Lateral dominance and aesthetic preference

Neuropsyehologia, 1976, Vol. 14, pp. 431 to 445, Pergamon Press. Printed in England,

LATERAL DOMINANCE AND AESTHETIC PREFERENCE*

JERI~ LEVY

Department of Psychology, University of Pennsylvania, Philadelphia, Pa, U.S.A.

(Received 23 October 1975)

Abstract--Left and right handers were found to differ in their preferences for mirror versions of vacation slides. Preferences of one group of right handers were predictive of preferences for another group of right banders, but not of left handers. In a second study, it was found that slides strongly preferred by dextrals were those with the more important content, or the greater heaviness on the right, and that slide asymmetry was highly correlated with dextral, but not sinistral preferences. Results were interpreted as reflecting a left field perceptual bias induced by selective right hemisphere activation in right handers.

THERE ARE various references throughout the art history literature to the relationship between lateral orientation of a painting, tapestry, or etching and the aesthetic quality of a work of art [1-5]. WOLFFLIN had first suggested in 1928 in Paul Wolters Festschrift (re- printed in [2]) that there is a strong tendency to scan a picture f rom left to right, the eye terminating its scan on the right half of the picture where the most important content is represented. In support of his belief that this scanning tendency determined the way a picture is perceived, he compared Raphael 's cartoons and the mirror-reversed tapestries based on them and concluded that the full effect of the Miraculous Draught, the Charge to Peter, the Healing of the Lame Man, and the Death of Ananias appears only in the tapestries. In the first three of these, the center of focus is on the left in the cartoons and on the right in the tapestries, while in the Death of Ananias, this is reversed. However, W61fftin believes that the position of the dying Ananias on the left of the tapestry, which, according to him, is an abnormal position, was a deliberate attempt by Raphael to induce shock in the viewer.

Although OPP~ [4] agrees that left to right scanning is an important determinant of the aesthetic response induced by a picture or its mirror image, he also believes that the position of the viewer, on the left or right of the picture, has important aesthetic consequences. He says, " I f . . . reproductions in reverse are moved so that they are looked at f rom the right instead of as normally from the left, all the disconcerting features (arising from the reversal) • . . disappear."

SCHLOSSER [1], also, believes that the differential aesthetic responses produced by mirror- symmetric pictures stem from a pronounced tendency to scan from left to right. He cites the example of Pieter Brueghel's Blind Men in which the cripples seem to tumble into the ditch after their leader, falling from left to right• When reversed, the figures seem to be pressing upon the leader, their movement, instead of being automatic and involuntary, becoming a conscious effort.

*The author expresses her appreciation to Dr. Norman Adler for allowing her to duplicate 32 of his vacation slides, and to Drs. Raquel and Ruben Gur for helping her to select the 97 slides used in this study from a total of almost 500. To Misses Joni Lacks and Gloria Possumato, thanks are due for gathering the data reported in Study I.

431

432 JERRE LEVY

GAFFRON [5], though emphasizing the importance of the way a picture is scanned, believes that the scan involves not only movement through the two dimensions of the picture plane, but into its perceived third dimension. She postulates that there is a certain fixed path, the "glance-curve," which viewers follow within a picture space of three dimensions. The glance-curve begins in the left foreground, moving up and toward the background, then curving toward the right. She states, "All objects within the range of this path are recognized spontaneously, while we must look separately for those located outside, i.e. in the right foreground or upper left background," concluding, further, that viewers are psychologically located at the beginning of the glance curve in the left foreground of the picture. Gaffron goes on to say that " . . . the well-known dominance of the left brain cortex . . . in a normal right-handed person . . . means there exists a difference in our awareness of visual data in favour of those which are perceived within the right visual field. The asymmetric glance-curve compensates for this asymmetry in our perceptual field and permits the most complete, unfalsified impression of three-dimensional space by visual space perception." (Her italics.)

All the foregoing writers underline the importance of directional scan in determining which of two mirror symmetric pictures is more aesthetically pleasing, relating the effect of this to the preferred location of important content, and attributing the scan direction to Western reading habits or to brain lateralization. Irrespective of the causes of the scan pattern, it is not obvious that left to right scanning implies either that the more important content of a picture should be located on the right for maximal aesthetic effect, or, as Gaffron contends, along the "glance-curve". Whether preferences for mirror-symmetric pictures derive from Western reading habits, brain lateralization, or both, the mechanisms by which these factors may affect aesthetic responses have not been specified. To assert that a particular distal cause is operative is not to suggest the nature of the causal chain by which the consequence is effected. Though Gaffron hypothesizes that a certain fixed scan pattern compensates for an inherent right visual field bias induced by brain lateralization, knowledge gained over the last 35 yr regarding the specialized functions of the left and right hemispheres, proves beyond question that it is the left visual field for which a perceptual bias exists.

Studies of people with unilateral lesions [6, 7] with total forebrain commissurotomy [8-12], and with intact neurological function (see [13] for review) all confirm that in the typical right-handed individual, though the left hemisphere is predominant for verbal functions, the right is predominant for visuo-spatial tasks, including spatial relations, spatial location, visual matching, face recognition, and imagistic encoding. It is particularly salient to point out that when two different pictures, whether of faces, nonsense shapes, common objects, or patterns of X's and squares, are simultaneously tachistoscopically projected to the left and right hemispheres of commissurotomy patients, and patients are directed to select, by pointing, a matching picture from a set displayed in free vision, they overwhelmingly select a single matching picture, and the picture selected is that which was seen by the right cerebral hemisphere [8]. The investigators concluded that " . . . the primary tendency is for all visual stimuli to be perceived in the minor (i.e. right) hemisphere." Thus, under tachistoscopie presentation conditions, right-handed people would tend to be more aware of the left visual field than of the right with respect to nonverbal, pictorial content. This conclusion has been amply confirmed, in fact, by KIMtlRA [14, 15], among others.

However, in free vision, when no restraints are placed on eye movements, it is not immediately obvious why a left-right asymmetry of perception should arise, since either

LATERAL DOMINANCE AND AESTHETIC PREFERENCE 433

half of space, relative to the body midline, can be projected to either hemisphere. Neverthe- less, an asymmetrical left field bias has, in fact, been demonstrated by GILBERT and BAKAN [16] and by NELSON and MACDONALD [17]. Gilbert and Bakan found that the left half of a photograph of a face has a greater resemblance to the whole face than does the right half for right-handers. This was demonstrated by reversing the face and comparing the original and reversed faces to the same face composed of two left halves or two right halves. Left-handers did not display any matching asymmetry, while right-handed Israelis showed the same asymmetry as right-handed North Americans. Nelson and MacDonald found that when subjects were asked to select titles for a series of pictures, a significant majority selected titles corresponding to the pictorial content in the left half of the picture. These two studies strongly suggest that during the act of visual perception attention is biased to the left, giving rise to the perceptual asymmetries observed. The underlying basis of this attentional bias was first discussed by TREVARTI~N [18].

It had been known since the 19th century that asymmetrical stimulation of an animal's brain leads to an orientation reflex toward the contralateral field [MoTr and SI-In~a~ 19], while unilateral brain damage often results in orientations toward the side ipsilateral to the lesion [SARNAT and NETSKY 20]. The effect of sensory stimulation in one of the other sensory field, necessarily resulting in asymmetric activation of the contralateral cerebral hemisphere, almost always produces an orientation reflex toward the source of stimulation. Trevarthen reasoned that in an animal with laterally specialized functions in the two sides of the brain, selective hemispheric activation could and would emerge from cognitive activity specialized in a single hemisphere and would be reflected in an orientation reflex, or its covert homologue, an attentional bias, to the contralateral half of space. KINSBOOgNE [21], BAKAN [22], and Gtm [23] have confirmed this hypothesis, finding leftward eye movements during visuo-spatial thinking, and rightward eye movements during verbal thinking in right-handed people. KINSBOtmNE [24] confirmed, as Trevarthen suggested, that processing of information in the left or right side of space is enhanced when the contralateral hemisphere is selectively activated by laterally specialized thinking. LEVY and TREVARTHEN [24], in work with split-brain patients, concluded that even expectancies as to what task is to be performed selectively activate one or the other hemisphere.

It would thus appear that visuo-spatial processing or the expectancy of it, arouses the right hemisphere, biasing attention toward and awareness of the left half of space, so that even if sensory input is not strictly confined to one side of the brain, nevertheless, a lateral bias in attention ensues. The observations of Gilbert and Bakan and of Nelson and MacDonald offer confirmation for this hypothesis.

Their results, however, say nothing directly about the determinants of preference for mirror-symmetric pictures, nor even whether cerebral lateralization plays any role at all in aesthetic choice. They may even seem to conflict with the conclusions of Schlosser, Wblfflin and Opp6 to the effect that the most important content of works of art are located in the right half of the picture. However, it is quite conceivable that if an asymmetrical inherent biasing of attention into the left half of space during visual perception exists, those pictures which tend to correct for this imbalance by having the center of focus, or most important content, on the right, would be perceived as the more aesthetically pleasing. This interpretation would be not only consistent with Wflfflin's, Schlosser's and Oppf's conclusions, but also with the stress on balance in paintings as a first determinant of good art. In this case, balance would be achieved via a stimulus correction for a psychological imbalance. On the other hand, it is possible that perceived balance is less important in determining

434 JERRE LEVY

aesthetic preference than is congruence between the focus o f interest in a pic ture and field o f greater at tent ion. I f so, Schlosser 's , Wrlf f l in ' s , and O p p r ' s opinions would be wrong and prefer red pictures would be those with the more impor t an t content on the left.

Whe the r the loca t ion of impor t an t content , opera t ing th rough any possible mechanism, is a significant preference-determining factor , it is o f some interest to ascer ta in the role o f hemispher ic special izat ion in aesthetic judgements o f pictures. Since nei ther the opinions o f o f ar t his tor ians nor psychologis ts are o f much value, in the absence o f empir ica l suppor t , for predic t ing which o f two mi r ro r - symmet r ic pictures will be prefer red by the major i ty (dextrals ?) o f viewers, o r whether, even, a given mir ror - symmetr ic pa i r will yield an asymmetr ic preference at all, in the first s tudy to be described, no assumpt ions regarding what was o r was no t aesthetic in terms o f left-r ight a symmet ry were made. I t was assumed tha t i f di rect ion o f hemispher ic la tera l izat ion affects preferences for mi r ro r - symmet r i c pictures left- and r ight-handers should d isp lay different response pat terns. This fol lows f rom the fact tha t a m o n g dextrals , over 99 % have language funct ions organized in the left hemisphere, and visuo-spat ia l funct ions in the right, while among sinistrals, some 44 ~o have a reversed pa t t e rn o f organiza t ion [26].

In o rder to test these hypotheses, preferences for one or the other o f pairs o f m i r r o r - image vaca t ion slides were compared for left- and r igh t -handed Amer i can univers i ty s tudents as descr ibed below.

U p o n comple t ion o f the first s tudy, a second study, involving a new group o f subjects, was conduc ted in an a t t empt to gain some unders tand ing o f the visual factors responsible for preference a symmet ry in mi r ro r pairs o f pictures.

M E T H O D - - S T U D Y I Subjects

Subjects were 145 American-born native readers of English who were students in an introductory psychology class. There were 31 left-handers and 114 right-handers, and 61 females and 84 males. Handedness was determined by self-classification into one of three categories: Right-handed, ambidextrous, left-handed. All subjects calling themselves "ambidextrous" were included with the left-handed group for theoretical reasons [26].

Materials Ninety-seven vacation slides, whose subject matter included street scenes, architecture, wildlife, and

natural scenery, 65 contributed by the author and 32 contributed by a colleague, were placed in a slide projector carousel (labeled "A"), random as to order and random as to front-back orientation. A second set of duplicate slides was placed in a second slide projector carousel (labeled "B"), in the same order as those in Carousel A, but in the opposite front-back direction. The slides were selected from a total set of approximately 500 by the author and two colleagues on the basis of their subjective judgements regarding the relative aesthetic values of the slides (viewed in both orientations). An attempt was made to select slides which, rather than merely recording some vacation scene, might be perceived as appealing or interesting by viewers. The purpose of this selection was two-fold: (1) to attempt to ensure that scenes could be judged along an aesthetic dimension, and (2) to attempt to maintain interest on the part of the viewers.

Slides were projected through two duplicate Sawyer 2 x 2 in. projectors, arranged on a double platform stand, one platform located above the other, on to a standard projection screen.

Procedure Slide pairs were simultaneously projected, one above the other, on to a standard projection screen located

in a classroom. Subjects were tested in one of four group sessions, in two of which Carousel B was located in the top projector and in two of which Carousel A was located in the top projector. In one of each of the pairs of sessions Projector 1 was the top projector and in the other, Projector 2 was the top projector. The latter control was instituted to correct for possible differential optical qualities of the two projectors.

Subjects were simply asked to view the 97 slide pairs and to choose the member of each pair that they preferred. Choices were marked on an answer sheet.


Each slide pair was exposed for 15 sec, followed immediately by the next slide. Subjects marked their answer sheets during the 15 see exposure.

Scoring procedure Upon completion of data collection, 31 right-handed males were randomly selected to serve as the score-

determination group (Group SD). The number of subjects in Group SD choosing the " B " version of each of the 97 slide pairs was noted, and X z tests were performed on each of the 97 distributions. All pairs deviating significantly from a 50/50 distribution were selected as the slides to be scored for the experimental groups. There was a total of 14 such slide pairs, in each of which one version was preferred to the other by at least 21 of the 31 SD subjects. The preferred version was labeled " P " and was assigned a weighted score which was the Z-equivalent of the obtained X2, while the non-preferred version, " N o n - P ' , was assigned a negative Z- equivalent. Each Z-score was multiplied by a constant so that the total score for any subject choosing the " P " version on all 14 slide pairs was 100, while that of a subject choosing the "Non-P" version on all 14 slide pairs was --100. Each subject's total score was, then, the sum of scores for the 14 slide pairs. Thus, for any subject, a positive score indicated that his preference was similar to that for Group SD, a negative score indicated the opposite preference, and a score of zero indicated no relationship with the preferences of Group SD.

It is realized, of course, that some of the 14 slide pairs so selected yielded significant X2's simply due to chance, while the failure to select other of the pairs represents a Type II error. However, these inferential errors can only result in a conservative underestimate of the differences in left- and right-handers, if such exist.

RESULTS

Scores were determined for each of the 114 subjects constituting the experimental groups, as well as for subjects in Group SD.

Because the numbers of subjects participating in each session, the numbers of males and females, and the numbers of left- and right-handers were unequal, and because the main hypothesis to be tested concerns possible differences in dextrals and sinistrals, statistical tests were first run to determine whether there was any effect of sessions on subjects' scores as a function of whether Carousel A or B was on top or whether Projector 1 or 2 was on top. Neither of these factors even approached significance for any of the four groups classified by handedness and sex, and subjects within each category were consequently combined across sessions. Males and females within each handedness category were then compared, and for neither right-handers nor left-handers did they differ significantly, nor even ap- proach significance. The sexes within each handedness category were, therefore, combined.

Table 1 displays the results.

Table 1. Preferencescoresfor right-and le~-handers

Comparison d ff ~ SE M !

Dextrals vs. zero

Sinlstrals vs. zero

Dextrals vs. slnIstrals

82 22.69 2.98 7.61"*

30 6.89 5.64 1.22

112 15.70 5.97 2.65**

**~ <0.005, one-tailed test (a two-tailed test was performed

on the sinistrals-vs.-zero comparison since no prediction was

made regarding direction of deviation).

436 JERRE LEVY

The mean score of right-handers deviated very significantly from zero in the positive direction, showing that their preferences were correlated with those of Group SD, and offering support for the validity of the scoring system. It should be noted, however, that the mean score of Group SD (45.47) was significantly more positive than that of the right- handed experimental group (t = 3.39, df---- 112, P < 0.005, one-tailed test), an expected result in view of the fact that the scoring procedure necessarily maximized the scores of Group SD, and in view of the undoubted Type I errors contribuing to their scores.

The mean score of left-handers did not differ significantly from zero, indicating a lack of relationship between the choices of the left-handed group as a whole and those of Group SD. However, of greatest interest is the finding that the left- and right-handers in the experimental group showed a significant preference difference, and in the direction predicted by the hemispheric asymmetry hypothesis.

There was considerable variablity across subjects, particularly among left-handers, but the variances of the two handedness groups did not differ significantly (F = 1.33, df= 30, 82, P > 0.05). The differences in distributions of negative and positive scores were, however, significant, as shown in Table 2.

Table 2. Distribution of negative and positive scores

Number of Subjects with:* Postive Scores Negative Scores Total

Right-handers 66 17 83

Left-handers 18 13 31

*Z = 2.31, ~ = 0.011, one-tailed (H o = ~(pos) L ~ P(pos)N)

Left- and right-handers were partitioned into those having negative or positive scores, and the magnitudes of the negative scores, and of the positive scores, were compared between handedness groups. Neither comparison yielded a significant difference, although the mean score of positive-scoring dextrals (32.97) was slightly greater than that of positive- scoring sinistrals (28.94), while the mean score of negative-scoring dextrals (--17.21) was slightly less negative than that of negative-scoring sinistrals (--23.64). On the basis of this analysis, and of the negative-positive distributions of the groups (Table 2), it would seem that the overall difference between the two groups (Table I) is mainly attributable to the larger fraction of sinistrals who received negative scores, rather than to differences in the magnitudes of the positive and negative scores themselves. This conclusion is consistent with the hypothesis that left- and right-handers would differ on asymmetry of preference due to the larger fraction of the former group having visuo-spatial skills integrated in the left hemisphere.

DISCUSSION

The results of this study support the hypothesis that lateral specialization of the cerebral hemispheres affects preference for one of two mirror symmetric pictures. While preferred choices of one group of right-handers predict choices of another group of right-handers, the predictive validity for a group of left-handers is essentially zero. Slightly over 40 of sinistrals prefer mirror versions of pictures opposite to the choice of dextrals. It is


of some interest to consider that among sinistrals, some 44 ~o or so have language integrated in the right cerebral hemisphere and visuo-spatial functions in the left. These observations are similar to those of SWARTZ and HEWITT [27] who found that a very small, but significant, majority of right-handers preferred the original versions of famous paintings as compared with their mirror images, while left-handers did not. However, the deviations of both groups from random expectation were so small that the left- and right-handed groups did not differ from each other, and the interpretation of their findings remains in doubt.

Though the present investigation offers strong confirmation for the role of cerebral lateralization in determining aesthetic preference for mirror image pictures, it provides no clue as to why this should be so, the nature of any invariances which may be present in preferred pictures, nor how the direction of cerebral lateralization interacts with such invariances if they are present.

It is obvious that the stimulus determinants of preference reside in some aspect of left- right asymmetry. What is not clear is just what this aspect is, or even whether it is suffici- ently simple that it can be defined. It is conceivable that there are many stimulus determinants of preference and that they interact in such complex ways that within any set of preferred pictures no invariances can be discovered. Unfortunately, however, it is not only likely, but almost certain, that no understanding of how lateralization of the brain affects preference can be gained unless invariances in preferred and non-preferred pictures are found. There are two aspects of asymmetry which have been discussed by art historians and which, at least, offer the possibility of at, tack on the problem. W6lfflin, as noted, emphasized the location of the most important content of a picture, and felt that this was the major factor underlying the asymmetries depicted by artists. In addition, the question of balance, i.e. relative heaviness or weightiness on left and right, has been repeatedly emphasized in discussions of what constitutes good art, though usually not in the context of direction of asymmetry, but rather degree of asymmetry.

In Study II, a group of 23 right-handed judges was shown a random selection of 7 of the 14 P slides, the remaining 7 shown in the non-preferred orientation, and 14 slides randomly selected from those pairs which showed almost totally symmetric preference for Group SD. They were asked to judge the location of the more important content, and if this was symmetric, then the side of greater heaviness. If this, also, was symmetric, they were to call the slide equally balanced on left and right. If either of these dimensions is an important determinant of preference asymmetry, certain predictions can be made.

First, both P and Non-P pictures should be perceived as significantly more asymmetric than the control slides which had elicited no preference asymmetry. Secondly, the direction of asymmetry should be in opposite directions on P and Non-P slides. Third, there should be a slight perceived left asymmetry in the control slides due to a left field bias of attention, found by other investigators to occur during visual processing. Fourth, whichever category, P or Non-P, has left asymmetry, should be perceived as more asymmetric than the category having right asymmetry, since in the former case, psychological asymmetry will add to stimulus asymmetry, and in the latter case will subtract from it. Fifth, the degree of perceived asymmetry in P and Non-P slides should be equal when compared after the control slide baseline has been subtracted out, since, once this is done, psychological asymmetry is removed.

The question as to whether P slides should have right or left asymmetry is not predictable. As discussed, it is conceivable that a picture with its more important content or greater

438 Ima~ LEvY

heaviness on the left will be p re fe r r ed because o f a c o n g r u e n c e be tween field o f g rea te r

a t t en t ion a n d l oca t i on o f s t imulus con ten t . H o w e v e r , such a p ic ture , theore t ica l ly , w o u l d be

pe rce ived as m o r e u n b a l a n c e d t h a n its m i r r o r image . W h e t h e r P slides shou ld be lef t o r r igh t

a s y m m e t r i c depends on the re la t ive p sycho log ica l i m p o r t a n c e o f c o n g r u e n c e versus ba l ance

in d e t e r m i n i n g aes the t ic taste. In a cer ta in sense, i t w o u l d be m o r e sa t i s fac tory i f P slides

were to s h o w r ight a s y m m e t r y because , i f this were so, the k n o w n n e u r o p s y c h o l o g i c a l

effects o f l a t e ra l i za t ion c o u l d be in tegra ted qui te d i rec t ly wi th the ar t i s t ' s c o n c e p t o f ba lance .

If, on the o the r hand , P slides were to s h o w lef t a symmet ry , the adhoc concep t o f c o n g r u e n c e

w o u l d have to be ca l led up a n d w o u l d r equ i re a grea t dea l o f fu r the r inves t iga t ion in o rde r

to ref ine its def in i t ion and to d e t e r m i n e the va r i e ty o f cond i t i ons u n d e r wh ich i t d id o r d id

n o t occur .

M E T H O D - - S T U D Y II Subjects

Subjects were 23 right-handed university students, 13 female and 10 male, none of whom had participated in the previous study.

Materials Stimulus materials consisted of 7 randomly selected slides, chosen from the 14 P slides of the previous

investigation, the mirror versions (Non-P slides) of the remaining 14 slides, and 14 additional slides (called S), randomly selected from the 28 slides preferred by at least 15 and no more than 16 of the 31 subjects in Group SD. (See Fig. 1 for examples of slide pairs from each category.) Seven of these latter were the " A " version and 7 the " B " version. The 7 P, 7 Non-P, and 14 S slides were then randomly ordered in a slide carousel and projected in sequence on a standard projection screen.

Procedure Subjects were given the following written instructions: You will see a set of 28 slides showing a variety of scenes. Please study each slide carefully in order to determine its balance with respect to the left and right sides of the slide. Balance is to be determined on the basis of two criteria:

(1) Where is the most interesting or important content located? (2) Which is the "heavier" side of the slide?

(A) Balance should be said to be UNEQUAL if either of the following conditions holds: (a) One side of the slide has more interesting or important content or more salient features toward

which the eye is drawn. (b) One side of the slide is "weightier" than the other, and this is not compensated by there being

more interesting or important content on the other side. (B) Balance should be said to be EQUAL if neither of the above conditions hold. Please study the slides carefully and decide whether the slide is UNEQUAL or EQUAL according to the criteria given. Check your answer on your answer sheet under the appropriate column. Note that there are two UNEQUAL columns, one marked LEFT and one marked RIGHT. If the focus of importance or interest, or if these are equal on left and right, the greater heaviness, is on your left, check the column under LEFT and if on your right, the column under RIGHT. After subjects had read the instructions, the experimenter summarized the most important aspects of their

content, and asked if there were any questions. There were none. Each slide was then projected for 15 sec on to a standard projection screen located in a classroom, followed

immediately by the next slide in the sequence, with subjects usually marking their answer sheets during the last 5 see of each projection. All subjects were tested in a single group session.

Scoring procedure An EQUAL answer was given zero points, an UNEQUAL-RIGHT answer was given a score of ÷ 1,

and an UNEQUAL-LEFT answer was given a score of --1. Two separate analyses of the data were performed. In one, average P, Non-P, and S asymmetry scores were determined for each subject, averaging over slides in each category, and in the other, average asymmetry scores were determined for each slide, averaging over subjects.

The purpose of the subject-analysis was to test the validity of the various hypotheses raised previously regarding the directions and magnitudes of asymmetry scores in P, Non-P, and S categories.

LATERAL DOMINANCE AND ~ T I ( ~ PREFERENCE 439

Although the averages of 7 items constitute P and Non-P scores, while the averages of 14 items constitute S scores, since the a priori probability of a score of q- 1 for a slide equals that of a score of -- 1, the averages so obtained have an expected value of zero and are independent of the set size, N, on which they are based. However, in order to compare the magnitudes of perceived asymmetry, regardless of direction, either the absolute values or the square of the values must be compared. In both these cases as set size, N, increases, it can be proved that the expected values decrease (unless the apriori probability for a +1, --1 or 0 score on any slide is unity, in which case average scores are independent of N). Consequently, any statistical test showing that the absolute magnitude or the square of P and Non-P exceeds that of S cannot be interpreted to mean that the probability of a + 1, --1, or zero score on a slide varies as function of the category to which it belongs.

Therefore, in order to compare the degree of perceived asymmetry, ignoring direction, two S scores, SA and S~ were computed for these comparisons, based on the 7 "A" version S slides and the 7 "B" version S slides, and their absolute values averaged to yield a score, IS'I, whose expected value is the same as that of IPI and [Non-PI. If IPI and/or INon-PI are/is found to exceed IS'l, then it can be validly concluded that perceived asymmetry is less in the latter than the former.

In the slide-analysis, slide-specific asymmetry scores (A) were assigned to each of the 28 slides. The preference data from Study I for these slides were then reanalyzed to determine slide-specific preference scores for the entire group of 114 right-handers (R), including both experimental and Group SD subjects, and for the group of 31 left-handers (L). These scores (R and L) were computed as follows. Each subject in Study I who preferred the version of a slide pair shown in Study II received a score of + 1 for the pair. A score of -- 1 was assigned if a subject in Study I preferred the version not shown in Study II. The average slide-specific preference score, R or L, was determined for each of the 28 slides by averaging across subjects within the right- or left-handed sample. Correlational analyses among A, R, and L were carried out in order to determine the proportions of variance in average slide preference scores which could be accounted for by A and by slide preferences of the other handedness group. It should be noted that R and L scores were so assigned that any P slide shown in Study II should, on the average, have a positive R score, while any Non-P slide should, on the average, have a negative R score.

R E S U L T S I. Subject analysis

All hypotheses , as well as W~51ftlin's conclusion, were confirmed. Al l 23 subjects had posi t ive P scores. Twenty subjects had negative N o n - P scores, 2 had zero scores, and 1 had a posi t ive score. The difference in P and N o n - P scores was highly significant (t = 9"76, d f = 22, P < 0.0001). I t is apparen t ly the case that , in general , when an asymmetr ic preference is manifes ted by r ight-handers , the prefer red scene is the one with the more impor - t an t content , or, i f this is equal, the greater heaviness, on the right.

As predicted, [P[ a n d / N o n - P / w e r e larger than IS'l (t = 5.36, P < 0.001 and t = 5.47, P < 0.001, respectively, bo th wi th 22 degrees o f freedom). This was true o f 17 o f 23 subjects on P scores, 4 subjects having equal IPI and IS'l scores, 2 subjects having smal ler IPI than IS'l scores. On N o n - P scores, INon-P] was larger than IS'l for 19 subjects, while 2 subjects had a zero difference, and 2 subjects had smal ler /Non-P/ than IS'l scores. These results demons t ra te conclusively tha t the average perceived a symmet ry in P and N o n - P slides was greater than tha t in S slides.

On S slides 17 o f 23 subjects had a negative score, 4 had zero scores, and 2 had posi t ive scores. The mean S score ( - -0 .09) across subjects was significantly less than zero (t = 4.45, P < 0.001), suppor t ing the hypothesis tha t a left field bias exists which results in asymmetr ic weight ing o f the left field. Consis tent wi th this in terpre ta t ion , and as predicted, the absolute magni tude o f the negative sign ca tegory (Non-P) was larger than tha t o f the posit ive sign ca tegory (P). F o r 12 subjects the absolute value o f N o n - P was larger than tha t o f P, for 5 subjects there was a zero difference and for 6 subjects the absolute value o f N o n - P was smal ler than P. The mean difference in the absolu te magni tudes was 0"105 in favour o f N o n - P (t = 1.93, P = 0.035, 1-tailed test). W h e n each subject 's S score was subt rac ted f rom his P and N o n - P scores, the absolu te magni tudes o f the two differences d id no t differ.

440 JEssa LEvY

Table 3 summarizes the results.

Table 3. Summary of results of subject-analysis in study 1I

C o m p a r i s o n M e a n SE M

P v s . N o n P 0 . 8 2 6 0 . 0 8 5 9 . 7 6 * *

P v s . Z e r o 0 . 3 7 3 0 . 0 3 7 1 0 . 1 8 " *

N o n P v s . Z e r o - 0 . 4 5 3 0 . 0 6 5 - 7 . 0 2 * *

S v s . Z e r o - 0 . 0 8 9 0 . 0 2 0 - 4 . 4 5 rr

Ipi vs. Is'l 0 .201 0 .031 5.36,r

INonP~ vs. Is'l o.3o6 o.o51 5.47"

tpl vs. INonpl - o . l o 5 0 .055 -l .93'

I p - s l vs. INonP-sl 0 .079 0 .064 1.22

Values in italics are correlated t's.

**£ < 0.0001, two-tail

~fp < 0.0001, one-tail

r~ = 0.035, one-tail

The nonsignificant contrast was tested by a two-tailed test.

There were 22 degrees-of-freedom in all comparisons.

I I . Slide analysis Simple, partial, and multiple correlations were computed for the A, R, and L scores of the

28 slides. Results are shown in Table 4.

Table 4. Relations between asymmetry (A) and right-handers' (R) and left-handers' (L) preference for 28 slides

Correlations

Simple r Partial r Multiple r

RA 0.58** RA.L 0.62** R.AL 0.69**

EL 0 . 4 0 * RL.A 0 . 4 7 * A.RL 0 . 6 2 * *

AL 0 . 0 3 AL.R -0.27 L.RA 0.47*

d f 26 25 2 , 2 5

* * p < 0 . 0 1

*p< 0 . 0 5

It is fairly obvious that preferences are under control o f at least two factors, one measured by A and affecting dextral preferences, but not sinistral preferences, and one uncorrelated or possibly even negatively correlated with A, and affecting dextrals and sinistrals similarly. Presumably, the latter factor is independent o f cerebral lateralization and possibly reflects


an environmental invariant such as direction of reading. The increases in the RA and RL correlations when L and A, respectively, are partialled out, mean that the two factors affecting dextral preference in the slides are negatively correlated, as seen in the AL correlation when variance due to R is eliminated.

The total variance in R. which can be accounted for by A and L is 48 ~, of which A alone accounts for 32~o, L alone, 14~o and A and L convariance 2~.

In an attempt to gain a better understanding of the factors controlling preferences, further analyses were performed on the 14 P and Non-P slides and the 14 S slides separately. For right-handers, almost all the preference variance (R scores) in the set of 28 slides was, as expected, contributed by P and Non-P slides (S 2 = 1.10), with S slides contributing very little (S 2 = 0.11). The differing variabilities for P and Non-P vs S slides (F = 9.93, df = 13, 13, P < 0.001, one-tailed) is due both to the inclusion of Group SD in the right- handed sample, as well as to the correlated preferences for the other right-handers, as shown in Study I. Since, by definition, P and Non-P slides showed opposite preferences, while S slides had almost no preference variability around a mean R score of zero for Group SD, the differing variance of preference for the entire dextral group for the two sets of slides are predictable.

The preference variances for left-handers (L scores), however, did not differ (F----- 1.02, d f = 13, 13, P > 0.20), being 0.89 for P and Non-P slides and 0.87 for S slides, neither differing from R on P and Non-P slides, both differing from R on S slides (P < 0.002). Left-handers, in other words, unlike right-handers, displayed quite variable preferences among the 14 S slides, on some slides, 75~ preferring the version shown in Study II, on others, 65 ~o preferring the version not shown in Study II. In contrast, the largest preference asymmetry shown for any S slide by right-handers was one in which 58 preferred the version not shown, while 43 ~ preferred the version shown.

A not unreasonable inference from these results is that the preference in a given S slide pair by some right-handers was mainly affected by one factor, producing a particular choice, while for other right-handers, preference was mainly affected by another factor, producing the opposite choice, the net R score being close to zero, while for left-handers the net L score on S slides was predominantly a function of a single factor. One would guess that one of the two factors derives from cerebral lateralization, that the other is extrinsic to it, and the two variables are negatively correlated in S slides for subjects with typical dextral brain organization.

Separate correlational analyses for P and Non-P slides (Table 5) and for S slides (Table 6) revealed that A was the sole significant predictor of dextral preference (R) in P and Non-P slides, being unrelated to sinistral preference (L), while A had no predictive capacity for preferences in S slides, but L and R were fairly strongly related. In the multiple correlation for P and Non-P slides, predicting R from A and L, A accounts for 46 ~o of the variance, L for 5 ~o, and A and L covariance 6 ~o, suggesting a positive correlation between the determining factors for dextral preference. However, in the comparable multiple correlation for S slides, A accounts for only 6 ~ of the variance in dextral preference, L for 45 ~, and A and L covariance is negative, subtracting 9 ~ from the total predictable variance, offering support for the suggestion that dextral preference in S slides is under control of two negatively correlated factors. Apparently, however, the net preferences manifested, although weak, were, to the extent they were predictable, almost entirely due to the same factor which controlled sinistral preferences.

442 JERRE LEVY

Table 5. Relations between asymmetry (A) and right-handers' (R) and left-handers' (L) preferences for 14 P and non-P slides

Correlat ions


RA 0.72** RA.L 0.71"* R.AL 0.76**

RL 0.36 RL.A 0.33 A,RL 0.72*

AL 0.19 AL.R -0.II L.RA 0.37

d f 12 ]I 2 , 1 1

* * p < 0 . 0 1

* p < 0 . 0 5

Table 6. Relations between asymmetry (A) and right-handers' (R) and left-handers' (L) preferences for 14 S slides

Correlations


HA 0 . 0 7 RA,L 0 . 3 0 R.AL 0 . 6 5 *

RL 0 . 6 0 * RL,A 0 . 6 4 * A .RL 0 . 4 0

AL - 0 . 2 7 AL,R - 0 . 3 9 L.RA 0 . 6 8 *

df 12 II 2,11

* £ < 0 . 0 5

DISCUSSION

The results of Study II offer strong support for the view that at least two factors affect aesthetic preference for mirror-symmetric pictures, one deriving from the direction of hemispheric specialization and operating to produce strong preferences in right-handers and one extrinsic to this, affecting dextral preferences only weakly, but sinistral preferences, strongly. In the particular sample of pictures shown, it appears that preferred pictures by right-handers are those in which the more important content or the greater heaviness is on the viewer's right. When dextral preferences are very weak (S slides), there is a slight, but significant, perceived left asymmetry. These results, taken together, are interpreted to mean that, in viewing pictures, the right visuo-spatially specialized hemisphere is selectively activated, producing a bias of attention toward and a psychological weighting of the left side of space. Pictures which correct for this imbalance by having their more important content or greater heaviness on the right are considered, for this reason, to be more aesthetically pleasing.

Left-right scanning, probably a function of reading habit, emphasized by WOLFFUN [2], SCHLOSSER [1], OPP~ [4] and GAFFRON [5], may well play a role in determining preference,

LATERAL DOMINANCE AND AESTHETIC PREFERENCE 44~

and may, in fact, be the extrinsic preference determinant for left- and right-handers. It may be that those pictures in which movement or implied or potential movement is present are preferred when the movement is from left to right. If so, monolingual readers of Hebrew or Arabic would prefer the opposite versions of such pictures from those preferred by readers of European languages. Left-handed Israelis or people from Arabic countries would be expected to show preference scores on the S slides which would be negatively correlated with L and R scores in the present investigation, if scanning is the main determinant of preference for these. It is planned to carry out a replication of the current study, using the same sets of slides, in Israel.

Future investigations, directed toward understanding of the factors affecting preference for mirror-symmetric pairs of pictures, could profitably utilize specially designed stimuli in which heaviness, location of important content, and movement are all controlled. If the ideas which have been put forward have any validity, it should be possible to predict preferences of dextral and sinistral populations based on controlled asymmetries in the stimuli.

A recent completed study of Lv.vy and REID [28] demonstrates beyond question that direction of cerebral lateralization can be indexed quite simply from measures of handed- hess in conjunction with hand orientation when writing, confirming earlier pilot results of LEvY and MANDEL [26]. It should be the case that the asymmetry dimension, A, would be as predictive of sinistral as dextral preferences when sinistrals are partitioned according to direction of cerebral lateralization.

The asymmetry of human neurological organization was, for over a century, of concern only to neurologists and neurosurgeons. During the last 10 yr, and particularly during the last 5 yr, more and more psychologists have come to recognize the pervasive consequences for psychological function which result from cerebral lateralization. Various factors, structures, and processes discovered by psychometricians, cognitive and clinical psychologists, behaviour geneticists, and those concerned with the phylogeny and ontogeny of behaviour, independently of any knowledge or interest in brain asymmetry, are now seen to relate quite directly to cerebral lateralization of function. The results of the present investigation seem to show also that the human aesthetic sense is profoundly affected by the fact that the human brain is laterally differentiated.

A final point which deserves emphasis relates to the evolution of lateral specialization. AS KIN6SBOURN~ has discussed [29], bilateral symmetry of both the body and the nervous system must have been under very strong selective pressures in several phyla for tens of millions of years. A left-right asymmetry in either receptor or effector systems would be clearly disadvantageous to an animal with the capability of directed movement in response to a reality in which the direction of asymmetry is random. It may, therefore, be concluded that lateralization of the human brain either produces no asymmetries in perception or motor response, and, consequently, emerged due to a simple relaxation of selective pressure, as KINSBOtraNE postulates [29], or that the hominids were under a special set of such strong selective pressures for cerebral asymmetry, related to the optimization of cognitive function, that the disadvantages of lateralization, associated with concomitant perceptual and/or motoric asymmetries, were outweighed by the cognitive advantages.

The results of the present study, along with those of NELSON and MACDONALD [17] and of GILBERT and BAIC~N [16], demonstrating perceptual asymmetries, and of BAKAN [22], IONsaotrmqE [21], and GuR [23], demonstrating motoric asymmetries, arising from lateral specialization of the cerebral hemispheres, disconfirm the selective relaxation model of

444 JEP, RE LEVY

h u m a n b ra in evolut ion, and suppor t the view tha t la tera l different iat ion o f the nervous system was a direct consequence o f posi t ive selective pressure [LnvY, 26]. This conclus ion is p red ica ted on the a s sumpt ion tha t a symmet ry o f m o t o r or ienta t ion, a t ten t ion or pe rcep t ion is adapt ive only i f the a symmet ry has been induced by an a symmet ry o f s t imula t ion in the r ight o r left sensory fields. When a bias o f m o t o r or ien ta t ion or percept ion arises pure ly as a funct ion o f cerebra l processing, independent o f asymmetr ic s t imulus events, as can and does occur when cognit ive capaci t ies are la tera l ly specialized, the p robab i l i t y o f adap t ive responses to envi ronmenta l events is reduced. I t fol lows, therefore, tha t i f la tera l specializa- t ion leads to s t imulus- independent la teral biases in behavior , such la tera l iza t ion mus t for this reason have been under negative selective pressures. I f so, in o rder for cerebra l la teral i - za t ion to have evolved, the cognit ive gain i t b rough t mus t have increased fitness more than la tera l behaviora l biases decreased it.

R E F E R E N C E S

1. ScrmossEg, J. W. Intorono alia lettura dei quadri. Critica, 28, 72-79, 1930. 2. W6LFFLIN, H. "Ober das Rechts und Links im Bilde", pp. 82-90, and "Das Problem der Umkehrung in

Raffaels Teppichkartons", pp. 90-96. In Gedanken zur Kunstgeschichte, Schwabe, Basel, 1941. 3. W6L~rLIN, H. Classic Art (2nd Ed.) p. 118, Phaidon Press, London, 1953. 4. O1,P~, A. P. Right and left in Raphael's Cartoons. J. Warburg and Courtauldlnst. 7, 82-94, 1944. 5. G~Ro~r, M. Left and right in pictures. Art Quarterly 13, 312-331, 1950. 6. LEm~ET, G. La dominance lat6rale. Annde Psychol. 65, 411--438, 1965. 7. H~C~N, H. Clinical symptomatology in right and left hemisphere lesions. In Interhemispheric Relations

and Cerebral Dominance, V. B. MotrrcrCASTLE (Editor), Johns Hopkins Press, Baltimore, 1962. 8. LEVY, J., TREvArtTHEN, C. and SPERRY, R. W. Perception of bilateral chimeric figures following hemi-

spheric deconnexion. Brain 95, 61-78, 1972. 9. N~ES, R. D. Superiority of the minor hemisphere in commissurotomised man for the perception of part-

whole relationships. Cortex 7, 333-349, 1971. 10. LEVY, J. Information processing and higher psychological functions in the disconnected hemispheres of

commissurotomy patients. Unpublished doctoral dissertation. California Institute of Technology, 1970. 11. SPERRY, R. W. GAZZA~aGA, M. S. and BO~EN, J. E. Interhemispheric relationships: The neocortical

commissures; syndromes of hemispheric disconnection. In Handbook of Clinical Neurology. P. J. VINr,~N and G. W. BRtr~r (Editors), Vol. IV, North-Holland, Amsterdam, 1969.

12. BeGEts, J. E. and G~ZZAmGA, M. S. Cerebral commissurotomy in man: Minor hemisphere dominance for certain visuospataial functions. J. Neurosurg. 23, 394-399, 1965.

13. Wreak, M. J. Laterality differences in perception: a review. Psychol. Bull., 72, 387--405, 1969. 14. Kn~aJRA, D. Dual functional asymmetry of the brain in visual perception. Neuropsychologia 4, 275-285,

1966. 15. KI~tJRA, D. Spatial localization in left and right visual fields. Can. J. Psychol. 23, 445--458, 1969. 16. GILBERT, C. and BAKAN, P. Visual asymmetry in perception of faces. Neuropsychologia 11, 355-362, 1973. 17. NELSON, T. and MAcDoNALD, G. Lateral organization, perceived depth and title preference in pictures.

Percept. Mot. Skills 33, 983-986, 1971. 18. TR~VARTHEN, C. Brain bisymmetry and the role of the corpus callosum in behaviour and conscious

experience. Paper presented at the Int. CoIL Cerebral Hemispheric Relations, Smolenice, Czechoslavakia, 1969. In Cerebral Hemisphere Relations, J. CERNACEK and F. PODIWNSKY (Editors), Slovak Academy of Science, Bratislava, 1972.

19. MOAT, F. W. and SH~R, E. A. On associated eye movements produced by cortical faradization of the monkey's brain. Brain 13, 165-173, 1890.

20. SARr~AT, H. B. and NErSKY, M. G. Evolution and the Nervous System, Oxford University Press, p. 186, New York, 1974.

21. KIr~snouRN~, M. Eye and head turning indicates cerebral lateralization. Science 176, 539-541, 1972. 22. BAKA~, P. Hypnotizability, laterality of eye movement and functional brain asymmetry. Percept. Mot.

Skills 28, 927-932, 1969. 23. Gt~, g . E. Conjugate lateral eye movements as an index of hemispheric activation. J. Person. See.

Psychol. In press, 1975. 24. KzNsnotrRr~, M. Direction of gaze and distribution of cerebral thought process. Neuropsychologia 12,

279-282, 1974.

LATERAL DOMINANCE AND AF.6q~ETIC PREFERENCE 445

25. LEVY, J. and TREVARTHEN, C. Meta-control of hemispheric function in human split-brain patients. 8. exp. Psychol. Human Performance.

26. LEvY, J. Psychobiological implications of bilateral asymmetry. In Hemisphere Function in the Human Brain, S. DIAMOND and J. G. BEAUMONT (Editors), Paul Elek, London, 1974.

27. SWARTZ, P. and HEW~TT, D. Lateral organization in pictures and aesthetic preference. Percept. Mot. Skills 30, 991-1007, 1970.

28. LEvY, J. and I~ID, M. Hand orientation in writing and correlated variations in cerebral and cognitive organization. To be submitted.

29. KINSSOORNE, M. Lateral orientation and the lateralization of cognitive function. Paper presented by the Symposium on Lateralization, New Jersey Chapter of the Society for Neuroscience, Rutgers University Medical School, Piscataway, New Jersey, May 19-20, 1975.

R6sum6 :

On a constat6 que des droitiers et des gauchers pr6sen-

taient des pr6f~rences diff~rentes pour des versions en miroir de

diapositives de"vacances". Les pr6f6rences d'un groupe de droi-

tiers permettaient de pr~dire les pr6f~rences d'un autre groupe

de droitiers mais non celles des gauchers. Dans une seconde 6tude,

on a trouv~ que les diapositives fortement pr~f6r6es par les droi-

tiers @taient celles qui avaient le plus important contenu ou le

plus grand poids sur le c6t@ droit, et que cette asym~trie des

diapositives ~tait hautement corr~16e avec les preferences des

droitiers mais non avec celles des gauchers. Ces r~sultats sent

interpr~t~s comme t6moignant d'un biais perceptif du cSt~ gauche,

biais induit par une activation s61ective de l'h6misph~re droit

chez les droitiers.

Deutschsprachige Zusammenfassung:

ES zeigte sich, dab Links- und Rechts-H~nder in der Be-

vorzuo~ung fHr spiegelbildliche Versionen yon Feriendias

differieren. Die Bevorzugung einer Gruppe yon Rechtsh~ndern

war vorhersagbar fur eine andere Gruppe von Rechtsh~ndern,

dagegen nicht bei den Linksh~ndern. In einer zweiten Studie

fand sich, da~ die Dias, die stark yon Rechtsh~ndern bevor-

zugt wurde~, die waren mit dem wichtigeren Inhalt oder dem

st~rkeren Gewicht auf der rechten Seite, und dab die Dia-

Asymmetric hoch korrelierte mit der Bevorzugung von Rechts-

h~ndern, dagegen nicht mit der van Linksh~ndern. Die Ergeb-

nisse ~rden in der Weise interpretier%, dan sic eine Nei-

gung zur Perzeption im linken Gesiehtsfeld widerspiegelten,

die dutch eine selektive reehts-hemisphirisehe Aktivati0n

bei Rechtsh~ndern hervorgerufen ist.

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Lateral dominance and aesthetic preference