BRAIN AND COGNITION 2, 404-419 (1983)

Asymmetry of Perception in Free Viewing of Chimeric Faces

JERRE LEVY, WENDY HELLER, MARIE T. BANICH, AND LESLIE A. BURTON

University of Chicago

We have devised a new free-vision task to index functional cerebral asymmetry for processing facial characteristics. Confirming its sensitivity to properties of lateralized hemispheric functions, left- and right-handers were clearly differentiated on this task with respect to several aspects of performance that conform with known differences between handedness groups in hemispheric asymmetry. Ad- ditionally, there were highly reliable and stable individual differences in perceptual asymmetries within handedness. Analyses of items in the task revealed that most of the differences between items in the asymmetries they elicited were random.

In split-brain patients (Levy, Trevarthen, & Sperry, 1972), in people with agenesis of the corpus callosum (Jeeves, 1979), and in normal right- handers (Milner & Dunne, 1977; Schwartz & Smith, 1980), there is a bias to respond to or identify correctly a half-face shown in the left visual field (LVF) during tachistoscopic presentation of face chimeras constructed of two half-faces of different posers. A similar LVF bias in dextrals has been found for tachistoscopic presentation of face chimeras composed of an emotional and neutral half-face of the same poser (Campbell, 1978; Heller & Levy, 1981), a bias that is not found in left-handers (Heller & Levy, 1981).

In addition, Gilbert and Bakan (1973) found that in right-handers, but not left-handers, bisymmetric composite faces shown in free vision looked more similar to the original when the composite was made of the half- face to the viewer’s left. These results demonstrate that it is not always necessary to restrict initial input to a single hemisphere to elicit a perceptual bias. Thus, the LVF bias for chimeric faces that are tachistoscopically presented may reflect an attentional bias toward the left induced by selective activation of the right side of the brain (see Kinsbourne, 1974) rather than unilateral hemispheric projection. Greater right- than left hemisphere activation in right-handers would be expected since the right

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CHIMERIC FACES 405

hemisphere is specialized for face processing (Benton & Van Allen, 1968; De Renzi, Faglioni, & Spinnler, 1968; Mimer, 1968; Narrington & James, 1967; Yin, 1970). This includes not only face recognition, but also the interpretation of emotional facial expressions (Buchtel, Campari, De Risio, & Rota, 1978; Ladavas, Umilta, & Ricci-Bitti, 1980; Safer, 1981).

Based on this consideration, pairs of chimeric face photographs composed of happy and neutral half-faces (Campbell, 1978; Heller & Levy, 1981) should result in perceptual biases even when shown in free vision, and in spite of their strangeness (see Fig. I). In particular, right-handers would be expected to perceive the chimera with a smiling half-face to the left and a neutral half-face to the right as happier than its mirror image, and the leftward perceptual bias should be significantly reduced for left-handers, who are diverse in hemispheric specialization (Hecaen & Sauguet, 1971). We should note that if handedness itself simply biases attention toward the nondominant hand, left- and right-handers would show opposite biases. If it is brain asymmetry itself that induces the perceptual bias, then since a majority of left-handers are lateralized in the same direction as right-handers, sinistrals should either show no perceptual bias or a leftward bias that is significantly less than that for right-handers.

The first question we investigated, therefore, was whether right-handers would show a leftward perceptual bias for chimeric faces presented in free vision and whether left-handers would show a different pattern, and of what nature. In addition, we examined individual differences within handedness groups since previous studies have shown that, even for right-handers, perceptual asymmetries vary in both magnitude and direction for individual subjects. For example, Heller and Levy (1981) gave two versions of their tachistoscopic task to each subject, and asymmetry scores on the two versions had a correlation of .751 for right-handers and .873 for left-handers, indicating quite stable individual differences. This is particularly interesting in view of the fact that for one version, dextrals had no asymmetry as group and for the other, they had a strong LVF bias. For left-handers, one version produced a nonsignificant RVF bias, and the other produced a significant LVF bias. Yet, relative perceptual biases between subjects remained stable over the two versions.

The diversities among right-handers in perceptual asymmetries on stan- dard measures of lateralization are not well understood. However, Levy, Heller, Banich, and Burton (1983) review evidence and present data indicating that such variations reflect characteristic and task-independent individual differences in asymmetric hemispheric arousal.

The perceptual asymmetry shown by an individual subject results, then, from the joint effects of hemispheric specialization itself and the subject’s characteristic pattern of asymmetric arousal. Based on this model, Levy et al. (1983) suggested that the group-typical asymmetry

FIG. 1. Item 1 from the free-vision task. In this example, the top face is a normal print and the bottom face is a mirror-reversed print of the same negative. The same chimeric pair reoccurs in the test booklet with top and bottom positions reversed, and the same poser appears in two other pairs, but with the smile produced by the left half of his face and the neutral expression by the right half.

406

CHIMERIC FACES 407

for right handers was indicative of hemispheric specialization, whereas most of the diversities among individual right-handed subjects were due to individual differences in characteristic asymmetries of hemispheric arousal.

If so, similar variations should be observed for asymmetries of perception in viewing chimeric faces in free vision, and indeed, stabilities of these individual differences would be likely to be even greater than for tach- istoscopic studies. These latter generally require lengthy test sessions, a high level of attention on the part of the subject, are affected by a subject’s alertness or fatigue, are subject to the effects of learning, and often generate asymmetry scores that are confounded with performance levels. A simple judgment of which of two chimeric faces looks happier, with unlimited time to view the stimuli, avoids these sources of noise. For this reason, if the free-vision task is sensitive to individual differences in asymmetric functioning of the two hemispheres, a subject’s degree of perceptual asymmetry should remain highly stable.

METHOD

Subjects The subjects were 111 right-handers and III left-handers who were university under-

graduates, graduate students. faculty members, or school teachers who were each tested individually. An additional 69 right-handed undergraduates were tested as a group. with stimuli presented by slides, to compare their asymmetries to the core group who were tested individually, but for these 69, handedness was merely determined by self-assessment. For the core group, handedness was assessed by a nine-item questionnaire, and dextrals were restricted to individuals who wrote with the right hand, had a noninverted handwriting posture, and preferred the right hand for all activities on the questionnaire. There were 41 dextral females and 70 dextral males who received individual testing. Sinistrals all wrote with the left hand, but were variable in the degree of left-hand preference for other manual activities and in handwriting posture. Left-handers using the noninverted handwriting posture (LN subjects) consisted of 22 females and 12 males. Those using an inverted handwriting posture (LI subjects) included 24 females and 37 males. Sixteen (16) left-handers had an ambiguous handwriting posture (LA subjects) and included three females and thirteen males. There were. then, 49 sinistral females and 62 sinistral males.

We should note that of the 49 left-handed males whose handwriting posture could be unambiguously classified, 75.5% fell into the LI category. whereas of the 46 left-handed females with an unambiguous hand posture. only 52.2% fell into the LI group. These percentages do not differ from those found by McKeever (1979) who also classified hand posture by experimenter examination in an adult North American sample. When subjects classify their own hand posture by choosing one of two prototypical examples most similar to their own (Coren & Porac. 1979). dimensions different from ours are being measured. This is because the prototypical examples show a combination of variables, some of which are critical for our classification, and others of which are irrelevant (e.g., paper orientation, whether the wrist is bent or straight, whether the pen tip points toward or away from the body).

The proportion of males and females did not differ between left- and right-handers (p = ,274, two-tailed), but among the total core sample of 222 subjects, males were more prevalent (JI = ,004s. two-tailed).

408 LEVY ET AL.

Procedure We adapted stimuli from Heller and Levy (1981) to construct 36 pairs of chimeric face

photographs for presentation in booklet form or by slide. Each of nine male posers (five right-handed, four left-handed) was photographed with a smiling and with a neutral expression; the two photographs from each poser were cut in half down the midsagittal axis and recombined to make two different chimeras: in one, the smile was produced by the left half of the poser’s face and the neutral expression by the right half: in the other, this was reversed. Each of the two chimeras was paired with its mirror image, once with the normal print at the top of the page and the mirror print at the bottom and once with positions reversed. This yielded 4 pairs of chimeras from each poser. For any given pair, the smiling half-face, in both members of the pair, had been produced by one side of the poser’s face and the neutral expression by the other, but one member of the pair was a normal print and the other was its mirror image.

Each poser appeared once in each successive set of nine pairs in the booklet (or in succession in the slides). Subjects looked at each of the 36 pairs and were encouraged to judge which chimera in the pair looked happier. They were allowed to give a “can’t decide” response if they felt a discrimination was impossible. Right-biased (left-biased) responses were those in which the chimera with the smile to the viewer’s right (left) was judged happier. The laterality measure was the number of pairs (N,) in which a rightward bias was shown minus the number of pairs (NJ in which a leftward bias was shown, divided by the total number of pairs (N = 36). This asymmetry measure, A, for subject i, has a standard error that is estimated by

under the null hypothesis that no bias is present. Split-half reliabilities of laterality scores were calculated for each of the core handedness

groups by deriving, for each subject, an asymmetry measure based on the 18 odd-numbered items and another asymmetry measure based on the I8 even-numbered items. In addition, we were able to relocate 26 of the original 111 dextral subjects to whom the test was readministered or who had been given the test a second time by other investigators at the University of Chicago. The two administrations were at least I week apart and ranged from 1 week to 6 months, with an average test-retest separation of 2 weeks. From this subset, we were able to estimate the test-retest reliability.

The 36 items were also treated as random variables, each with an asymmetry score as determined by the responses of the 111 right-handers. Thus, each item’s asymmetry score was given by the difference in the number of subjects who had a rightward bias and the number who had a leftward bias, divided by 111. We sought to determine whether items varied in the asymmetry they elicited from right-handed subjects as a function of the side of the poser’s face producing the smile, whether the chimera with the smile to the left was at the top or bottom of the page, whether the item appeared in the first or second half of the task, and the particular poser producing the stimulus. This analysis was important to assess whether specific item characteristics were central to the results or whether, instead, item variations were predominantly random. If item variations are random, this would mean that any set of chimeric pairs, constructed from any set of posers, should yield results similar to those from our particular test.

RESULTS

Left- versus Right-Handers

Preliminary analyses of the core groups revealed that the mean for dextral females (- .328) did not differ from that of dextral males (- .288),

CHIMERIC FACES 409

and that the mean for sinistral females (- .095) did not differ from that of sinistral males ( - .166). Consequently, sexes within handedness groups were collapsed in this analysis to compare right- and left-handers. Table 1 summarizes the results. As expected, dextrals displayed a highly sig- nificant leftward bias (t(ll0) = -7.255, p < lo-“, one tailed). A left- ward bias also appeared in left-handers (t(l10) = - 3.144, p = ,002, two tailed) that was smaller than that in right-handers (t(220) = 2.832, p 3- .003, one tailed).

The 69 dextral undergraduates who saw the stimuli in slides from different locations in a classroom had a mean asymmetry score of - 0.243 2 0.054 (SD = .446), showing a clear leftward bias (t(68) = -4.526, p = 1.2 x IO-‘, one tailed) that did not differ significantly from the leftward bias of the core group of dextrals (t(178) = 0.885, ns). However, the difference with the sinistrals also failed to reach significance (t(178) = 1.473, p = .074, one tailed). Since the core groups of dextrals and sinistrals differed at a highly significant level, this means that the inference that the two groups of dextrals do not differ is either a Type II error or that the inference that sinistrals and dextrals seeing the stimuli by slides do not differ is a Type II error. Given the differing magnitudes of the t ratios, the most likely interpretation is that, though the difference between

TABLE 1 ASYMMETRY SCORES OF RIGHT-BIASED, NO-BIAS, LEFT-BIASED, AND TOTAL DEXTRALS

AND SINISTRALS

Dextrals Sinistrals

Mean SD N Mean SD N

Classification I Right Bias .314 ,243 26 ,388 ,246 43 No Bias 0 0 3 0 0 2 Left Bias - .510 .275 82 - ,479 ,282 66

Classification II Right Bias ,484 .206 14 ,496 ,201 31 No Bias - .004 ,127 28 - ,008 ,123 28 Left Bias - .584 .232 69 - ,578 .232 52

Classification III Right Bias .484 ,206 14 ,514 ,194 29 No Bias - .024 ,132 31 - ,020 .147 34 Left Bias - ,601 ,223 66 - ,607 ,217 48

Total - ,303 .440 111 -.I34 .499 111 -

Note. The observed distribution is shown in Classification I. Subjects are allocated to the No-Bias category if their asymmetry scores did not differ from 0 at p = .lO, one- tailed (Classification II) or at p = .05, one-tailed (Classification III).

410 LEVY ET AL.

the 69 dextrals and the sinistrals would have occurred by chance 7.4% of the time, the difference, in fact, was not a chance finding.

As Table 1 shows, we classified subjects according to whether they showed a leftward bias, a rightward bias, or no bias. Typically, studies divide subjects into categories simply on the basis of their observed asymmetry scores. However, when investigating magnitudes of asymmetry, it is important to identify the extent to which scores deviate significantly from zero. Consequently, we decided to use three classifying criteria. The first simply employed subjects’ observed asymmetry scores (Clas- sification I). The other placed subjects into bias groups according to statistical criteria. This was done by determining the significance of the deviation of Ai from 0, using the Gaussian approximation corrected for continuity. For Classification II, the criterion for rejection of the null hypothesis was set a p = .lO, l-tailed, a level that provides a reasonable control for Type I errors without entailing an unreasonably high level of Type II errors. For Classification III, the level for rejection of the null hypothesis was set at the more stringent p = .05, l-tailed, which reduces Type I, but increases Type II errors. As will be seen, the dis- tributions change very little from Classification II to Classification III, indicating that the criteria chosen give a fairly accurate representation of population distributions. For the statistical categorizations, subjects were allocated to the No-Bias group if their asymmetry scores did not differ significantly from 0.

For Classifications I, II, and III, respectively, the fourfold point cor- relations, rp , between right-handed/left-handed and left-biased/right-biased (No-Bias subjects omitted), were .165 (p = .0075, one tailed), .230 (p = .0022, one tailed), and .226 ($ = .0023, one tailed). Thus, with respect to both mean asymmetry scores and frequency distributions, left- and right-handers were found to differ significantly. A further distinction between the groups is that, for Classification I, the magnitude of asymmetry for left-biased dextrals was larger than for right-biased dextrals (t(106) = 3.252, p = .0028, two tailed), whereas for sinistrals, the magnitude of asymmetry was either weakly or not associated with its direction (t(107) = 1.730, p = .087, two tailed).

The fact that both dextrals and sinistrals had significant leftward biases, that the bias of dextrals was larger than that of sinistrals, and that the distributions of asymmetry scores differed for the two groups is concordant with known differences between left- and right-handers in hemispheric specialization. The results, therefore, justify the inference that the free- vision test is sensitive to hemispheric specialization for the processing of faces.

Possible Hand-Posture Effects in Left-Handers

For male left-handers, there was no sign of an effect of the hand- posture variable: LN, LI, and LA men had asymmetry scores of - 0.157,

CHIMERIC FACES 411

-0.161, and -0.186, respectively. The variances for the three groups were similar and did not differ significantly, as indicated by standard deviations of 509, .566, and 537, respectively, for groups LN, LI, and LA. Further, for none of the classifying criteria did the distributions of LN, LI, and LA males into Left-Bias, No-Bias, and Right-Bias categories differ.

For female left-handers, relations were more complex. Since there were only three LA females, these had to be excluded from analysis. Scores and distributions of LI and LN females are shown in Table 2. Although the difference in asymmetry scores did not differ for the two groups (t(44) = 1.538, p = .13, two tailed), they differed in a number of other ways.

Omitting the No-Bias category, the frequencies of LI and LN women in the Left-Bias and Right-Bias categories differed for Classification I (p = .023, two tailed), Classification II 0, = ,018, two tailed), and Clas- sification 111 (p = .034, two tailed): compared to LN women, LI women were overrepresented in the Right-Bias category and underrepresented in the Left-Bias category. Second, asymmetry scores were significantly more variable for LI than for LN women (F(23, 21) = 2.219, p < .05). Inspection of Table 2 suggests that, for a given direction of asymmetry, the magnitudes of asymmetry were greater for LI than for LN women.

TABLE 2 ASYMMETRY SCORES OF LI AND LN FEMALES, DISTRIBUTED INTO RIGHT-BIAS, NO-BIAS, AND

LEFT-BIAS CATEGORIES AND FOR TOTAL SAMPLES

Classification I Right Bias No Bias Left Bias

Classification II Right Bias No Bias Left Bias

Classification III Right Bias No Bias Left Bias

Total

LI females LN females

Mean SD N Mean SD N

.433 ,221 - -

- ,608 ,271

,487 ,182 .046 ,085

- ,680 ,173

,512 ,083

- ,680

.043

,166 ,101 ,173

,566

IS 0 9

13 3 8

12 .403 ,133 4 - ,052 ,168 8 - ,470 ,204

24 - ,178 ,380

,324 ,161 6 0 0 1

- ,391 ,221 15

,403 .I33 ,017 .148

- ,432 ,208

4 5

13

4 7

11

22

Note. Classification I is the observed distribution. Subjects are allocated to the No-Bias category if their asymmetry scores did not differ from 0 at p = .lO, one-tailed (Classification II) or at p = .05, one-tailed (Classification III).

412 LEVY ETAL.

Although the magnitude of asymmetry did not differ significantly for LI and LN women in the Right-Bias group for any of the classifications, the magnitude of asymmetry was larger for LI than for LN women in the Left-Bias group, for Classifications I (t(22) = 2.141, p < .05, two tailed), II (t(l9) = 2.818, p < .02, two tailed), and III (t(17) = 2.356, p < .05, two tailed). For women in the LN group, the magnitude of asymmetry did not vary as a function of direction for any classifications, but for LI women, the magnitude of asymmetry was greater for Left-Bias than for Right-Bias subjects in Classifications II (t(l9) = 2.403, p < .05, two tailed) and III (t(l8) = 2.181, p < .05, two tailed).

Thus, although fewer LI women fall into the Left-Bias than Right-Bias category, those in the former have the larger asymmetry, and larger asymmetries than comparable LN subjects. The LI women seem to be composed of relatively distinct subgroups characterized by a majority with small rightward biases and a minority with large leftward biases. The differences between LN and LI females require replication in future studies, but if they are real, they could be due to differences in hemispheric specialization, differences in arousal patterns that are characteristically present, and/or differences in field dependency that, in right-handed females, affect perceptual asymmetries (Rapaczynski & Ehrlichman, 1979).

Possible Sex Differences in Right-Handers

It was already noted that male and female right-handers had statistically equal asymmetry scores, but as the foregoing comparison of LI and LN women shows, such equality of means does not necessarily mean that performance patterns are the same. Table 3 shows asymmetry scores and distributions of dextral males and females.

First, for Classification I, the magnitude of asymmetry is smaller for females than males in the Right-Bias category (t(26) = 2.033, p = .053, two tailed). Second, a greater proportion of females fall into the No- Bias category, both for Classification II (x’(1) = 4.447, p = .035) and Classification III (x2( 1) = 3.978, p = .046) as compared to male subjects. Thus, relative to men, women are overrepresented in the No-Bias category and underrepresented in the combined Right-Bias and Left-Bias categories. Although mean asymmetry scores for dextral males and females are the same, the similarity is due to a greater bimodality in males and smaller asymmetry scores in females, particularly among right-biased subjects.

Stable Diversities among Subjects

The split-half reliabilities of asymmetry scores were .925 for right- handers and also .925 for left-handers, indicating that, within both hand- edness groups, little of the variation could be attributed to random error. Instead, subjects’ bias scores are evidently reflective of true individual differences. For the subset of 26 dextral subjects for whom we had retest

CHIMERIC FACES 413

TABLE 3 ASYMMETRY SCORES OF DEXTRAL MALES AND FEMALES, DISTRIBUTED INTO RIGHT-BIAS,

NO-BIAS, AND LEFT-BIAS CATEGORIES AND FOR TOTAL SAMPLES

Dextral females Dextral males

Mean SD N Mean SD N

Classification I Right Bias No Bias Left Bias

Classification II Right Bias No Bias Left Bias

Classification Ill Right Bias No Bias Left Bias

Total

.I17 .I31 0 0

- .479 .289

,347 .I37 - .006 ,129 - ,586 .235

,347 .I37 - .019 ,136 - ,601 .227

- ,328 ,373

8 .375 ,259 18 2 0 0 1 31 - ,528 ,267 51

2 ,507 ,211 15 - .002 ,130 24 - .583 .233

2 .507 .211 16 - ,028 ,138 23 - ,601 ,222

41 - ,288 ,476

12 13 45

12 15 43

70

Nofe. Classification I is the observed distribution. Subjects are allocated to the No-Bias category if their asymmetry scores did not differ from 0 at p = .lO, one-tailed (Classification II) or at p = .05, one-tailed (Classification Ill).

scores, the test-retest reliability was .87, not significantly different from the split-half reliability of the total sample. The split-half reliabilities show that subjects are consistent in their asymmetries for different stimulus items, and the test-retest reliability shows that they are stable over time in their asymmetry scores for the same items.

Do Test Items Differ in the Asymmetries They Elicit?

Item asymmetries, as determined from the responses of dextrals, did not differ as function of the top/bottom position of the smile-left chimera. When the smile-left chimera was at the top of the page, the average asymmetry of items was -0.295, and when the smile-right chimera was at the top of the page, the average asymmetry of items was - .316, not significantly different (t(34) = .637, ns). The average asymmetry for the first 18 items was -0.298, and for the second 18 items was -0.313. The difference was not significant (t(34) = .456, ns). For items in which the smile had been produced by the left side of the posers’ faces, the asymmetry was -0.318, and for those where the smile was produced by the right side of posers’ faces, it was - 0.294(t(34) = .733). The mean asymmetries for left-handed and right-handed posers were -0.325 and -0.290 and did not differ (t(34) = 1.07, ns).

414 LEVY ET AL.

All 36 items yielded negative asymmetry scores. ‘This means that for every item, more dextrals had a leftward bias than had a rightward bias. However, different items produced larger or smaller asymmetries, and we sought to determine whether these variations were random or were due to real differences among items. For each item, we calculated two asymmetry scores, one based on half the dextral subjects and the other based on the other half of dextral subjects. Males and females were balanced in each half sample. This enabled us to calculate a split-half reliability for items. The raw correlation between the two sets of asymmetry scores was r = .268, giving a split-half reliability of r’ = .423. This means that item asymmetries should have a correlation of approximately .423 when calculated on two independent samples of dextrals having I1 1 subjects in each sample, and with all subjects administered the test in the same way. The correlation of .268 is not significant (t(34) = 1.622, p = .058, one tailed), although it approaches it.

The reliability coefficient tells us that approximately 18% of the variance among items reflects real item differences, whereas 82% of the variance is random. Given the close-to-significant correlation, we could not de- finitively rule out the possibility that the nonrandom variation is meaningful. We therefore investigated whether the asymmetry elicited by a given poser whose smile was produced by a given side of the face was reliable. Since each poser produced two items that were identical except for top/ bottom location of the smile-left chimera, we could determine if the two identical items resulted in the same asymmetries. This was done by calculating the intraclass correlation between identical items. There were 18 sets of these, and the intraclass correlation was ri = .342 (F(17, 18) = 2.038, p > .05).

Possibly, top/bottom position interacts with poser, so this possible contamination was eliminated in the following manner. For a given poser, we selected an item in which the smile had been produced by the right side of the poser’s face and in which the left-smile chimera was at the top of the page, and a second item from the same poser in which the smile had been produced by the left side of the poser’s face and in which the left-smile chimera was at the bottom of the page. An average asymmetry score was calculated over these two items, and a second asymmetry score for the same poser was calculated over the remaining two items. This yielded nine pairs of asymmetry scores, one pair for each poser. The intraclass correlation for these was r; = .460, but again, this was not significant (F(8, 9) = 2.703, p > .05).

The foregoing analyses reveal that item asymmetries are unaffected by the top/bottom location of the smile-left chimera, by whether the smile is produced by the left or right side of a poser’s face, by whether an item appears in the first or second half of the task, or by a poser’s handedness. Possibly, there was some tendency for different posers to

CHIMERIC FACES 415

elicit different magnitudes of asymmetry, but none of our analyses ex- amining this question yielded significant effects. The large proportion of variance in item asymmetries, and possibly all, was random. Thus, it is fairly safe to assume that almost any random collection of posers could supply an adequate stimulus set that would yield results similar to those we found.

Since all 36 items had negative asymmetry scores (that were statistically significant for 35 of these at p = .05 or better by a one-tailed test) for right-handers, it is possible that a reduced test, with fewer items, would be adequate. However, the item eliciting the largest leftward bias produced left-biased responses in 80 dextrals, no bias in 10 dextrals, and right- biased responses in 21 dextrals, whereas the item with the smallest asymmetry produced left-biased responses in 49 dextrals, no bias in 27 dextrals, and right-biased responses in 35 dextrals. Clearly, if only a very few items are used, sampling factors could radically change the distribution of scores observed in subjects. A second consideration is that asymmetry scores of subjects were continuously variable, but highly reliable and stable. The fewer the items in the task, the less its sensitivity to continuous variations in asymmetries among subjects. Nonetheless, a test of 16 or 20 items may be satisfactory.

DISCUSSION

Although the free-vision task is extremely simple to administer, takes only a brief time to complete, and requires no equipment except 36 pages or slides showing pairs of chimeric faces, it yields highly reliable laterality scores for left- and right-handers and consistent leftward biases in right- handers, whether they see the stimuli in booklet form or by slides.

The difference in average asymmetry scores and in distributions of scores for left- and right-handers confirms the view that the free-vision task is sensitive to hemispheric specialization for face processing. There is no way to prove that the differing asymmetries of the two handedness groups do not reflect some peripheral bias to direct attention toward the nondominant hand, but there is no evidence in the scientific literature to suggest the operation of such a factor, and massive evidence that hemispheric asymmetry affects perception and differs in left- and right- handers. Further, concordant with evidence that a majority of left-handers (about 70%) are lateralized in the same direction as right-handers, left- handers in our sample had a significant leftward bias, and this is inconsistent with the peripheral explanation.

In addition, right-handed subjects displayed highly stable and reliable asymmetry scores that showed considerable variation across the group. One possibility is that right-handers are variable in hemispheric spe- cialization either for cognitive or emotional processing of faces. In this case, one would have to posit that in the minority in whom the left

416 LEVY ET AL.

hemisphere is specialized for these functions, lateral specialization is not as strong as in the majority with right hemisphere specialization. This follows from our finding that the magnitude of asymmetry was smaller in right-biased than in left-biased dextrals.

An alternative possibility, and one we believe finds a great deal of support in the literature, is that the vast majority of right-handers have right hemisphere specialization for processing faces and their emotions, and that dextral subjects who fail to show a leftward bias on our task have characteristically higher left hemisphere than right hemisphere arousal. Highly stable individual differences in hemispheric arousal asymmetries have been identified in studies of electrocortical activity (Ehrlichman & Weiner, 1979; Morgan, McDonald, & MacDonald, 1971) and cerebral blood flow (Dabbs & Choo, 1980). Further, such studies indicate that characteristic arousal asymmetries in favor of the left hemisphere are correlated with high verbal and reduced spatial performance, and those in favor of the right hemisphere are correlated with high spatial and reduced verbal performance (Dabbs, & Choo, 1980; Davidson, Taylor, & Saron, 1979; Furst, 1976; Gale, Davies, & Smallbone, 1978; Glass & Butler, 1977; Gur & Reivich, 1980; Rebert, 1977). These physiological indices of cerebral arousal are associated with lateral eye movements contralateral to the hemisphere with higher arousal (Gur & Reivich, 1980), and individual differences in eye-movement directionality show the same relation with verbal and spatial function (Tucker & Suib, 1978) as do direct physiological measures of brain activity.

Levy et al. (1983) also showed that perceptual asymmetries for tach- istoscopic identification of nonsense syllables in right-handers were si- multaneously sensitive to hemispheric specialization and to a subject’s characteristic pattern of asymmetric hemispheric arousal. They concluded that a great part of the variation among dextrals in perceptual asymmetries on standard behavioral measures of lateralization were due to the arousal factor. Their right-handers with large RVF advantages showed better overall verbal performance than those with weak or no asymmetries, concordant with the physiological evidence, and additionally, the larger the RVF advantage for syllable identification, the smaller the leftward bias on the free-vision face-processing task.

Burton and Levy (Note 1.) found that asymmetries on the free-vision task were substantially correlated with reaction-time asymmetries for tachistoscopic identification of faces, and that the larger the leftward bias or LVF advantage, the better was overall performance on the tach- istoscopic task. In brief, right-handers show stable individual differences in characteristic patterns of asymmetric hemispheric arousal, as phys- iologically measured, differences that are correlated, in the expected direction, with verbal and spatial performance. Behaviorally, these arousal asymmetries are manifested in both biased lateral eye movements and

CHIMERIC FACES 417

biased lateral attention toward the side of space contralateral to the hemisphere with higher arousal, and they have the same association with verbal and spatial performance as do the direct physiological measures.

Although the data we report in this study are compatible either with the view that right-handers are diverse in hemispheric specialization itself or with the view that the variations in their perceptual asymmetries reflect variations in hemisphereic arousal patterns, we favor the latter inter- pretation. It accounts for the fact that right-biased dextrals have smaller asymmetries than left-biased dextrals (since in the former, arousal factors and hemispheric specialization go in opposite directions), and it is fully concordant with previous literature. The much weaker association in left-handers between the magnitude and direction of perceptual asymmetries would presumeably reflect the fact that left-handers are diverse in hemi- spheric specialization itself.

Obviously, the proposal that behavioral laterality tasks are simultaneously sensitive to the effects of individual differences in hemispheric arousal patterns and to hemispheric specialization itself (Levy et al., 1983) in- troduces new difficulties in interpreting differences between various subgroups of the population. The differences we found between male and female dextrals and between LN and LI females may reflect the arousal factor, hemispheric specialization, or both. Our data do not allow conclusions regarding these questions. One point, however, that should be considered is that the functional competence of a hemisphere is de- termined both by its specialized characteristics and by its arousal level. This means that if some individual has one hemisphere specialized for a given function, but it is the other hemisphere that has the characteristically higher arousal level, the potential of the specialized hemisphere will not be realized in performance. To the extent that a researcher is interested in the output of a system, perhaps the fact that biases in perception reflect both hemispheric specialization and the arousal pattern that is characteristic of the subject is not as distressing as might appear at first sight.

The free-vision task can be of value in many applications. Because of its brevity and simplicity, it would be a feasible instrument for large- scale investigations of familial correlations in laterality patterns or for preselection of subjects from a large potential sample. The evidence that it is sensitive to attentional asymmetries, whether task-induced or inherent, suggests that it could be of value in identifying the lateralization of brain damage. In addition, although the task is similar to that of Gilbert and Bakan (1973) in that asymmetries of perception are elicited in free viewing of faces, their task has greater stimulus complexity and higher cognitive demands. Stimulus items in their task consist of a normal face photograph (or its mirror image) and two bisymmetric composites, each made from half the original face. Subjects are required to judge which of the two

418 LEVY ET AL.

composites looks more similar to the normal face. Thus, two separate comparisons must be made, and then the two judgments must themselves be compared. This contrasts with the single comparison required in our task. The Gilbert and Bakan (1973) task may also entail more conscious evaluation than a simple immediate perception of relative happiness in two chimeric faces. Thus, given its minimal cognitive requirements, the free-vision task may be capable of providing new insights into hemispheric functioning in retarded populations, children, the aged, and psychiatric patients.

ACKNOWLEDGMENTS The support of The Spencer Foundation is gratefully acknowledged. We thank Jennifer

Christian0 for help in data collection and Professor Robert Butler for allowing us to present our stimuli by slides to 69 right-handed undergraduates in his introductory psychology class.

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REFERENCE NOTE

1. Burton, L., & Levy, J. 1982. Lateral asymmetry for processing facial stimuli and speed of performance. Unpublished manuscript,