LATERAL ASYMMETRY OF POSITIVE AND NEGATIVE EMOTIONS

Peter D. Duda and Julie Brown

(Department of Psychology, University of Guelph, Guelph, Ontario)

INTRODUCTION

Visuo-tachistoscopic studies examining the lateralization of emotion­ality and face recognition have reported that the right hemisphere is dominant for the processingof emotional expression 5Dekosky et al., 1980; Suberi and McKeever, 1977). These studies did not systematically examine how the valence (positive versus negative) or ghe intensity of emotional expressions influence perceptual asymmetries. More recent studies have been concerned with hemispheric differentiation in process­ing negative versus positive emotion (Ley and Bryden, 1979; Reuter­Lorenz and Davidson, 1981; Strauss and Moscovitch, 1981) and two contradictory findings, which reflect opposing views of how emotions are lateralized in the cerebral hemispheres, have emerged from their data. Ley and Bryden (1979) reported that the right hemisphere plays a crucial role in processing both positive and negative emotion. This finding has clinical and experimental support (Dekosky et al., 1980; Ley and Bryden, 1979, Suberi and McKeever, 1977). However, Reuter-Lorenz and Davidson ( 1981) have shown that the left hemisphere processes positive affect whereas the right hemisphere is dominant for processing negative emo­tion. There is clinical evidence for this result as well (Galin, 1974; Hecaen, 1962; Wexler, 1980).

Tachistoscopic evidence for right hemisphere processing of emotion was reported initially by Suberi and McKeever (1977). They required female subjects to discriminate previously memorized target faces from non-target faces in a reaction time paradigm. The facial stimuli were either neutral or emotional in expression. They found a faster reaction time favoring the left visual field-right hemisphere for those subjects who had memorized emotional faces as compared to those who had memorized neutral faces. Ley and Bryden (1979) used cartoon line drawings of five adult male characters each with five emotional expressions ranging from

Cortex (1984) 20, 253-261

254 Peter D. Duda and Julie Brown

estremely negative to extremely positive. The subject's task was to deter­mine whether the emotional expression of a target face presented unila­terally to the right or left visual field matched a subsequently centrally presented face. Once again a significant left visual field-right hemisphere superiority for processing of emotional expression was reported. These studies were criticezed (Strauss and Moscovitch, 1981) for such short­comings as the use of line drawn faces as stimuli (Ley and Bryden, 1979) and females as subjects (Ley and Bryden, 1979; Suberi and McKeever, 1977). In their study, Strauss and Moscovitch (1981) used female and male faces as stimuli, with each expressing three different emotions: happy, sad and surprised. The expressions were posed by models who were trained to control particular facial muscles. Simultaneous pairs appeared either to the right or left of a central fixation. Using a go-no-go procedure, the subjects' (males and females) task was to indicate as quickly as possible whether the two expressions were the same or different. They reported a left visual field superiority for emotion with no differences between positive and negative valences. Furthermore they reported no sex differ­ences in the perceptual asymmetry of emotional expressions of faces.

In contrast to these findings of right hemisphere superiority in pro­cessing facial expressions of emotion, Reuter-Lorenz and Davidson (1981) did report visual field differences in discrimination of happy and sad faces. Emotional (sad or happy) and neutral facial expressions of the same person (male or female) were presented bilaterally to each visual field by a tachistoscope with one face of the bilateral pair always being neutral. Subjects were required to discriminate which visual field con­tained the emotional expressions as quickly as possible. They found faster reaction times when the happy faces were presented to the right visual field and a left visual field superiority for sad faces. One purpose of the present study was to partially replicate the Reuter-Lorenz and Davidson (1981) study.

A second purpose was to examine systematically the effects of sex differences on possible hemispheric asymmetries in positive and negative affect. Studies examining sex differences in the lateralization of face recognition have reported mixed findings (Hilliard, 1973; Patterson and Bradshaw, 1975; Rizzolatti and Buchtel, 1977; Young and Bion, 1980). Rizzolatti and Buchtel (1977) and Young and Bion (1980) reported that males showed a significantly stronger left visual field superiority whereas females showed no hemispheric asymmetry. Contrary to these findings are published reports that female subjects showed right hemispheric superiority for accuracy of recognition of faces (Hilliard, 1973; Patterson and Bradshaw, 1975). Similarly, studies dealing with laterality of emo­tional expression of faces have reported mixed results (Ladavas et al.,

255 Lateral asymmetry of emotions

1980; McKeever and Dixon, 1981; Safer 1981). Safer (1981), using a face discrimination task, reported males to be significantly superior in accu­racy of emotional expression for stimuli presented to the left visual field­right hemisphere whereas females showed no hemispheric asymmetry. On the other hand, two separate studies found females to be superior to males when emotional expressions were presented to the left visual field-right hemisphere (Ladavas et al., 1980; McKeever and Dixon, 1981). Strauss and Moscovitch (1981) reported no sex differences in the asymmetry of face and emotion perception. Thus the evidence for sex differences in recognition of faces and emotional expressions is unclear and requires further examination.

MATERIALS AND METHOD

Subjects

The subjects used in this experiment were 40 University of Guelph Intro­ductory Psychology students with a mean age of 19.9 years. There were 20 males and 20 females who were right-handed (as measured by the Oldfield inventory) with normal or corrected-to-normal vision.

Stimuli

The stimuli were 18 black anc white full face photographs of three male and three female faces. Each person expressed a sad and happy emotion as well as a neutral pose. These facial expressions were obtained from the Eckman et al. (1972) collection of standardized pictures which were posed by trained models and empirically tested for their affective content. The slides that received the highest ratings from the Eckman scale were chosen as stimuli. A neutral and an emotional expression (sad or happy) of the same person were paired on each slide. There was one practice block of 32 trials and two experimental trial blocks consisting of 48 trials each making a total of 96 trials (192 faces). The facial slides were in different random order for each block. Each emotional expression for each person was presented twice to each hemifield within each bloc, for a total of four times to each hemifield. Each slide set was counterbalanced for emotion and sex of expressor. Each stimulus slide also contained a central fixation number which ranged from 1 to 5 and was randomly assigned to each stimulus card.

Apparatus

Stimuli were presented by a Scientific Prototype GB three-filed tachistoscope with the viewing distance being approximately 137 em. The faces were presented 2.5 degrees to the left and right of the central fixation. Exposure duration was individually set for each subject to get approximately a mean accuracy of 75%. A

256 Peter D. Duda and Julie Brown

novel approach was developed to obtain manual response times. A 9.9 cm.lever which could be gripped comfortably by hand and rotated both to the left and right of an upright, neutral starting position was used for each subject's res­ponses. Preliminary tests showed no bias or latency differences in subjects' abilities to move the lever to the left or right of vertical. The advantage of this response mechanism was that shifting the lever to the left and right when the emotional target was in the left and right visual field respectively preserved the spatial or stimulus-response compatibility in this task. This approach avoids the potential confounding effects of handedness, anatomical position of hand(s) and location of response keys that occur in this type of study (Nicoletti et al., 1975).

Procedure

The subject was alerted to a trial with a verbal "fixate" signal. The signal required the subject to concentrate on the center of the viewing screen. About one second after the fixation signal, a black fixation dot appeared in the centre of the viewing field for 500 msec. and immediately after the stimulus slide with the two lateralized faces and the fixation number was exposed for a brief duration. The fixation number was in exactly the same central location as the previously shown fixation dot. The faces in each pair were of the same person and one face expressed emotion (sad or happy) where the other was always a neutral pose. The subject's task was to indicate as quickly and accurately as possible which side contained the emotional face by moving the lever, loosely gripped by the right hand, to the left if the emotional face was in the left visual field and to the right if the emotional face was in the right visual field. Reaction time was recorded from slide onset to the subject's lever movement. Practice trials were used to familiarize the subject with the procedure and to determine an exposure duration yielding about 75% accuracy in subsequent experimental trials. The mean exposure duration in the experimental trial blocks was 315 msec. The one male and one female face used in practice were not used in experimental trials. After making the reaction time response, subjects were instructed to report the fixation num­ber. Trials where they failed to do so were diregarded. In the experimental blocks the error rate on fixation number reports was zero.

In summary this was a mixed factorial design with sex of subject a between factor and visual fields, emotional valence and sex of facial stimuli as within factors.

RESULTS

The data for accuracy and reaction time were analyzed separately. The accuracy data will be discussed first. The means for the number of faces correctly reponded to in the left and right visual fields by female and male subjects are presented in Table I. Analysis of variance showed a signifi­cantmaineffectforvalenceoremotionality(F = 241.54; d.f. = 1,38; p = 0.0001) with happy faces being responded to more accurately. A Signif­icant main effect for facial stimuli (F = 20.31; d.f. = 1,38; p = .00009) was obtained indicating that there was greater accuracy of response when

257 Lateral asymmetry of emotions

male facial stimuli were presented. A significant emotionality X sex of subject interaction (F = 9.66; d.f. = 1,38; p = .00362) was obtained. Multiple mean comparisons using Duncans New Multiple Range Test (p < .05) show that female subjects showed greater accuracy when iden­tifying sad faces as compared to male subjects.

TABLE I Mean Number of Faces Correctly Identified as a Function of Visual Fields,

Emotionality, Sex of Stimuli and Sex of Subject (N = 40)

Left Visual Field Right Visual Field Sad Happy Sad Happy

faces faces faces faces

Female stimuli 7.3 9.7 8.6 10.2Female subjects Male stimuli 8.1 10.0 9.0 10.1 Female stimuli 7.2 9.9 6.9 10.3Male subjects Male stimuli 8.0 10.9 8.0 11.0

(Maximum possible score = 12)

TABLE II Mean Number of Reaction Times as a Function of Visual Fields,

Emotionality, Sex of Stimuli and Sex of Subject (N = 40)

Left Visual Field Right Visual Field Sad Happy Sad Happy

faces faces faces faces

Female stimuli 720 636 733 681Female subjects Male stimuli 680 607 704 657 Female stimuli 753 659 737 652Male subjects Male stimuli 723 628 698 629

(Reaction time is in msec.)

Analysis of variance of the mean reaction times shown in Table II showedasignificantmaineffectforemotionality(F = 62.37; d.f. = 1,38; p = .00001) with happy faces being responded to faster than sad faces. A significant main effect for facial stimuli (F = 21.112; d.f. = 1,38; p = .0007) was obtained with male stimuli being reacted to faster than female stimuli. A visual field X sex of subject interaction (F = 5.02712; d.f. = 1,38; p = .02930) was obtained. Use of the Duncans New Multiple Range Test (p < .05) indicated that females were faster when stimuli were presented to the left visual field-right hemisphere compared to males and

258 Peter D. Duda and Julie Brown

that females were faster with stimuli presented to the left visual field-right hemisphere in comparison to the right visual field-left hemisphere. A visual field X emotionality interaction (F = 4.79017; d.f. = 1,38; p = 0.03306) was obtained. Again the Duncans New Multiple Range Test (p < .05) indicated that happy faces were responded to significantly faster when presented to the left visual field-right hemisphere compared to the right visual field-left hemisphere.

DISCUSSION

The reaction time data indicated a clear superiority for processing of positive emotions (happy faces) presented to the left visual field-right hemisphere. No significant visual field differences were found for sad faces. These results do not support the Reuter-Lorenz and Davidson (1981) findings of an emotional valence by visual field interaction in the opposite direction. In this connection it is instructive to note that our reation times were in the range of 600 to 700 msec. from onset of stimulus to response whereas the Reuter-Lorenz and Davidson (1981) study reported long latences of 2200 to 2600 msec. from offset of stimulus to response. Given that the mean exposure durations and percent accuracy in both studies were comparable, it is difficult to account for the large discrepancy in latency and Reuter-Lorenz and Davidson (1981) did not speculate as to the basis for their very long latencies. There are some further problems with the above study. First, the analyses seem to be based on only 32 of a total of 96 trials per subject. To elminate discre­pancies in emotional intensities and between spontaneous and posed expressions, they reduced analyzable data from six to four faces and four to two emotions. However, they did not identify the sex of those four faces nor did they identify the sex of the 28 adult subjects they tested. In addition, they did not report what visual field and emotionality effects, if any, were obtained across the full range of 96 trials. As such, their data base appears to be somewhat limited for a reaction time study.

The results of the present study partially support earlier findings that the right hemisphere plays a crucial role in processing both positive and negative emotions (Ley and Bryden, 1979; Strauss and Moscovitch, 1981, Suberi and McKeever, 1977). Although we found a left visual field-right hemisphere reaction time superiority for processing happy faces only, Ley and Bryden (1979) reported a left visual field-right hemispheric super­iority for extreme positive emotions and extreme negative emotions, with the largest visual field differences occurring for the extremely negative expression. Strauss and Moscovitch (1981) also reported a left visual

259 Lateral asymmetry of emotions

field-right hemispheric dominance for processing both positive and nega­tive emotions.

The accuracy and reaction time data do support the findings of Strauss and Moscovitch (1981) and Reuter-Lorenz et al. (1983) that performance was superior when happy faces were presented in comparison to sad faces. Reuter-Lorenz et al. (1983) suggest the possibility that discrimination of happy and sad faces may rely on different stimulus features rather than affective content. Happy faces may be identified by the lower face (i.e. mouth) whereas sad faces may be identified by the whole face. Therefore a subject would only have to look at the stimulus person's mouth to see if it was happy, but they would have to look over the whole face to see if it was sad. This could possibly be an explanation of why reaction time was found to be so much slower for perception of sad faces compared to happy faces. In our study, subjects spontaneously reported that sad faces were very difficult to distinguish from the neutral expressions. One of the problems with using the standardized Ekman faces is that the emotional expressions of sadness compared with neutral poses may not be as discriminable as the happy versus neutral poses.

Two futther findings of differences in sex of face presented and in sex of subject were of interest. Both accuracy data and reaction time data showed that facial stimuli were reponded to with significantly greater accuracy and faster reaction times in comparison to female facial stimuli. It is possible that the more salient external facial feature of hairline of female subjects distracte subjects from attending to inner facial features displaying emotional cues as compared to male stimulus faces with sig­nificantly less salient hairlines.

Two types of sex differences were also found in this study. First, females were faster than males in responding to emotional stimuli pre­sented to the left visual. field-right hemisphere. Several studies have reported a similar finding (Ladavas et al., 1980; McKeever and Dixon, 1981; Strauss and Moscovitch, 1981). These data do not support the hypothesis that females are not as welllateralized as males in terms of linguistic versus spatial ability. (Witelson, 1978). Second, females were more accurate than males in response to sad faces. This sex difference in perception of negative affect is difficult to assess at this time and requires further empirical support to be considered as reliable effect.

In conclusion, the data from this study failed to replicate the Reuter­Lorenz and Davidson (1981) findings of differential hemispheric process­ing of positive and negative emotions. As such, their somewhat tentative proposal of a motor theory of facial perception does not provide an adequate framework to examine emotionality in perceptual asymmetries of face recognition. Our results partially support the theory that emotions are primarily processed in the right hemisphere. The pattern of sex dif­

260 Peter D. Duda and Julie Brown

ferences in this study support existing findings with the interesting new finding that females were more accurate in response to sad faces com­pared to males. The further issue of whether these sex differences reflect underlying differences in hemispheric organization or culturally devel­oped biases in processing strategies is unresolved and offers a promising direction for future research.

ABSTRACT

This reaction time study examined whether the left and right hemispheres are differentially specialized to process positive and negative affect respectively in adult females and males. Either happy or sad facial expressions of emotion were presented simultaneously with neutral facial expressions to the two visual fields. Happy faces were responded to more rapidly in the left visual field-right hem­isphere with no visual field differences for sad faces. Females were superior to males when emotional facial stimuli were presented to the left visual field and were more accurate in recognizing sad faces compared to happy ones.

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Peter D. Duda, Department of Psychology, University of Guelph, Ontario, NIG 2Wl.