Eehav. Res. Thu. Vol. 27, No. 3, pp. 289-294, 1989 0005-7967/89 53.00 + 0.00 Printed in Great Britain. All rights reserved Copyright 0 1989 Pergamon Press plc

PSYCHOPHYSIOLOGICAL AND SUBJECTIVE REACTIONS OF SOCIAL PHOBICS AND NORMALS TO

FACIAL STIMULI

HARALD MERCKELBACH*, WILJO VAN HOUT, MARCEL A. VAN DEN HOUT

and PETER PAUL MERSCH

Department of Mental Health Sciences/Experimental Psychopathology, University of Limburg, P.O. Box 616, 6200 MD Maastricht, The Netherlands

(Received 7 October 1988)

Summary-Nine social phobics and 9 normal control subjects were exposed to slides of angry faces, happy faces, and neutral objects (i.e. flowers or mushrooms). Skin conductance responses (SCRs) to the stimuli and eyeblink rate (EBR) during stimulus exposure were recorded. In addition, subjects were asked to rate the stimuli in terms of pleasantness. While angry face stimuli elicited greater SCRs, stronger inhibition of EBR, and were evaluated more negatively than the other stimuli, there were no differences between social phobics and normals in these respects. Thus, the findings lend no support to the idea that social phobics are particularly sensitive to facial cues in general or to negative facial cues in particular.

INTRODUCTION

In a series of studies, Dimberg (1982, 1989, 1987) has explored the influence of different types of facial stimuli on psychophysiological responding and conditioning. Dimberg (1982) reported that stimuli depicting angry facial expressions and those depicting happy facial expressions elicit completely different patterns of electromyographic activity in normal Ss. Whereas the former class of stimuli induced more corrugator activity, which is associated with negative facial expressions, the latter class of stimuli induced more zygomatic activity, which is associated with positive facial expressions. Dimberg (1989) also showed that skin conductance responses (SCRs) conditioned to angry facial stimuli extinguish more slowly than SCRs conditioned to happy facial stimuli. A similar effect was obtained for cardiovascular reactions (Dimberg, 1987).

Dimberg (1987; t)hman and Dimberg, 1978) related his findings to the concept of “preparedness” formulated by Seligman (1971). According to this concept, humans are biologically prepared to respond to those stimuli that were once threatening to prehistoric man. Seligman (1971) argued that the idea of biological preparedness would explain why most phobias pertain to a relatively narrow class of stimuli (e.g. snakes, high places, thunder, etc.). In a similar vein, Dimberg (1987) reasoned that humans are predisposed to react to threatening social cues (e.g. angry faces).

In their theoretical papers, t)hman and Dimberg (ohman and Dimberg, 1984; ohman, Dimberg and t)st, 1985; ohman, 1986) have claimed that their experimental results sustain an interpretation of social phobias in terms of preparedness. This claim is based largely on the idea that the fear of facial cues plays a decisive role in social fears. For example, t)hman et al. (1985) suggested that “social fears are the result of a biologically determined readiness to easily associate fear to certain classes of stimuli, and perhaps particularly those originating in theface” (p. 31; italics ours). Those aspects of the face that are most likely to be prepotent are the eyes. First, fear and escape are induced by staring eyes and eye-like shapes in birds (Blest, 1957), chickens, and non-human primates (Redican, 1975). A wide range of species actually employ displays of eyes or eye-like spots to frighten others (Marks, 1987). Second, in normal humans, heart rate and skin conductance activity are higher when gazes are directed at, rather than away from, them (Ellsworth, 1975). Dimberg and ohman (1983) found that extinction of SCRs conditioned to angry face stimuli is only delayed when the angry face is directed at, rather than turned away from, the experimental S. Referring to these findings, Marks (1987) suggested that social phobics’ fear of being watched is an exaggeration of the normal human sensitivity to eyes.

*To whom all correspondence should be addressed.

289

290 HARALD MERCKELBACH er al.

While the idea that social phobics are particularly responsive to angry facial expressions corresponds with clinical impressions (Marks, 1969, 1987) it has, to the present authors’ knowledge, not been subjected to experimental tests. Dimberg, Frederikson and Lundqvist (1985; cited by ijhman et al., 1985) reported that Ss with high social fear gave significantly larger SCRs to social stimuli than Ss with low social fear. Yet, this study has two obvious limitations. First, it is not clear to what extent the social fear Ss resembled social phobics. Second, the social stimuli used in the Dimberg et al. study (1982) consisted of neutral facial expressions.

The present study sought to ascertain whether social phobics are more responsive to angry facial expressions than normal control Ss. SCRs to, and subjective evaluation of, facial and non-facial (neutral) stimuli were measured in social phobics and normal controls. In addition, endogenous eyeblink rates (EBRs) were recorded during stimulus exposure. As it has been shown that visual tasks requiring attention inhibit EBRs (Stern, Walrath and Goldstein, 1984) it was expected that angry facial cues would inhibit EBRs more than happy facial or neutral cues, and especially so in social phobics.

METHOD

Subjects

Eighteen Ss (4 men and 14 women) participated in the experiment. Their mean age was 22 yr (range 8-3 1).

Social phobics (N = 9: 2 men and 7 women) were recruited by an advertisement placed in both a local newspaper and a university magazine. The advertisement described social phobia symptoms (e.g. shyness, tension in social situations, etc.) and invited readers with these symptoms to contact the researchers by telephone. Persons who responded were first screened in an interview that took place over the phone. They were later asked to come to our laboratory and to complete several questionnaires, among them the fear questionnaire (FQ; Marks and Mathews, 1979). Ss with a primary diagnosis of depression, agoraphobia, or simple phobia were excluded from the experi- ment. Of the remaining Ss with a primary diagnosis of social phobia, one used minor tranquilizers. None of the Ss were undergoing any kind of behavioral treatment at the time.

Control Ss (N = 9: 2 men and 7 women) were recruited from undergraduate psychology courses. The control group was matched on age, sex, and educational level. Control Ss were also invited to complete the FQ.

Apparatus and stimulus materials

During the experiment, psychophysiological and subjective responses to angry facial expressions, happy facial expressions, and neutral stimuli were recorded.

A number of psychophysiological responses, namely skin conductance levels (SCLs), SCRs, EBRs, and respiration rate (RR), were measured. SCLs and SCRs were recorded using two Beckman Ag-AgCl electrodes (8 mm dia) attached to the medial phalanges of the second and third fingers of the right hand. The electrodes were connected to a Beckman Skin Conductance Coupler

(type 9844). EBRs were derived from the vertical electro-oculogram. Two Beckman Ag-AgCl electrodes, one

placed 2 cm above the right eye and one placed 2cm below the right eye, were d.c. coupled (Beckman Voltage Coupler), with the low-pass filter set at 50 Hz.

RR was recorded using a Beckman Respiration Belt connected to a Beckman Voltage/Pulse/ Pressure Coupler (type 9853A).

SCRs, EBRs, and RRs were recorded on a Beckman R 711 polygraph (paper speed 5 mm/set). Stimuli were scored in terms of “pleasantness”, using a 10 cm visual analog scale, with - 5

indicating “extremely pleasant”, 0 indicating “neutral” and + 5 indicating “extremely unpleasant”. All Ss saw one slide of an angry face, one slide of a happy face, and one non-facial, neutral

slide (i.e. either a flower or a mushroom). The facial stimuli were taken from Ekman and Friesen (1975). All gazes from the facial cues were directed at the S.

The slides were projected onto a white screen, 2 m in front of the S. The size of the projected image was approx. 75 x 110 cm. A Kodak Carousel was used for stimulus presentation.

Reactions of social phobics and normals to facial stimuli 291

A PDP Mint 11 Microcomputer controlled stimulus presentation and registration of psycho- physiological responses.

Design

The experiment was conducted according to a 2 (group) x 3 (stimulus type) design, with the first factor being a between-S factor (social phobics vs normal controls) and the second factor being a within-S factor (angry face; happy face; neutral stimulus).

Ss saw not only facial stimuli but also a non-facial, neutral stimulus in order to control for the possibility that social phobics react more strongly to facial cues (friendly and happy faces) per se.

Each slide was presented to the S 6 times, and consequently a trial factor, containing 6 levels, was included in the statistical analysis.

Procedure

Ss sat in a comfortable chair that was placed in a dimly lit, sound-attenuated chamber. The apparatus was located in an adjacent room. The stimuli were projected through a hole in the wall.

The Ss were told that the experiment dealt with subjective and physiological reactions to facial and non-facial cues. They were instructed to watch the stimuli and to avoid unnecessary movements. After the Ss had given their consent and recording sites had been cleaned with distilled water, electrodes were attached with adhesive collars. A 4-min resting period then followed, during which baseline values of SCL and EBR were obtained. The slides were then presented to the Ss in a random order. The only restriction was that no more than two successive slides of one type (e.g. angry face) would be shown. Slide duration was 8 sec. The intertrial interval varied from 15 to 21 set, with a mean of 18 sec.

In order to correct for the influence of irrelevant stimulus details, different Ss within a given group saw different people exhibiting the facial expressions and different neutral stimuli. However, for each S, the angry face stimulus, happy face stimulus, and neutral stimulus were kept constant across trials.

At the end of the experiment, the three stimuli were presented once more, and the S was asked to rate the stimulus in terms of pleasantness.

Data reduction and analysis

t-Tests were used to evaluate group differences in fear questionnaire scores and in baseline SCL and EBR.

SCRs to the stimuli were defined as the maximal deflection occurring l-8 set after stimulus onset. SCR and SCL values were measured in micromho and square root-transformed. RR was recorded as a control variable. That is, trials with respiratory irregularities were excluded from the data analysis. Such irregularities occurred on less than 3% of all trials. SCR values for these trials were estimated on the basis of the adjacent trials.

As for EBR, the total number of eyeblinks between stimulus onset and offset were counted. Both SCR and EBR values were subjected to 2 (group) x 3 (stimulus type) x 6 (trials) analysis of variance (ANOVA) with repeated measures on the last two factors. For effects involving a trial factor, Greenhouse-Geisser corrected probability levels were used.

RESULTS

FQ -scores and psychophysiological baseline values

As is evident from Table 1, the social phobia group and the control group did not differ in FQ-agoraphobia scores or FQ-blood/injury scores. The groups also had comparable baseline values for SCL and EBR. The only notable difference between the groups appeared on the FQ-social phobia scale, with social phobics scoring significantly higher than normal controls. Thus, it can safely be concluded that the social phobia group was characterized specifically by social fears, rather than by an elevated level of generalized fear. The mean FQ-social phobia score of the social phobics came close to 4, which corresponds to “would definitively avoid it” (Marks and Mathews, 1979).

292 HARALD MERCKELBACH er al.

Table I. Mean FQ subscale scares (agoraphobia. blood/injury phobia. and

social phobia) and square rooted basal conductance (XL) and cycblink rate

(EBR) during the baseline period far the social phobia and control group.

Standard deviatlans are gwcn in parentheses

FQ-agoraphobia FQ-social

FQ-blood/injury phobia phobia SCL EBR

Social phabics I .o I.6 3.5. 2.1 20.0

(1.4) (1.4) (1.7) (0.7) (6.4) Normal controls 0.6 I.8 1.9 2.0 16.5

(0.9) Cl.01 Cl.51 (0.5) (8.2)

l P < 0.05, d/ = 16, one-tailed.

SCRs to facial and non-facial stimuli

Figure 1 shows SCRs to non-facial, neutral stimuli (left panel), happy face stimuli (middle panel), and angry face stimuli (right panel). A 2 (groups) x 3 (stimulus type) x 6 (trials) ANOVA revealed a significant main effect of trials [F (580) = 15.0, P < 0.051, due to an overall response decrement over trials. Furthermore, a significant main effect of stimulus type was found [F (2,32) = 4.0, P < 0.051. This effect was due to the fact that angry face stimuli elicited greater SCRs than either happy face stimuli or neutral stimuli. Post hoc t-tests confirmed that responses to angry face stimuli were larger than those to neutral stimuli [t (17) = 1.6, P = 0.06, one-tailed] or to happy face stimuli [t (17) = 3.5, P c 0.05, one-tailed], with means (JSCR) of 0.28 (SD = 0.22), 0.23 (SD = 0.22), and 0.21 (SD = 0.18), respectively. However, no significant main effect of group [F (1, 16) = 1.4, P = 0.261 or interaction effect of group with stimulus type [F (2,32) < l] was found. Thus, social phobics and normal controls were not found to be different in their SCRs to facial stimuli.

EBRs during facial and non-facial stimuli

Mean EBRs during presentation of neutral stimuli (left panel), happy face stimuli (middle panel), and angry face stimuli (right panel) are shown in Fig. 2. A 2 (group) x 3 (stimulus type) x 6 (trials)

l -* Social phobic5

l - --0 Normal controls

. .

\ t \ l

\ \ \

\ .\ l \

l -0-e

Trials

Neutral t+wy Awry

Fig. 1. Mean JSCRs as a function of trials (6) to non-facial, neutral stimuli (left panel), happy face stimuli (middle panel), and angry face stimuli (right panel) of the social phobia and control group.

0-o Social phobics

a---• Normal controls

I L I I I I , , I / I I , , I 1 I I,

Trials

Neutral How Angry

Fig. 2. Mean EBR as a function of trials (6) during non-facial, neutral stimuli (left panel), happy face stimuli (middle panel), and angry face stimuli (right panel) of the social phobia and control groups.

Reactions of social phobics and normals to facial stimuli 293

ANOVA revealed a borderline significant effect of trials [F (5,80) = 2.5, P = 0.081, caused by the slight increase in EBR over trials Furthermore, a significant main effect of stimulus type emerged [F (2,32) = 6.2, P < 0.051. Post hoc r-tests indicated that the EBR during presentation of angry face stimuli was lower than the EBR during presentation of happy face stimuli [t (17) = 1.5, P = 0.08, one-tailed], while the EBR during presentation of happy face stimuli was si~ifi~antly lower than that during presentation of neutral stimuli [t (17) = 2.0, P < 0.05, one-tailed], the means being 2.8 (SD = 1.7), 3.1 (SD = 1.6), and 3.6 (SD = 1.9), respectively. Again, no significant differences were found between social phobics and normals.

At the end of the experiment, Ss were asked to rate the stimuli in terms of pleasantness (- 5 to +5). A 2 (groups) x 3 (stimulus type) ANOVA revealed no significant group differences. The only effect reaching significance was a main effect of stimulus type [J’ (2,32 = 12.8, P < O.OS]. Post hoc t-tests showed that angry faces were rated as significantly more negative than non-facial, neutral stimuli [t (17) = 4.6, P c 0.05, one-tailed], while these latter stimuli were rated as relatively more negative than happy faces [t (17) = 2.5, P < 0.05, one-tailed]. The means were +4.0 (SD = 2.9), 0.5 (SD = 1.4), and -2.0 (SD = 2.5), respectively (positive values indicate negative evaluation).

DISCUSSION

If angry facial expressions were to function as primary conditioned stimuli in social phobics (ohman, 1986), one would predict stronger responses to these cues in social phobics than in normal controls, and if it were human eyes in particular that social phobics were hypersensitive to, then social phobics and normals would respond differently to facial vs non-facial stimuli. The present study failed to substantiate either of these predictions. No significant difference was found between social phobics and normal controls for either the psychophysiological or the subjective data. Consequently, the current findings run counter to the idea that social phobias can be understood as the outcome of prepared conditioning to negative facial expressions (ohman and Dimberg, 1984; ohman, 1986).

A second point to be noted is that the present study found significant effects of stimulus content on psychophysiological responses. Angry faces elicited greater SCRs and fewer eyeblinks than either happy faces or neutral, non-facial stimuli. While this finding is in line with the idea that angry facial stimuli constitute “prepotent” or “salient” cues (Marks, 1987), that is, cues which elicit an intensified orienting response that is unlearned, it contrasts with earlier reports (Dimberg, 1982, 1986). Thus, Dimberg (1982) found that angry and happy face stimuli evoked divergent electromyographic patterns, yet elicited equal SCR magnitudes in normal Ss. Similarly, Dimberg (1986; Exp. I-Exp. 4) found no differences in SCRs to angry and happy face stimuli during the habituation phase of his conditioning experiments. It is unclear why the present study succeeded in finding SCR differences for angry and happy face stimuli whereas Dimberg failed to observe such differentiation. However, the fact that differential effects of stimulus content were not restricted to the SCR mode but also appeared for EBR underlines the validity of the present results.

As far as EBR is concerned, some remarks are in order. Solid evidence has indicated that endogenous eyeblinks are associated with a partial loss of information intake (Wittenmeyer, Stem and Chen, 1983). It has also been shown that the greater the attentional demands of a task, the greater the inhibition of EBR (see review by Stern et al., 1984). Given the reduction in visual info~ation processing associated with each eyeblink, the inhibition of EBR during a difficult visual task would seem to be adaptive (Stem et al., 1984). The reduced EBR during exposure to angry face stimuli reported above suggests that the attentional demands of such stimuli are greater than those of happy face or non-facial, neutral stimuli. The gradual increase in EBRs over trials that was found in the present study can best be explained as a time-on-task effect. Several experiments have demonstrated that repetitive tasks that lead to boredom or fatigue are a~ompanied by an increase in EBR (Stern et al., 1984).

In sum, the present study found that angry face stimuli elicit greater SCR magnitudes and fewer eyeblinks than do either happy face stimuli or non-facial, neutral stimuli. This finding is in line

BRT 2?!?-F

294 HARALD MERCKELBACH et al.

with the suggestion that angry face stimuli are salient, prepotent cues (Marks, 1987). Yet, no evidence was found to suggest that facial stimuli in general, or angry face stimuli in particular, are more salient for social phobics than for normal controls.

Acknowledgemenr-We gratefully acknowledge the technical advice regarding eyeblinks that we received from Dr John A. Stern (Washington University).

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