ERP evidence for differential processing of visuallyŽ . Ž .presented pleasant and unpleasant affectively valent words mood adjectives for both supraliminal 40 ms and

Ž .subliminal unmasked, 1 ms stimulus durations. Unpleasant words elicited a more positive amplitude than pleasantŽ .words in both durations. ERP components P1, N1, P2, P3, and a late positive potential; LP were measured at six

Ž .electrode sites F3, F4, P3, P4, CzPz, Oz . ERPs to subliminal stimuli demonstrated differences between pleasant andunpleasant words in the left hemisphere across all measured components. Supraliminal processing showed similar

Ž .differences in the left hemisphere for early components P1 and N1 , but bilateral differences for late componentsŽ .P3 and LP . Activity in the P2 time window was associated with the divergence between supraliminal and subliminalaffective responses. Implications for the study of affect and consciousness are discussed. � 2001 Elsevier ScienceB.V. All rights reserved.

Keywords: Emotion; Affect; Valence; Words; ERP; Subliminal; Unconscious; Non-conscious

1. Introduction

The main purpose of this study is to investigatebrain responses to affective semantic visual sti-muli presented consciously, and to extend thisresearch to brain responses to these stimuli pre-

� Corresponding author. Tel.: �1-612-624-5063; fax: �1-612-624-2079.

sented outside conscious awareness. Event-Ž .related potentials ERPs have been shown to be

sensitive to the conscious affective perception ofŽwords Begleiter et al., 1979; Cacioppo et al.,

1996, 1994; Chapman et al., 1980; Skrandies and. Ž .Weber, 1996 , faces Kayser et al., 1997 , and

Žpictures Johnston et al., 1986; Yee and Miller,.1987 . Additionally, there is now considerable

psychological and physiological evidence suggest-ing that affective processing occurs without con-

0167-8760�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 1 6 7 - 8 7 6 0 0 1 0 0 1 3 3 - 7

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�3412

Žscious awareness e.g. LeDoux, 1995; Kunst-Wilson and Zajonc, 1980; Shevrin et al., 1992,

.1996; Wong et al., 1997 .This study is focused on the affective dimen-

sion of valence, also referred to as a pleasant�unpleasant dimension, the hedonic quality, or

Žpositive affect and negative affect Russell and.Carroll, 1999 . Consistently in the literature, af-

fective valence has been identified as the mostpowerful single measure of the emotional qualityof stimuli, generally explaining more variance inaffective responses than other single dimensionsŽLang, 1984; Lang et al., 1993; Osgood et al.,

.1975; Russell, 1978 . Affect theorists from differ-ent vantage points include some measure of af-fective valence as a fundamental to their modelŽBrenner, 1973; Davidson, 1993; Heller, 1993;Lang et al., 1992; Izard, 1993; LeDoux, 1995;Miller, 1996; Ortony and Turner, 1990; Osgood et

.al., 1957; Smith and Ellsworth, 1985 . While someform of valence is widely considered to be afundamental affective dimension, notable debateexists as to whether it is best represented by a

Žsingle bipolar pleasant�unpleasant vector e.g..Osgood et al., 1957; Russell and Carroll, 1999 or

by a bivariate model with separable activation ofŽnegative and positive evaluative processes e.g..Cacioppo and Berntson, 1994; Davidson, 1998 ,

Although valence has been shown to be thestrongest single dimension, many models of affectincorporate additional dimensionality beyondvalence. Findings from a substantial body of workbased on factor analysis of semantic ratings sug-gest that one or two additional dimensions areimportant. In order of variance accounted for, themost common empirically derived model containsthree factors: valence, arousal, and dominance. It

Ž .is worthy of note that Wundt 1897 postulated asimilar set of three dimensions underlying affect.Arousal usually has a U-shaped quadratic rela-tionship to valence in which extreme pleasant orunpleasant stimuli are associated with increased

Žarousal Lang et al., 1993; Russell and Carroll,.1999 . The third dimension, generally referred to

as dominance or potency, is statistically moreunstable and less widely accepted than the va-

Žlence and arousal dimensions Russell, 1978;.Smith and Ellsworth, 1985 . Currently, most con-

sider the two top dimensions of valence andarousal to constitute the most compelling model

Žto emerge from this work Russell and Carroll,.1999; Watson et al., 1999 . An important variant

of the valence�arousal model is based on rotatingthe two dimensions by 45� and labeling them

Ž .Positive Affect PA; excited vs. sluggish and Neg-Ž . Žative Affect NA; distressed vs. relaxed Watson

.and Tellegen, 1985; Watson et al., 1999 . Theauthors argue that this rotation captures the sub-jective components of broader biobehavioral sys-tem of approach and withdrawal.

1.1. Physiological studies of affect

Several ERP studies have found that ERPs aresensitive to emotional qualities of visual stimuliŽe.g. Begleiter et al., 1979; Cacioppo et al., 1996,1994; Chapman et al., 1980; Johnston et al., 1986;Kayser et al., 1997; Skrandies and Weber, 1996;

.Yee and Miller, 1987 . While these studies pro-vide evidence that ERPs can be used to differen-tiate the affective dimensions of a variety of sti-muli, they vary widely in their methodology, in-cluding the type, modality and valence of stimuli,experimental design, required tasks, number andlocation of recording electrodes, statistical analy-ses, and the ERP components evaluated. Notably,the general notion that hemispheric lateralizationis important in affective processing has continued

Žto gain support from ERP paradigms e.g. Ca-.cioppo et al., 1996; Kayser et al., 1997 .

One consistent finding among these ERP stud-Ž .ies is that emotional stimuli elicit a P300 P3

Ž .component and a late positivity LP amplitudedifference, although the nature and direction of

Žthese differences are not yet clear Cacioppo etal., 1994, 1996; Chapman et al., 1980; Lang et al.,1990; Roschmann and Wittling, 1992; Yee and

.Miller, 1987 . Although most ERP studies ofemotion have analyzed the later endogenouscomponents, there is also evidence that emotionalprocesses can be differentiated in earlier time

Ž .windows. Begleiter et al. 1979 reported that atrough-to-peak measure of the N1P2 componentdifferentiated pleasant from unpleasant word cat-egories at electrode P3. Skrandies and WeberŽ .1996 found significant valence word class effects

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�34 13

Ž .in an early time region 130 ms , which weresimilarly located towards the left hemisphere.

Most ERP studies of emotion have examinedthe role of affective valence within the context ofa behavioral response task, such as stimulus rat-ings, stimulus discrimination, or target detection.Many also use reaction time responses, whichtypically evoke confounding motor potentials inaddition to brain potentials. In this study, nobehavioral response to stimuli was requested ofthe participants. This was done partly to avoidconfounding motor potentials, but also becausebrain potentials resulting from a more complexcognitive task might overwhelm the variance in

Žthe ERPs to the subliminal stimuli ERPs tosubliminal stimuli tend to be quite small, as de-

.tailed below in the methods section . Further-more, asking for a response to stimuli which arenot being consciously seen can easily becomenonsensical to participants. Research on the P300component suggests that individuals need not be

Ž . Žwilling Allen et al., 1992 or able Renault et al.,.1989 to declare how they evaluated a stimulus to

elicit an ERP that can be used to differentiateŽ .their responses. Additionally, Crites et al. 1995

have demonstrated that individuals do not have tomake an explicit judgment for what they call a

Ž .late positive potential LPP to distinguish themagnitude of evaluative differences among affec-tive stimuli. Furthermore, they have shown thatwhen participants are instructed to misreport theirevaluative ratings, the LPP amplitude varies onlyas a function of correctly reported evaluativejudgments, not of intentionally misreported evalu-ative judgments. This suggests that ERPs can beused to assess the affective dimension of attitudes

Ž .people are unwilling to report Crites et al., 1995 .

1.2. Unconscious affecti�e processing

Although the vast majority of research on emo-tion has been on stimuli available to consciousawareness, there is now considerable evidencesupporting the notion that significant affectiveprocessing happens outside conscious awarenessŽe.g. Bunce et al., 1999; Esteves et al., 1994; Fox,1991; Izard, 1993; Kunst-Wilson and Zajonc, 1980;LeDoux, 1989; Mogg et al., 1993; Ohman et al.,

1995; Shevrin and Fritzler, 1968; Shevrin et al..1996; Wong et al., 1994, 1997 . This is consistent

with a broader understanding developing in psy-chology that a great many mental processes occur

Žoutside conscious awareness Kihlstrom, 1987;.Miller, 1996; Shevrin and Dickman, 1980 . A

number of theorists give serious consideration tothe premise that complex affective processes can

Žoccur without the benefit of consciousness Ep-. Ž .stein, 1994; Lazarus, 1991 . LeDoux 1989 has

argued that the core of the emotional system is abrain mechanism that computes the affective sig-nificance of a stimulus. He argues that this brainmechanism is part of what gives rise to the con-scious experience of emotion, and that it neces-sarily operates outside of conscious awareness.

1.3. Subliminal stimulus presentations

Subliminal presentations of visual stimuli havebeen used to demonstrate that unconscious pro-cessing can be complex, influence consciousprocesses, and can operate differently from con-

Žscious processes Shevrin and Fritzler, 1968;Shevrin and Dickman, 1980; Shevrin et al., 1969,

.1992; Snodgrass et al., 1993 . Kunst-Wilson andŽ .Zajonc 1980 have demonstrated how uncon-

scious affective stimuli can influence appraisals ofconscious stimuli. Evidence is also available sug-gesting that aversive conditioning can occur with-out conscious awareness, as indexed by brain re-

Ž .sponses Wong et al., 1997 , EMG responsesŽ . ŽBunce et al., 1999 , and skin conductance e.g.

.Ohman et al., 1995 . Finally, Bernat, Shevrin andŽ .Snodgrass 2001 present evidence that subliminal

stimuli in an oddball paradigm can produce aP300 response.

The method of measuring unconsciousprocesses by presenting visual stimuli subliminallyhas a long history marked by methodological dif-ficulties and advances which are beyond the scope

Žof this paper see footnote 1 for a brief account of.these issues . Subliminal stimuli in the current

study meet objective detection threshold criteriaŽ .using a 2AFC detection procedure . According

Žto the signal detection theory SDT, Macmillan,. Ž1986 , when detection sensitivity is at chance d�.�0 , it is unlikely that there is conscious aware-

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�3414

ness of the stimulus.1 Other research has reliedon a subjective criterion of awareness in whichthe participant reports no awareness of the stimu-lus, but d� can still be above zero, thus increasingthe likelihood of conscious awareness.

1.4. Hypotheses

The purpose of the present study was to con-tribute to the understanding of emotional

1Although many researchers accept a definition of sublimi-nal which relies on stimuli which meet the objective threshold

Žusing a detection procedure Holender, 1986; Macmillan,. Ž .1986 , Reingold and Merikle 1988, 1990 have suggested that

even detection may not ‘exhaustively’ measure all task-rele-vant conscious perception. The essence of Reingold and

Ž .Merikle’s concern see also Jacoby, 1991 is that wheneverdifferent tasks are used to index conscious and unconscious

Žperception e.g. detection and subliminal priming, or detection.and a physiological response , task differences and process

differences are confounded. Thus, even if the objective detec-tion threshold was satisfactorily attained, effects on the indi-rect measure might merely reflect aspects of conscious percep-tion not indexed by the detection task, rather than truly

Ž .unconscious effects. Snodgrass 2000 addresses theseŽ .concerns, and on the basis of signal detection theory SDT

Žargues that detection is sufficiently exhaustive see also Bernat.et al., 2001, for a more brief review of these issues . Snodgrass’s

reasoning is as follows: almost every indirect effect requires atŽleast partial stimulus identification in order to occur at all e.g.

whenever such effects require semantic analysis, as for de-.termining affective valence . In other words, for semantic

analysis to take place, at least partial word identification mustoccur. In this sense, these measures are identification-depen-

Ž .dent. This is consistent with SDT Green and Swets, 1966which models identification discriminations as simply multidi-

Žmensional detection see, e.g. Green and Birdsall, 1978;.Macmillan and Creelman, 1991 . Identification is conceived of

as the multidimensional distance between the detection sensi-tivities possessed by the individual response alternatives. Inthis way, identification arises from, depends on, and cannotoccur without detection of the individual response alterna-tives. In short, given the SDT framework, detection is exhaus-tively sensitive to all identification-relevant information. Inparticular, when the objective detection threshold is attained,identification should be impossible given standard, single-pro-cess SDT theory. Thus, when effects requiring some degree ofstimulus identification are nonetheless obtained, this is strong

Ž . Ževidence for unconscious processes. As Macmillan 1986 p.. � �39 concluded, ‘Above-chance recognition i.e. identification

Ž .performance or other evidence of activation when detectiond��0 would be, for almost everyone, persuasive evidence forunconscious perception.’

processes by demonstrating that ERPs are sensi-tive to the affective valence of a stimulus, whetherprocessed in awareness or outside of consciousawareness. Amplitude measures of ERP compo-nents elicited by the visual presentation of moodadjectives were predicted to discriminate betweenpleasant and unpleasant words, in the absence ofany behavioral responses from the participants.

Ž .Based on previous research discussed earlier ,primary effects were expected to occur for the P3and LP components. Additionally, the ERPs werepredicted to discriminate between the affectivevalences of the stimuli, whether they were

Žprocessed in conscious awareness supraliminal.ERPs or below the objective detection threshold

Ž .subliminal ERPs . Finally, examinations of theeffects of affective stimuli on hemisphericasymmetries for both supraliminal and subliminalstimulus durations were planned.

2. Method

2.1. Participants

Seventeen undergraduate students were re-cruited through the Introduction to Psychologysubject pool at the University of Michigan as part

Ž .of a larger data collection Bunce et al., 1996 ,and received course credit for their participation.Nine of the participants were women, 15 wereright-handed,2 and all had vision correctable to20�20. The age range was 17�23 years with a

Ž .mean of 18.41 S.D.�1.28 , and all denied anyhistory of neurological or mental abnormalities.

2 Because hemispheric lateralization was an important mea-sure in this study, there is a question of whether the left-handed participants would evidence the same pattern of re-sults as the right-handed participants. To assess this possibil-ity, affective lateralization results from the full 17 participantsŽ .presented in ‘Results’ were compared with such results fromonly the 15 right-handed participants. The results werestronger with the inclusion of these two participants, bothsupraliminally and subliminally, indicating that the left-hander’s pattern of affective lateralization was consistent withthe right-hander’s.

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�34 15

2.2. Mood adjecti�e stimuli

Visual stimuli were 32 mood adjectives chosenŽ .as part of the larger study Bunce et al., 1996 in

which participants completed daily emotionalŽstate ratings of the 32 words and 10 other mood

.adjectives for 28 consecutive days prior to partic-ipation in this study, twice daily. One form wascompleted at the midpoint of the day, coveringexperiences during the first half of the day; asecond report was completed close to a partici-pant’s normal bedtime, covering the second halfof the day. Emotional experience was rated on a

Ž .7-point Likert scale ranging from 0 not at all toŽ .6 extremely . A written definition of each mood

adjective was provided for daily referral. Al-though the daily ratings are not the focus of thisreport, it is important to keep in mind that partic-ipants had substantial exposure to the stimuliprior to collection of the ERP data presentedhere.

2.3. Affecti�e �alence ratings

At the end of the experiment, the participantsrated each of the 32 stimulus words using a5-item subset of Osgood’s evaluative dimensionŽfollowing Chapman et al., 1980, the items were:nice�awful, sweet�sour, good�bad, heavenly�un-

.heavenly, mild�harsh . Pleasant and unpleasantblocks were created using the 10 mood adjectivesreceiving the most positive and most negative

Žaverage Osgood rating across participants i.e. 20.of the 32 stimulus words, see Table 1 for a listing .

To ascertain the reliability of these pleasant�un-pleasant blocks, a larger sample of 111 partici-pants drawn from the University of Michiganundergraduate subject pool also completed Os-good evaluative ratings for the 32 words. Ratingsfor all 32 words between the two samples were

Ž .highly correlated R�0.99 . The pleasant andunpleasant blocks replicated almost identically.One word in the 111 participant sample pleasantblock fell from 10th to 11th in ranking, and thusoutside the pleasant block of 10 used in thisstudy. All 32 words and their associated grandaverage rankings from the current 17 participantsdataset and the larger 111 participants datasetare presented in Table 1. Frequency of usage was

also assessed as a possible explanatory factorŽ .Kucera and Francis, 1967 , which yielded nosignificant difference between the pleasant and

� Ž . �unpleasant word blocks F 1,18 �0.125, ns .

2.4. Stimulus presentation

The stimuli were printed in 18 pt HelveticaLight centered on 4�6 inch white cards andpresented in a 3-field Gerbrands Model T3-8tachistoscope. One field of the tachistoscope wasused for the presentation of all stimulus words. Asecond field was used to present a fixation point,visible at all times except during stimulus presen-tations. Field brightness was tested for luminancelevel and pulse width and equated for both fields.Luminance levels for the stimulus and fixationfields, as well as the ambient light levels in theexperimental booth, were set at 5 footlamberts.

ŽThe 32 stimuli were presented six times each a.total of 192 presentations , in randomized order,

by an experimenter blind to their content. Toavoid confounding the subliminal presentation ofthe words with their supraliminal exposures, thestimulus set was first presented at 1 ms durationŽ .unmasked subliminal condition , and then at 40

Ž .ms supraliminal condition , for all participants.

2.5. Small amplitude ERPs to subliminal stimuli

Because the tachistoscopically presented sub-Žliminal 1 ms stimuli black print on a white back-

.ground were preceded and followed only by aŽ .fixation field black dot on a white background of

equal luminance, there was very little disturbancein the visual field.3 This resulted in an ERP

3The method employed here differs from ‘energy’ maskingin which the stimulus is rendered subliminal by following it

Ž .with a much brighter stimulus. Andreassi et al. 1976 havereported on the use of energy masking and its results. In thestudies described, the index for subliminality was based on theP’s failure to report the masked stimulus, thus amounting to asubjective threshold. This method also differs from patternmasking insofar as the stimuli are exposed for a very brief

Ž .duration 1 ms and are not followed by a conscious patternmask. The shortest stimulus duration generally used in pattern

Žmaking is one screen refresh of a computer monitor generally.not faster than 10 ms , and stimuli are rendered subliminal by

being followed with a supraliminally presented pattern.

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�3416

Table 1Stimuli and Osgood evaluative scale means for 17 Ps in this study and 111 Ps in questionnaire only replication, sorted by evaluative

aratings

17 Ps in this study 111 Ps in questionnaire only replication

Affect words Osgood evaluative mean Affect words Osgood evaluative mean

Unpleasant UnpleasantHostile �2.48 Hostile �2.29Angry �2.24 Angry �2.08Jealous �2.12 Depressed �1.94Unhappy �2.09 Jealous �1.85Distressed �2.09 Unhappy �1.62Depressed �1.99 Envious �1.55Envious �1.67 Agitated �1.50Sad �1.65 Distressed �1.48Agitated �1.48 Fearful �1.38Fearful �1.39 Sad �1.31

Middle MiddleTense �1.38 Lonely �1.25Lonely �1.07 Tense �1.16Nervous �1.07 Worried �1.09Jittery �0.87 Sluggish �0.94Sluggish �0.79 Nervous �0.79Worried �0.75 Jittery �0.60Anxious �0.51 Anxious �0.55Drowsy �0.31 Drowsy �0.18Wild 0.29 Wild 0.29Still 0.91 Still 0.74Quiet 0.98 Quiet 0.99Aroused 1.29 Excited 1.21

Pleasant PleasantEnthusiastic 1.60 Aroused 1.37Calm 1.64 At rest 1.39At rest 1.66 Enthusiastic 1.52Tranquil 1.74 Elated 1.57Excited 1.75 Calm 1.79Relaxed 2.05 Tranquil 1.85Elated 2.12 Relaxed 2.03Affectionate 2.40 Affectionate 2.20Warm-hearted 2.60 Loving 2.48Loving 2.66 Warm-hearted 2.51

a Ž . Ž .Note: means for 17 Ps and 111 Ps were strongly correlated 0.99, P�0.001 . The unpleasant top 10 and pleasant blocksŽ .bottom 10 form the 17 Ps were the data analyzed in this study.

waveform which is substantially smaller in ampli-tude and ‘noisier’ than conventional ERP wave-forms to supraliminal stimuli. However, multiply-ing the ERPs to subliminal stimuli by a factor of 4Ža simple transformation which does not alter themeasurement relationships for the underlying

.ERP component values reveals a visually appar-

ent component structure nearly identical to thatof supraliminal ERPs. Fig. 1 illustrates this withgrand averages across electrodes and participantsfor supraliminal, subliminal, and subliminal multi-plied by four. A bivariate correlation between thesupraliminal and subliminal grand averages de-

Žscribes this similarity in structure numerically R

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�34 17

Ž . Ž .Fig. 1. Grand averages digitally filtered at 25 Hz across all 32 stimuli, participants, and electrodes for supraliminal solid ,Ž . Ž .subliminal dash , and subliminal multiplied by four dash�dot . Arrows point to components for the supraliminal duration.

.�0.74, P�0.001 . Correlations for the individualŽelectrodes were similarly positive and sizable F3,

R�0.91; F4, R�0.91; CzPz, R�0.85; P3, R�0.66; P4, R�0.80; Oz, R�0.48; P�0.001 for

.all . These correlations indicate that positive andnegative peaks in the ERPs to supraliminal andsubliminal stimuli tend to occur at the same la-tency and have the same form. It should be notedthat the statistical analyses reported below arebased on the actual ERPs and not on the datapoints multiplied by four.

2.6. Conscious perception index

After all stimulus presentations, a 30-itemforced-choice detection task was administered at

Ž1 ms duration and 5 footlamberts luminance the.same conditions as during the experiment to

determine if stimulus presentations met the crite-ria for the objective detection threshold. Afterbeing shown a word in a supraliminal presenta-tion, participants were told that either a word or ablank card, which may be difficult to see, wouldbe presented to them an equal number of times,

in random order, for a total of 30 presentations.They were asked to state after each presentationwhat they believed had been presented, the wordor blank card, and to keep their responses roughlyequally divided between the two choices. Detec-tion did not differ from the chance mean of 15� Ž . �Mean�14.35; t 16 �0.94; ns . A discordancy

Ž .test for single outliers Barnett, 1994 indicatedthat all participants were performing within anexpectable chance distribution.

2.7. Physiological measurement apparatus

The recording sites were F3, F4, P3, P4, CzPzŽbetween Cz and Pz, modeled after Chapman et

.al., 1980, and Shevrin et al., 1996 , and Oz, ac-cording to the International 10�20 ElectrodePlacement System. Electrode sites were cleanedwith a mild abrasive solution, and Grass Instru-ment silver�silver chloride cup electrodes wereaffixed with Grass electrode paste. All electrodeswere referenced to linked ears with a mastoidground. Electrode impedance was less than 5kOhms. Eye activity was monitored by electrodes

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�3418

placed on the outer canthus and suborbital ridgeof the right eye. Individual trials contaminated by

Žartifacts eye blinks, muscle tension, or any ap-.parent activity that would render a trial unusable

were rejected by visual inspection and replacedŽon-line this process is explained in greater detail

.below in Section 2.8 .All signals were collected utilizing a Grass

Model 8-24D polygraph linked to a Macintoshcomputer. Signals were digitized at 2500 Hzthrough a National Instruments NB-MIO-16X

ŽA�D board controlled by LabVIEW 2.2 Na-.tional Instruments software, then stored in com-

puter files for off-line analysis. Signals were ana-log filtered online through Grass Model 8A5 ACamplifiers with a low-pass frequency of 200 Hz,and a high-pass frequency of 0.1 Hz. ERPs weresampled for 1200 ms, including a 200-ms pre-stimulus interval. ERPs were digitally downsam-

Žpled from 2500 to 500 Hz using the Matlabresample command to handle filtering before

.downsampling . A digital lowpass filter at 10 Hzwas applied to the data before selecting compo-

Žnents P3 and LP fifth order Chebychev filter witha ripple parameter of 20 dB down applied using

.the Matlab filtfilt command .

2.8. Experimental procedure

After a brief introduction to the laboratory,participants completed an informed consentstatement. All electrodes were attached prior toseating participants in a sound-proof, electricallyshielded, temperature-controlled booth. As notedearlier, all participants received subliminal pre-sentations of all words first, followed by thesupraliminal presentation of the stimulus set. Forall trials, participants were instructed to remainas still as possible, focus on the fixation point, payattention, and keep eye blinks to a minimum.Participants were told that at some point soonafter saying ‘ready’, there would be a quick flashof something on the screen that they may or maynot be able to see. They were not required torespond in any way, simply to pay attention andfocus on the fixation point. Participants were re-minded periodically throughout the trials to focus

on the fixation point and to minimize eye blinksduring trials.

The stimulus delivery sequence was as follows.First, participants heard a single tone indicatingthat the experimenters were ready to presentanother trial. Participants would then respond bysaying ‘ready’, after which the stimulus deliverywas triggered by an experimenter monitoring the

ŽEEG record who was blind to the stimulus con-.tent . After the stimulus was presented and 1 s of

ERP activity was recorded, a double tone soundedindicating the end of that trial. Participants weretold they could move, blink, and relax betweenthat double tone and the next single ready tone.During the time between the double tone and thenext single ready tone, the data from that trialwas written to disk, and an experimenter in the

Ž .booth also blind to the content changed thestimulus card. Based on visual inspection, if arti-

Žfacts were present in the trial i.e. EOG or other.apparent disruptive potentials the trial was dis-

carded rather than saved to disk and the trial waspresented again during the next sequence ratherthan advancing to the next stimulus. Participantswere not informed when trials were retaken vs.advancing normally to the next trial.

After the supraliminal phase of the experiment,the 30-item forced-choice detection task wasadministered. Participants then completed theOsgood semantic differential evaluative scale rat-ings of the experimental stimuli. All electrodeswere then removed, the participants were de-briefed, paid, and dismissed.

2.9. Data reduction

Individual ERP trials were averaged across thesix presentations for each of the 32 words sepa-rately, and components selected from these wordaverages. This process was completed within par-ticipant, electrode, and duration. Component val-ues from the 20 word averages in the pleasantand unpleasant blocks were then compared statis-tically.

Component windows were defined for supral-iminal and subliminal durations based on grandaverage ERP wave forms from each duration

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�34 19

Ž .Fig. 1 across all 32 stimuli, participants, andŽ .electrodes Hoormann et al., 1998 . First, compo-

nent amplitudes and latencies for the supralimi-nal duration were apparent and similar to stan-dard components for P1, N1, P2, P3 and a late

Ž . Ž .positive component LP Coles and Rugg, 1995 .For the subliminal duration, the P3 and LP com-ponents are most apparent and similar in latencyto the supraliminal components. Less clearlyidentifiable in the subliminal curves, but of inter-est, were the P1 and N1 components. In compar-ison to supraliminal duration components, the P1,N1 and P2 components in the subliminal durationappeared to have longer latencies. Based on theseobservations, P3 and LP component windows weredefined identically for the subliminal and supral-iminal durations, while P1, N1 and P2 componentwindows were defined with longer latencies in thesubliminal than the supraliminal duration.

Baselines were calculated as the mean ampli-tude from �200 to �20 ms prior to stimuluspresentation.4 The P3 and LP components werecalculated from baseline to peak amplitude withinthe same time windows for ERPs to both sublimi-

Žnal and supraliminal stimuli P3 200�500 ms; LP.500�900 ms . For P1, N1 and P2 amplitude was

calculated as the difference between the baselineŽand the mean amplitude sum of the amplitude

.values, divided by the number of bins in theirrespective time windows which differed for the

Žtwo durations: P1 20�80 ms supraliminal; 40�120. Žms subliminal , N1 supraliminal: 52�150 ms; sub-

. Žliminal: 80�170 , P2 80�190 ms supraliminal;.100�210 ms subliminal . Means for components

P1, N1 and P2 were measured before the applica-tion of the 10-Hz digital filter, whereas peaks forcomponents P3 and LP were calculated after theapplication of the 10-Hz digital filter.5

4A difference in baseline activity for the subliminal durationŽis apparent in the grand averages most notable at electrode

.F3 , suggesting that differences in the baseline may accountfor findings in the subliminal duration. Analyses were sepa-rately conducted covarying the 10-ms preceding stimulus onsetto verify that this apparent difference was not of statisticalimport.

5A similar pattern of results was found using peak measuresfor all components and using filters of 25 and 100 Hz.

There were several reasons for using the aver-age amplitude for the early component windowsŽ .P1, N1, P2 and the peak value for the later

Ž .component windows P3 and LP . Recalling thatcomponent values for ERPs to subliminal stimuliare smaller than for ERPs to supraliminal stimuli,this amplitude difference becomes more impor-tant for fast, low amplitude components such asP1 and N1. Also, the component peaks for P1 andN1 in the subliminal condition were less clearlydefined than for those in the supraliminal condi-tion. Because there was interest in direct compar-isons between the supraliminal and subliminaldurations, the average activity within these earlytime windows was used. Mean measures are gen-erally less sensitive to possible peak amplitude

Žsize differences and latency jitter Hoormann et.al., 1998 ; as such, they are generally less sensitive

to noise and will tend to underestimate ratherthan overestimate differences between conditionsŽ .van Boxtel, 1998 .

Although affective processing as early as P1 isnot a well understood or well documented pheno-menon, recent work suggests that affectiveŽ . ŽLeDoux, 1995 and semantic Schendan et al.,

.1998 processing can happen in a very early timewindow. Clearly, other studies have assessed scalprecorded early ERP activity with greater specific-

Ž .ity e.g. P30 and P50 components than the meanP1 measure employed here. However, becauseunderstandings of early ERP affective processingare not well-developed, the intention was to use agross measure of activity in this time window, andfindings for the P1 region are considered moreexploratory in nature than other components.

2.10. Statistical analyses

To allow for the direct comparison of supral-iminal and subliminal durations, all data were

Ž . Žroot mean square RMS transformed normal-.ized within duration prior to analysis. Additio-

nally, to assess amplitude shifts spanning the en-tire ERP epoch, the RMS transform was appliedseparately to each component so that all compo-nents would have similar weights in analyses ofthe average across components. The process ofRMS transforming the data was as follows: for

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�3420

Ž .each component e.g. P3 the individual values inthe dataset were divided by the square root of theaverage of squared component values across allsubjects. This process was applied separately toeach component within each duration, normaliz-ing each component separately within the dura-tion.

To assess lateralization, coronal and lateralŽelectrode factors were created modeled after Ca-

. Žcioppo et al., 1996 : coronal factor F3 and F4 vs.. ŽP3 and P4 and a lateral factor F3 and P3 vs. F4

. Ž .and P4 . The other two electrodes CzPz and Ozwere each analyzed separately.

Supraliminal and subliminal durations were firstassessed in separate analyses, and then togetherin one analysis to directly compare durations. Thesupraliminal duration was analyzed separately toallow for comparisons with previous studies, whichhave dealt with supraliminal stimuli. The sublimi-nal duration was analyzed separately to directlytest the hypothesis that pleasant and unpleasantwords would show differences in the subliminalcondition.

Analyses were conducted in the following man-ner. First, the RMS transformed component am-

Ž .plitudes P1, N1, P2, P3, LP were analyzed usingŽ .the following factors: affect pleasant, unpleasant

Ž .and electrode coronal, lateral . Next, the RMStransformed component amplitudes were ana-lyzed using the same affect and duration factorsfor CzPz and Oz separately. These analyses werefirst conducted for the supraliminal and sublimi-nal durations separately, and then together in one

Žanalysis including duration as a factor supralimi-.nal, subliminal . To identify any amplitude shift

across components, the mean for the five RMStransformed component measures was also as-

Žsessed presented with the component measures.as the average .

3. Results

Grand average ERP waveforms by affectiveŽ .valence unpleasant, pleasant for the supralimi-

nal and subliminal durations are presented inFigs. 2 and 3, respectively. On inspection of thesefigures, the most obvious difference is that the

unpleasant stimuli evoke a larger positive ampli-tude than the pleasant stimuli. This difference isapparent both supraliminally and subliminally. Forthe subliminal duration, the difference betweenunpleasant and pleasant appears to be greater inthe left hemisphere than the right. For the supral-iminal duration, P3 and LP appear to show dif-ferences in all electrodes, while components priorto P3 show more differential activity on the leftthan the right. Statistical analyses to be presentedbelow will confirm these impressions.

3.1. Coronal�lateral analysis

3.1.1. Supraliminal durationThe supraliminal RMS transformed component

Ž .amplitudes P1, N1, P2, P3, LP were subjected toŽ . Žan affect pleasant, unpleasant � lateral left,

. Ž .right �coronal frontal, parietal analysis of vari-ance. The results of this analysis are presented inTable 2a and are described here. Two findingsrelated to the affect differences emerged fromthis analysis. First, analysis revealed an affect

Žmain effect for P3 and LP unpleasant �.pleasant . Second, an affect � lateral interaction

was significant for P1, N1, and the mean acrossŽcomponents the difference in favor of unpleasant

over pleasant was greater on the left than the.right . Notably, while the P2 component shows

Žthe same direction in the grand means unplea-sant greater than pleasant overall, with the dif-

.ference greater on the left side than the right , P2showed no statistically significant effects involvingaffect for the supraliminal duration alone.

3.1.2. Subliminal durationThe subliminal RMS transformed component

amplitudes were subjected to the same analysis.The results of this analysis are presented in Table2b. This analysis revealed an affect � lateral in-

Žteraction for each individual component P1, N1,.P2, P3, LP and the mean across components

Žunpleasant greater than pleasant overall, withthe difference greater on the left side than the

.right . In addition to the affect � lateral interac-tion, a main effect for affect was significant for

ŽP2, and trend level for N1 and P3 unpleasant �.pleasant . Thus, in the subliminal duration affec-

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�34 21

Ž . Ž . Ž .Fig. 2. Supraliminal responses digitally filtered at 25 Hz : unpleasant solid and pleasant dash�dot .

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�3422

Ž . Ž . Ž .Fig. 3. Subliminal responses digitally filtered at 25 Hz : unpleasant solid and pleasant dash�dot .

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Table 2Ž . Ž .Significant F-ratios for a supraliminal duration; b subliminal duration

Ž .a Component AverageaEffect P1 N1 P2 P3 LP F P eff.

2 2 2 2 2F P eta F P eta F P eta F P eta F P eta

Ž .Affect A 11.38 0.004 0.42 4.70 0.046 0.23Ž .Coronal C 10.20 0.006 0.39 23.00 0.001 0.59 20.82 0.001 0.57 7.40 0.015 0.32 8.06 0.012 0.34Ž .Lateral L 13.78 0.002 0.46 12.51 0.003 0.44 7.36 0.015 0.32 3.67 0.073 0.19

A�CA�L 11.40 0.004 0.42 8.29 0.011 0.34 9.23 0.008 0.37C�L 18.53 0.001 0.54A�C�L

Ž .bŽ .Affect A 4.40 0.052 0.22 5.27 0.036 0.25 3.38 0.085 0.17 3.85 0.067 0.19Ž .Coronal CŽ .Lateral L

A�CA�L 6.63 0.020 0.29 5.47 0.033 0.25 4.93 0.041 0.24 8.78 0.009 0.35 14.89 0.001 0.48 6.53 0.021 0.29C�LA�C�L

aAll d.f.�1,16.

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Table 3ŽSimple effects statistics and corresponding RMS transformed means for affect main effect and affect � lateral interaction LU� left�unpleasant; LP� left�pleasant;

.RU�right�unpleasant; RP�right�pleasant

aEffect Unpleasant vs. pleasant RMS means Left vs. right

Left Right LU LP RU RP Unpleasant Pleasant2 2 2 2F P eta F P eta F P eta F P eta

SupraliminalP1 3.12 0.096 0.16 0.04 ns 0.93 0.20 0.62 0.71 4.05 0.061 0.20 5.40 0.034 0.25N1 1.79 ns 0.02 ns 0.64 0.40 0.85 0.82 6.70 0.019 0.30 16.33 0.001 0.50P2 2.84 ns 0.73 ns 0.72 0.52 0.83 0.73 5.10 0.038 0.24 11.60 0.004 0.42P3 9.06 0.008 0.36 9.23 0.008 0.37 1.03 0.96 0.96 0.88 8.60 0.010 0.35 5.12 0.038 0.24LP 4.30 0.055 0.21 4.26 0.056 0.21 1.04 0.89 0.95 0.83 1.48 ns 1.23 ns

Average 3.75 0.071 0.19 0.11 ns 0.87 0.59 0.84 0.79 0.47 ns 7.68 0.014 0.32

SubliminalP1 3.65 0.074 0.19 0.16 ns 0.27 �0.69 �0.00 �0.16 2.12 ns 6.06 0.026 0.27N1 6.52 0.021 0.29 1.50 ns 0.52 �0.82 0.24 �0.28 1.52 ns 6.02 0.026 0.27P2 6.66 0.020 0.29 2.48 ns 0.73 �0.61 0.48 �0.11 1.22 ns 5.27 0.035 0.25P3 5.66 0.030 0.26 1.27 ns 1.13 0.93 0.98 0.90 5.82 0.028 0.27 0.42 nsLP 2.10 ns 0.00 ns 1.11 0.98 0.91 0.91 4.80 0.043 0.23 0.86 ns

Average 5.97 0.026 0.27 1.18 ns 0.75 �0.04 0.52 0.25 2.89 ns 4.06 0.061 0.20

aAll d.f.�1,16.

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�34 25

tive valence was differentiated primarily laterallyfor all components assessed, with some tendencytowards bilaterality centered on the P2 compo-nent.

3.2. Simple effects for affect and lateral factors

To further assess the valence main effect andthe valence by lateral interaction, simple effectsanalyses were conducted for both the supralimi-

Žnal and subliminal data separately results are.presented in Table 3 . These analyses compared

unpleasant vs. pleasant responses separately inthe left and right hemispheres, and then left vs.right hemispheres separately for unpleasant andpleasant responses.

3.2.1. Supraliminal duration: unpleasant �s. pleasantwithin hemispheres

Inspection of the simple effects reveals that P3and LP are significant within both hemispheres,further underscoring the bilaterality found in thefull analysis of the supraliminal duration in whichboth P3 and LP were significant as a main effectfor affective valence. Earlier components P1, N1,and P2, which evidenced an affect by lateral in-teraction in the full analysis, have larger butnon-significant F values in the left hemispherethan in the right; only component P1 shows evena trend level differentiation in the left hemi-sphere alone. No significance is reached in theright hemisphere for the P1, N1, or P2 compo-nents in the supraliminal duration. Analysis ofthe average of the components reveals a trendlevel difference in the left hemisphere and nosignificant difference in the right hemisphere.

3.2.2. Supraliminal duration: left �s. right hemispherewithin affecti�e �alence

Both pleasant and unpleasant responses dif-fered significantly in lateralization for compo-nents P1, N1, P2 and P3, while component LPshows no such significant difference. Interest-ingly, the average of the components is significantfor lateralization only for the pleasant stimuli,suggesting the presence of stronger or more uni-form lateralization effects for the pleasant words.Inspection of the direction of differences presents

a more complicated picture. For the unpleasantwords the left hemisphere response is significantlygreater than the right for P1 and P3, but signifi-cantly greater on the right for N1 and P2. For thepleasant words the right hemisphere response isgreater than the left for P1, N1 and P2, but thehemispheric preference is reversed for P3.

3.2.3. Subliminal duration: unpleasant �s. pleasantwithin hemispheres

The differentiation between pleasant and un-Ž .pleasant stimuli unpleasant �pleasant is sig-

nificant in the left hemisphere for componentsP1, N1, P2, P3, and the average across all compo-nents. The means for component LP are consis-tent with this pattern as well, though not statisti-cally significant. No significant differences werefound in the right hemisphere.

3.2.4. Subliminal duration: left �s. right hemispherewithin affecti�e �alence

Pleasant stimuli elicited significant lateraliza-Ž .tion for components P1, N1 and P2 right� left ,

whereas unpleasant stimuli resulted in significantŽlateralization for components P3 and LP left �

.right . While these effects were only significantfor the above listed components, the direction of

Žthe means was consistent for all components un-.pleasant: left �right; pleasant: right� left . Like

the supraliminal duration, pleasant responsesshowed a trend level right greater than left dif-ference while the unpleasant responses did not.The implications of these findings will be dis-cussed below.

3.3. Supraliminal �s. subliminal duration comparison

The supraliminal and subliminal RMS trans-Žformed component amplitudes P1, N1, P2, P3,

. ŽLP were subjected to an affect pleasant, un-. Ž .pleasant �duration subliminal, supraliminal �Ž . Ž .lateral left, right �coronal frontal, parietal

analysis of variance. The results of this analysisŽare presented in Table 4 means in microvolts are

presented in Table 5 for both durations sepa-.rately by electrode and component . As might be

expected from the previous separate supraliminaland subliminal analyses, an affect �duration�

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Table 4Significant F-ratios of subliminal and supraliminal durations together

aEffect Component Average

P1 N1 P2 P3 LP F P eff.2 2 2 2 2F P eta F P eta F P eta F P eta F P eta

Ž .Affect A 4.18 0.058 0.21 5.54 0.032 0.26 6.80 0.019 0.30 4.07 0.061 0.20Ž .Duration D 4.63 0.047 0.22 4.68 0.046 0.23 3.48 0.081 0.18Ž .Coronal C 3.82 0.068 0.19 3.46 0.081 0.18Ž .Lateral L 9.52 0.007 0.37 3.02 0.101 0.16 4.28 0.055 0.21

A�D 3.23 0.091 0.01 4.31 0.054 0.21A�CA�L 20.74 0.001 0.56 9.11 0.008 0.36 6.15 0.025 0.28 5.25 0.036 0.25 10.38 0.005 0.39 13.81 0.002 0.46D�C 3.65 0.074 0.19 3.33 0.087 0.17D�LC�L 12.34 0.003 0.44 4.23 0.56 0.21A�D�CA�D�L 3.54 0.078 0.18 7.34 0.015 0.31 3.86 0.067 0.19D�C�LA�C�L�D

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Table 5Ž . Ž . Ž . Ž .Unpleasant UP and pleasant PL means in microvolts separately for supraliminal Sup and subliminal Sub durations

Comp. Electrode Average

F3 F4 P3 P4 CzPz Oz UP PL

UP PL UP PL UP PL UP PL UP PL UP PL

SupP1 0.73 0.39 0.56 0.69 0.47 �0.14 0.24 0.21 0.50 0.20 0.06 �0.33 0.43 0.17N1 0.30 0.04 0.58 0.57 1.68 1.16 2.04 1.96 0.98 0.62 2.71 2.37 1.38 1.12P2 0.59 0.23 0.85 0.56 2.58 2.07 2.83 2.68 1.93 1.45 3.50 3.30 2.05 1.72P3 8.41 7.89 8.33 7.67 10.92 10.02 9.70 8.75 12.16 10.86 7.72 6.90 9.54 8.68LP 5.18 4.29 5.07 4.10 5.55 4.97 4.80 4.48 7.56 6.48 4.19 3.61 5.39 4.66Average 3.04 2.57 3.08 2.72 4.24 3.62 3.92 3.62 4.63 3.92 3.64 3.17 3.76 3.27

SubP1 0.27 �0.38 0.14 �0.11 0.02 �0.38 �0.15 �0.07 �0.05 �0.09 �0.16 �0.23 0.01 �0.21N1 0.26 �0.58 0.16 �0.26 0.30 �0.32 0.10 �0.04 0.16 �0.07 0.17 �0.14 0.19 �0.23P2 0.48 �0.40 0.35 �0.10 0.35 �0.30 0.20 �0.03 0.27 0.03 0.17 �0.28 0.30 �0.18P3 4.29 3.52 3.99 3.45 4.38 3.65 3.54 3.43 4.43 4.29 2.92 2.68 3.92 3.50LP 4.20 3.80 3.78 3.77 4.55 3.88 3.40 3.36 4.49 4.26 3.20 2.78 3.94 3.64Average 1.90 1.19 1.68 1.35 1.92 1.31 1.42 1.33 1.86 1.68 1.26 0.96 1.67 1.30

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�3428

lateral three-way interaction was significant forP3, and was trend level significant for P2 and LP.Second, an affect �duration interaction was trendlevel significant for N1 and P2. The separateanalyses of the supraliminal and subliminal dura-tions allow us to understand the direction ofthese interactions. In particular, P2, P3, and LPevidence greater lateralization for responses tosubliminal stimuli than for supraliminal respon-ses. Interestingly, the early components P1 andN1 do not show such an interaction; instead theyboth show a similar left-lateralized response.These findings point to interesting differencesbetween supraliminal and subliminal processingof the same affective stimuli.

Based on this analysis, similarities in processingbetween supraliminal and subliminal durationscan also be assessed. First, a trend level maineffect for affect was found for the mean across

Ž .components unpleasant �pleasant . While thefive component means are in the same directionŽ .unpleasant � pleasant , separate componentanalyses reveal that P2 and P3 reach independentstatistical significance, and N1 reaches trend levelsignificance. Second, a highly significant affect �lateral interaction is present for each of the fivecomponents, and for the mean across compo-nents. Inspection of the means confirms that, likeaffect by lateral interactions in the supraliminaland subliminal durations separately, the differ-ence between unpleasant and pleasant is greaterfor the left hemisphere than for the right hemi-sphere.

3.3.1. Midline electrodesThe supraliminal and subliminal RMS trans-

Žformed component amplitudes P1, N1, P2, P3,.LP for the CzPz and Oz electrodes were sepa-

Žrately subjected to an affect pleasant, unpleas-.ant analysis of variance. Analysis of the CzPz

electrode revealed that affective responses showsignificant differences for the supraliminal dura-

Ž .tion unpleasant �pleasant at components P3� Ž . 2 �F 1,16 �18.27, P�0.001, eta �0.53 and LP� Ž . 2 �F 1,16 �8.06, P�0.012, eta �0.33 , but do notshow any significant differences for the subliminal

Ž .duration. Notably, grand means Table 3 for theCzPz electrode showed the same pattern of direc-

Ž .tion unpleasant �pleasant for all componentsand both durations, even if only P3 and LP in thesupraliminal duration reach statistical signifi-cance.

Analyses of the Oz electrode reveals only atrend level main effect of affect for component

� Ž .P3 in the supraliminal duration F 1,16 �3.81,2 �P�0.069, eta �0.19 . Like almost all compar-

isons in these data, grand means for unpleasantstimuli are greater in amplitude than pleasantstimuli for all components and for both durationsat the Oz electrode, however, these differencesdo not achieve statistical significance.

3.4. Summary

Two main findings emerge from the statisticalŽ .analyses: 1 for both durations, greater ampli-

tude positivity is elicited by unpleasant as com-pared to pleasant stimuli; this is particularly the

Ž .case for P3 and LP; 2 subliminal stimuli elicitthis affect valence effect on the left side for allcomponents, while supraliminal stimuli elicit bi-lateral effects for the later P3 and LP compo-nents and left-side effects for the earlier P1 andN1 components.

4. Discussion

Evidence from this study supports the growingconsensus that affective responses can happenwithout conscious awareness. On the basis ofevidence already available, it appears that affec-

Žtive meaning can be modified i.e. conditioned;.e.g. Ohman and Soares, 1994; Wong et al., 1997 ,

Žbe elicited e.g. Dimberg et al., 2000; Wong et al.,. Ž1994 , and influence conscious appraisals e.g.

.Murphy and Zajonc, 1993 , all without the directinvolvement of consciousness. The evidence inthis study additionally suggests that such processescan emerge within 100 ms, and can change frompleasant to unpleasant valence within seconds torandomly presented valenced words. The use ofsemantic stimuli also demonstrates that pictorialstimuli, considered more biologically preparedthan words, are not required. Given this evidence,it appears that a substantial range of affective

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�34 29

processes can occur without benefit of conscious-ness.

By obtaining subliminal and supraliminal ERPsto the same stimuli, the relationship betweenconscious and unconscious affective processes canbe examined. The following points will be dis-

Ž .cussed: 1 substantial similarities in the sublimi-nal and supraliminal ERP component structurewere apparent, suggesting that similar neural ac-

Ž .tivity is involved; 2 a difference between con-scious and unconscious processing in the degreeof hemispheric lateralization for later endogenous

Ž .components P3 and LP was also apparent; 3there is evidence to suggest that activity associ-ated with the P2 component is important in thechronology of the observed patterns of lateraliza-tion for both subliminal and supraliminal dura-

Ž .tions; 4 evidence of affective processing within100 ms raises questions about the nature of such

Ž .early processes; 5 theoretical implications ofusing semantic rather than pictorial stimuli; andŽ .6 finally, the possibility that the observed ERPcomponent differences are best characterized as aslow wave will be discussed.

Responses to stimuli in the subliminal andsupraliminal conditions were similar in waveformŽ .similar observable standard components , direc-

Žtion of difference unpleasant �pleasant ampli-.tude , as well as sharing early left-lateralized dif-

ferentiation of the affective stimuli. These simi-larities suggest that similar neural pathways dif-ferentiating pleasant from unpleasant stimuli wereengaged during the two conditions, implying thatsome mental processes may operate on the samebasis whether or not conscious perception is in-volved. This idea is bolstered by recent workshowing that a greater P300 endogenous compo-nent is present in response to subliminal oddball

Ž .stimuli Bernat et al., 2001 , suggesting that con-text updating, the most widely held explanation ofthe oddball effect, can operate outside consciousawareness on a fundamentally similar neural ba-sis as when consciousness is directly involved.Similarly, work investigating unconscious aversiveconditioning shows that, as in conscious aversiveconditioning, the CS� stimuli produce greater

Ž . ŽSCR Ohman et al., 1995 and P300 Wong et al.,.1997 responses. It may be that the conscious

aspects of affective processing in this study arenot central to the observed responses, but insteadonly enhance or add to some base response.

However, even if conscious and unconsciousaffective processing share some functional andneural basis, the overlap may be narrow. LeDouxŽ .1995 has suggested that at least some uncon-scious affective processing represents only‘valance tagging’, simple pleasant�unpleasant in-formation. Data in this study does not offer evi-dence beyond this interpretation. In order to as-sess more complex unconscious affective process-ing, or any form of ‘emotion’ such as is experi-enced consciously, further research would beneeded. For example, it would be of interest toassess other physiological concomitants of con-sciously rated emotional responses during uncon-scious affective processing, like hemispheric later-

Ž .alization of alpha e.g. Davidson et al., 1990 orEMG differentiation of valence combined with

Ž .SCR differentiation of arousal Lang et al., 1993 .The clearest lateralization differences were

found for P3 and LP. Unconscious affectiveprocesses were left lateralized for these two com-ponents, while conscious affective processes werebilateral. The role of late endogenous compo-nents in distinguishing subliminal from supralimi-

Ž .nal processing is of some interest. Libet 1978has suggested that the later ERP componentsŽ .after 100 ms in his research might be associated

Ž .with consciousness. Posner and Boies 1971 sug-gested that P300 might be a marker for con-sciousness itself. These findings are at best equiv-ocal on this score. Every component from P1through LP including P3 is associated with sub-liminal duration pleasant�unpleasant differences,thus offering both the Libet and Posner hypothe-ses little support. At the same time, P3 doesappear to distinguish between subliminal andsupraliminal processing. Subliminally, the P3 ef-fect is found only on the left side, while supra-liminally, it is found bilaterally. Perhaps the dif-ference between conscious and unconscious pro-cessing depends on the extent of brain activation,or some specific contribution made by the rightside to conscious processing. It should also beborne in mind that the predominantly left-sidedfindings could be attributed to the fact that verbal

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�3430

stimuli were used, although this would not ac-count for the bilateral supraliminal effects. Fu-ture research will be required to sort out theseand other possibilities.

Activity in the P2 time window appears to playan important role in this larger pattern and war-rants closer examination. Subliminally, while allcomponents evidenced significant left-lateralizedaffective valence effects, only P2 additionally evi-denced a significant main effect across hemi-spheres for affect valence, preceded and followedby trend level main effects for N1 and P3. Thispattern of findings suggests the possibility thataffective processing peaks in the P2 time windowfor the subliminal duration. Supraliminally, whilethe P2 component showed no significant effectitself, it appears chronologically as the point ofchange between the left-lateralized early effectsand the bilateral late effects. Notably, some ofthe first studies reporting subliminal ERP findingsŽ .see Shevrin, 1973 for a review found that the P2component carried the main statistical effect.Current findings support the idea that brain activ-ity associated with the P2 component may play asignificant role in unconscious processing and maybe important in the process by which affectiveresponses become conscious.

Prior to participating in the physiological datacollection session, participants rated a set of 42

Žwords which included the 32 presented in this.study each day for 28 days on a scale from zero

Ž . Ž .not at all to six extremely , indicating the de-gree of that emotion they were feeling. Althoughthese daily ratings were undertaken for purposes

Žunrelated to the findings reported here daily.mood findings will be reported elsewhere , it

seems reasonable to believe that these wordscame to represent emotions for these participantsmore strongly than a set of words not rated for 28days. It is also the case that words used in thisstudy are relatively common words that most na-tive English speakers will have had a great deal ofexperience with. At this point, one is left tospeculate about the influence of the daily ratings.It is conceivable that these daily ratings con-tributed to the affect differentiation apparent inthe earliest component windows. Both supralimi-nal and subliminal durations show left-lateralized

differentiation of pleasant from unpleasant emo-tion words in component windows P1 and N1.These findings are consistent with research inthese early processing stages. Schendan et al.Ž .1998 found that stimuli drawn from well-learnedcategories can evoke differential ERP compo-nents at least as early as 125 ms, and that wordscan elicit a different ERP response from faces orother objects at least as early as 90 ms. Thus, awell-learned and rehearsed set of words can beprocessed earlier than a less-learned set of wordsand that words can be processed at least as earlyas 90 ms. Another line of research supporting thepossibility of early affective processing is providedby studies of the thalamo-amygdala pathway,which have demonstrated that affective informa-tion can be processed in this early time frameŽ .LeDoux, 1995 .

Much of the research on unconscious affectiveprocessing has relied on pictorial stimuli. Resultshave been interpreted in terms of biologically

Ž .prepared fear reactions Seligman, 1971 and thusmore likely to occur to pictorial stimuli. For ex-ample, Ohman and colleagues suggest that un-conscious aversive conditioning can occur but islimited to biologically prepared fear relevant sti-

Žmuli Esteves et al., 1994; Ohman and Soares,. Ž .1994 . Dimberg et al. 2000 presented evidence

that EMG reactions to subliminally presentedpictures of emotional faces will be consistent withthe facial expressions � greatest zygomatic mus-cle activity in happy faces and greatest corrugatormuscle activity in angry faces. They suggest thatthis effect is driven by biologically prepared sensi-tivity to face-to-face communication, and that suchresponses can occur without awareness. In thisstudy, the stimuli were neither pictorial nor facial,but rather common mood adjectives with whichmost English speakers would be familiar as se-mantic stimuli. While one can argue that thereactions to these stimuli were based more onlearned iconic representations than genuine un-

Žconscious semantic decoding particularly due tofamiliarity from the daily rating task preceding

.data collection , it remains that, as visual stimuli,they are not biologically prepared to the samedegree as pictorial stimuli. Thus, the evidencepresented here represents an extension of the

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�34 31

kind of processing involved in unconscious affec-tive responses to more weakly biologically pre-pared processes or potentially to semantic pro-cessing. Some semantic processing did occur dur-ing at least some of the supraliminal presenta-tions, as participants could easily report themeaning of the words. Interestingly, the contribu-tion of the semantic processing supraliminally didnot produce a different direction of activation

Žthan subliminal stimulations unpleasant ampli-tude was greater than pleasant for both dura-

.tions , but may have contributed to the late bilat-eral activation evident only in the supraliminalduration. Finally, the consistent early left-sidedactivation across durations may itself be partlydue to the tendency for semantic stimuli to pro-duce left-hemisphere activation.

The consistency with which all five componentsshowed a greater positive amplitude for the un-pleasant stimuli suggests the possibility that aslow wave positivity for unpleasant stimuli span-ning all components could explain the findings

Ž .more parsimoniously Rosler, 1988 . This is sup-ported statistically by significant differences inthe average across all five RMS transformed com-ponent measures. If a slow wave hypothesis iscorrect, then the differential affective processingmay not be due to a sequence of separable pro-cessing activities by individual components, butmay instead be due to a quickly rising wavestaying active across the one second time window;with the individual components ‘riding on top’ ofthis slow wave. Consistent with this hypothesis isthe finding that negative components such as N1,rather than showing increased negative amplitudefor the unpleasant words, showed a greater posi-tive amplitude. Greater positive ERP amplitudehas been found to indicate more general physio-

Ž .logical arousal Polich and Kok, 1995 . On thewhole, more unpleasant stimuli may evoke greaterarousal than pleasant stimuli, and that this wouldapply to both subliminal and supraliminal stimu-lations. The hypothesized slow wave may reflect abrief change in state, perhaps based on generalphysiological arousal, which serves as the contextin which the more specific processing measuredby ERP components takes place.

In conclusion, cumulative evidence available

suggests that affective significance can be modi-fied, be elicited, and influence conscious ap-praisals, all without the direct involvement ofconscious perceptions. The similarity in responsesto stimuli in this study regardless of durationfurther suggests that some affective processes mayoperate on a basis in which conscious perceptionis not fundamental. Such evidence supports thenotion that affective processing happens on abroad continuum, in which conscious perceptionhas a smaller role. It will thus be of great interestin further research to begin to map the extent ofmental processing which may be happening out-side the purview of consciousness and its relation-ship to conscious processing. More specifically,what is the relationship between conscious andunconscious affective processes? For example, tothe degree that there is consistency between theneural activation during conscious and uncon-scious emotional reactions does this representfundamentally similar mental activity? Similarly,what is the extent to which ongoing unconsciousemotional processes form a context for consciousemotions or conscious emotional perceptions? Fi-nally, to what extent can conscious mentalprocesses modify or change overall emotional re-sponsivity, including activity unavailable to con-sciousness?

In summary, the main finding is that unpleas-ant emotion words elicited greater ERP positivitythan pleasant emotion words for both subliminaland supraliminal durations across all componentsP1, N1, P2, P3 and LP. Unconscious affectiveprocessing evidenced left lateralization across allcomponents from P1 to LP, while conscious af-fective processing is left lateralized for P1 and N1and then becomes bilateral for P3 and LP. Thisaffective processing occurred rapidly, beginningwithin 100 ms, and changed from one valence toanother within seconds. The early differentiationbetween unpleasant and pleasant stimuli may bea function of overlearned verbal stimuli, or mayindicate an early pathway for affective activation.These findings also suggest that P2 activity mayplay a unique role in unconscious processing andin some way may mark the point of transitionbetween conscious and unconscious processing.Because this study employed semantic stimuli

( )E. Bernat et al. � International Journal of Psychophysiology 42 2001 11�3432

these findings show that unconscious affectivestimulation need not be limited to pictures orfaces assumed to be more potent biologically pre-pared stimuli. Lastly, a slow wave may provide amore parsimonious account of the observed ERPdifferences, and may constitute evidence for abrief change in state reflecting heightened physio-logical arousal. It will be of great interest infurther research to begin to map the extent andnature of affective processing occurring outsidethe purview of consciousness, and how it relatesto conscious processing. These findings and hy-potheses may provide some guidance in this fur-ther research.

Acknowledgements

The research reported in this study was sup-ported in part by gifts from Robert H. Berry, theDepartment of Psychiatry, Department of Psy-chology, and the Rackham Graduate School ofthe University of Michigan. Portions of these datawere presented at the 36th Annual Meeting ofthe Society for Psychophysiological Research, inVancouver, Canada, October 1996, the 38th An-nual Meeting of the Society for Psychophysiologi-cal Research, in Denver Colorado, September1998, and published in the dissertation of E. B.Ž .University of Michigan . We thank MichaelSnodgrass, Philip Wong, William Williams, andWilliam Gehring for assistance in many theoreti-cal and technical aspects of this manuscript. Wealso thank John Cacioppo and members of hislaboratory group for several important sugges-tions about the analysis and interpretation ofthese findings, such as the root mean square

Ž .transform RMS used in this study and covaryingthe baseline in the subliminal duration to validate

Ž .findings footnote 4 .

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