Journal of Experimental Psychology: Human Perception and Performance 1975, Vol. 1, No. 4, 3S3-365

Individual Differences in Adult Foveal Visual Asymmetries M. Joseph Schaller and Gregory M. Dziadosz University of Wisconsin—Madison The existence and nature of asymmetries in the recognition of elements of a visually presented array have been topics of dispute. In the present study, 32 adults responded to a single vertical or horizontal bar embedded in one circle of a S X 7 array of circles by touching a plate corresponding to the orientation of the bar. Two thirds of the subjects were left superior, while one third were right superior. Performance was in general top superior and decreased with increasing distance from the center. Possible explanations for these asymmetries are examined in terms of sequential processing, acuity dominance, hemispheric specialization, and selective attention.

The human visual system is not uniformly accurate in detecting and recognizing tachistoscopically presented stimuli in all areas of the visual field. Acuity decreases as the distance of a target from the center of the fovea increases (Riggs, 1965). But in addition there may be asymmetrical changes in recognition accuracy which favor one part of a stimulus array over another. For example, when Glanville and Dallenbach (1929) tachistoscopically presented to their subjects two rows of letters, one row above fixation and one below, those letters on the left side of the top row were reported consistently more accurately than the others. Similar studies have appeared more recently (see White, 1969b and Dziadosz & Schaller, Note 1, for reviews). In general, Western European and North American adults have been found to report elements on the left and top of arrays more accurately than on the right and bottom (e.g., Braine, 1972; This research was supported by grants to the first author from the Wisconsin Alumni Research Foundation and Grant HD-00117 from the National Institute of Child Health and Development. We thank D. W. Becker for help in running the subjects, and L. J. Harris, W. E. Epstein, D. W. Massaro, and D. A. Grant for their helpful comments and suggestions. Gregory M. Dziadosz was supported by a National Science Foundation Predoctoral Fellowship during the period of this research. Requests for reprints should be sent to M. Joseph Schaller, Department of Psychology, University of Wisconsin, W. J. Brogden Psychology Building, Charter at Johnson Streets, Madison, Wisconsin 53706.

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Harcum, 1964; Heron, 1957; Kimura, 1959), and children have been found to do better on the top and right (e.g., Braine, 1972; Dyer & Harcum, 1961). These asymmetries within arrays have been seen by some (see White, 1969b, 1973; but cf. McKeever, 1974; McKeever & Ruling, 1970) as distinct from differences between hemifields (e.g., the Mishkin & Forgays effect, 1952). The latter have been hypothesized to be the result of hemispheric specialization of function, while several investigators have argued that recognition asymmetries within arrays provide evidence for some form of sequential processing of visual information (e.g., Braine, 1972; Bryden, 1967; Harcum, 1967b). Visual information processing in which precedence is given to one part of an array (e.g., a linear directional sequential processing from left to right) might play a role in the formation or maintenance of behaviors that involve a directional component, such as reading, overt scanning (Lincoln & Averbach, 1956), naming and recalling (Gottschalk, Bryden, & Rabinovitch, 1964; Haith, Morrison, Sheingold, & Mindes, 1970), and perceptual organization (Harris & Schaller, 1971, 1973; Rock, 1974; Schaller & Harris, 1975; Takala, 1951). The demonstration and understanding of a mechanism potentially related to so many behaviors would add much to any explanation of the manner in which the environment is perceived. Unfortunately, the evidence for recognition asymmetries is equivocal. Several

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M. JOSEPH SCHALLER AND GREGORY M. DZIADOSZ

researchers report no differences (Ayres, 1966; Smith & Ramunas, 1971), and the positive evidence for recognition asymmetries is open to alternative explanations. It is possible that these within-array asymmetries are only the result of particular characteristics of the stimuli, instructions, or response measures used in previous research. Studies that use verbal or written report of all elements in a multielement display (e.g., Crovitz & Schiffman, 1965; Harcum, 1964; Heron, 1957; Kimura, 1959; Mathewson, Miller, & Crovitz, 1968; White, 1969a) allow rapid forgetting during reporting and might encourage directional bias in covert rehearsal strategy. Since spontaneous report order is almost always left to right (Bryden, 1960), elements on the left of the array would enjoy an artifactual advantage. Studies which attempt to control order of report by requiring alternative strategies (center-out or right-left) seem not to have escaped the influence of long-established report and rehearsal habits. This is evidenced by reduction of performance during the unusual report orders, particularly with letters and numerals (Bryden, 1967; Bryden, Dick, & Mewhort, 1968; Mewhort, 1966; White, 1969c; however, cf. Freeburne & Goldman, 1969), and possibly with geometric forms (e.g., Harcum, Hartman, & Smith, 1963; cf, however, Bryden, 1960). The interaction between unusual report orders and position (cf. Wolford & Rollingsworth, 1974, Experiment II) probably masks or confounds any asymmetry which might be present. Several attempts to use partial report still include other sources of bias such as the hypothesized left-hemispheric (right visual field) advantage for processing and reporting verbal material (e.g., Coltheart & Arthur, 1971; Miller, 1971, 1972; Smith & Ramunas, 1971; Winnick & Bruder, 1968). Preexposure fixation errors have also been invoked as a potential source of artifactual results (Ayres, 1966; McKeever & Huling, 1970). But attempts to guarantee fixation have been inconclusive (Ayres, 1966; Harcum, 1967a, 1967b; Harcum et al, 1963, McKeever, 1974; White, 1973), and have generally resulted in such a reduction of

overall performance that floor effects may result. No direct measurements of eye position have been made during a study of end asymmetries, so it is not possible to relate fixation errors to performance. Under fixation instructions, however, eye movements are usually less than 5 min. of arc (Alpern, 1971). On the other hand, studies which do not use a fixation point at all (e.g., Braine, 1965, 1972) would seem most suspect on these grounds. Finally, interactions among known asymmetries of sensitivity in the peripheral visual system, such as acuity dominance (Crovitz, 1961; Hayashi & Bryden, 1967) and the greater number of fibers in the crossed versus uncrossed visual pathways (Hubel & Wiesel, 1959, 1962) may also play a role in producing end asymmetries (Dziadosz & Schaller, Note 1). To determine whether recognition asymmetries occur independently of the effects of response and stimulus characteristics and thus represent a more interesting perceptual phenomenon, the following steps are necessary : 1. The task should require report of only one item from an array (cf. Averbach & Coriell, 1961). This would eliminate position-dependent differential fading or destruction of subsequent elements from memory during report and the necessity to use sequential rehearsal strategies. Also, since we plan to test the evidence of change in asymmetries with age (Braine, 1972; Dyer & Harcum, 1961), the procedure must be useful with children. Both children and adults can easily remember one item from an array (Haith, 1971). 2. The stimulus array itself (rather than some other signal; cf. Sperling, 1960) should indicate which item is to be reported. Miller (1972) reported masking of the items by a postarray indicator. A subsequent signal may also require the subject to rehearse the entire array until the pointer appears. On the other hand, a prestimulus indicator, depending on the timing, may allow or cause shifts of attention or fixation. Cross-modal indicators (e.g., Smith & Ramunas, 1971) may introduce interactions with other func-

INDIVIDUAL DIFFERENCES IN ADULT FOVEAL VISUAL ASYMMETRIES tional asymmetries or interfere with the main processing task, and would present special difficulties in testing children (cf. Pick & Pick, 1970). 3. The array should be two-dimensional rather than linear. This would allow examination of the strength of asymmetries along both major axes. In addition, linear arrays (particularly of letters and numerals) seem to invite particular types of organization. 4. Fixation should be controlled. A fixation point with instructions to attend to it are minimal requirements. Distractions in the peripheral field should be eliminated. In addition, the stimuli should be structured to minimize systematic expectation of target position. For example, stimuli should appear equally often on the left and right, and in an unpredictable pattern. The use of additional "fixation-forcing" tasks appears to provide no guarantee of better fixation and so does not justify the increase in error rate and subsequent risk of floor effects. Ideally, eye position might be accurately monitored during stimulus presentation, although the need for such fine control has not been convincingly demonstrated (Dziadosz & Schaller, Note 1). 5. The stimulus should be nonverbal, since there are known left-right asymmetries in performance using verbal stimuli that are assumed to be due to hemispheric specialization. In addition, it is difficult, especially across age, to control for guessing based on expectations due to experience with word and letter frequency, similarity of shape and symmetry of letters, and letter ordering in the language. Letters and words are also directional in nature, biasing toward particular sequential organization, and numerals probably share this characteristic (Bryden, 1960). Highly directional nonsense forms such as those used by Braine (1972) or Schaller and Harris (1975) might also be inappropriate for similar reasons. There is no evidence for hemispheric specialization for processing circles, on the other hand (White, 1972), and they are nondirectional in nature. 6. The response should be nonverbal. The vocal output mechanism appears to be

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strongly localized in the left hemisphere in most people (Sparks, Goodglass, & Nickel, 1970). Written response may suffer the same laterality problems (Zaidel, Note 2). Verbal report is especially a problem with sequential output of multiple elements. The response to be used should instead be bilaterally controlled and relatively uncomplicated. A binary response is preferable. It is easy to specify and control the level of chance performance. In addition, such a response is simple and requires little training and is appropriate for both adults and children. The design of the present study evolved from these considerations. METHOD Stimuli Each stimulus was an array of 35 evenly spaced open white-line circular annuli on a dark ground, arranged in S rows by 7 columns. These were photographed and mounted as high contrast (80:1) slides. In each array, one of the 35 circles contained a white bar across either its vertical or horizontal diameter (see Figure Ib). A series of 280 such stimuli, 4 for each of the 35 target locations by 2 orientations, were arranged into 8 subseries of 35. In each of these, all 35 target locations were included in random order, with orientation counterbalanced between subseries and determined by Fellows (1967) series. Each stimulus array projected an image 2.79 X 4.00 in. (7.08 X 10.16 cm) in size, which subtended 2.130° vertically and 3.055° horizontally when viewed from 75 in. (1.91 m). Each circle subtended .278° and was separated from each neighboring circle by .185°. The width of all lines was .0212°.

Apparatus Stimuli were rear projected onto a dark screen by a projector fitted with a high-resolution lens. Presentation was controlled by a electromechanical surplus aerial camera shutter with operating time of less than 1 msec placed directly in front of the lens. A i-in. (.635-cm) red fixation dot was projected within the position occupied by the center circle of the array, and an oblique crosshatch pattern postexposure mask (see Figure Ic) was projected so that it could completely cover the area of the array. The fixation point and mask were controlled by Vincent Electromechanical shutters with operating times of 5 msec. An Automated Data Systems 1800A process control system regulated presentation of stimuli, fixation point, and postexposure mask, and also immediately recorded response and latency data on magnetic tape.

M. JOSEPH SCHALLER AND GREGORY M. DZIADOSZ

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Fixation Point

Stimulus

Dark Field

1-12 msec

99-88 msec

Fixation Point

100

600-

T I ME (msec) -»

FIGURE 1. Stimulus presentation sequence. (A red fixation point appeared at the beginning of each trial. The stimulus array appeared for 1-12 msec. Following a blank field interval for the remainder of 100 msec, the mask appeared and remained for a minimum of 500 msec or until a response was made.)

Luminance Projectors commercially available do not produce a uniformly illuminated field, and correction by varied-density filters and other means proved impracticable.1 The variation in field luminance was reduced by various adjustments to an average deviation of ±9.5% (log .039) with a maximum deviation of —24% (log .094).2 However, luminance still varied with target position. Additional modifications were made to provide two luminance levels across target position: A "dim" setting produced a field with a mean luminance of 212.0 cd/m2 (range 161.0 to 243.2 cd/m2, SD = 2Q.2 cd/m2) and a "bright" setting produced a mean luminance of 282.5 cd/m2 (range 229.5 to 321.9, SD — 26.7 cd/m 2 ). This arrangement allowed the testing and, if necessary, removal of any effects of perceived brightness by means of multiple regression analysis. Subjects were individually tested in a room illuminated to maintain photopic adaptation. A 90% reflectance white card in the plane of the screen reflected 479.5 cd/m2.

Procedure Each subject sat at a desk with his head 75 in. (1.91 m) from the screen and approximately level with the display area. Blank canvas walls 6 ft. (1.83 m) high extended from the sides of the screen to the rear of the room on either side of the subject and blocked distracting aspects of the room from view. 1 We thank William Moore and others of the Eastman Kodak Co. for technical assistance in this endeavor. 2 Given the limited range of intensity in terms of the sensitivity of the eye, and the high contrast ratio of the stimuli (80:1), the expected effect on accuracy would be minimal. Adult observers could not tell that the field varied in brightness (cf. Eriksen, 1966) ; however, we wanted to assure that we had some way to measure and control any such effects.

Each subject was shown a photoenlargement of the stimulus array and was told that on each trial a single vertical or horizontal bar would appear in one of the 35 circles, with equal frequency in each circle. The subject's task was to detect the orientation of the single bar and to respond immediately by touching with his right hand one of two metal plates in front of him to right and left of midline. One was marked with a prominent vertical line, the other with a horizontal line. A red light between the panels lit immediately when the subject was correct. After the fixation point reappeared, the subject began the next trial by pushing with his right hand a button on the midline in front of him. Each subject was instructed to look directly at the red fixation point. He was told that this would be the best position to detect the bars since they would appear equally often on all sides. A counterbalanced random selection of 80 stimuli was then presented at durations of 1, 4, 8, and 12 msec. This pretesting both served as subject warm-up trials and provided an estimate of exposure time necessary for each subject's 75% correct performance; this exposure time was then used for 280 experimental trials for each subject. All testing was completed in one session lasting 30-40 min. The cross-hatch pattern was presented 100 msec after the onset of the stimulus array. This mask was used to suppress reported afterimages which had been a problem in pilot testing. The 100-msec interval should have provided no interference with processing of the stimulus array (Turvey, 1973). The mask remained on until a response was made or until 500 msec had elapsed, whichever was longer, and was immediately followed by the reappearance of the fixation point, indicating that the next trial could begin (see Figure 1). Subjects Fifty-nine college introductory-psychology students who reported being right-handed were

INDIVIDUAL DIFFERENCES IN ADULT FOVEAL VISUAL ASYMMETRIES tested. From the pool of 59, 32 (16 of each sex) were selected by eliminating those whose performance on the experimental task did not differ significantly from chance and those whose performance was too close to perfect.3 This was done in order to avoid floor or ceiling effects in performance. Two experimenters each tested half of each sex group.

RESULTS Brightness Because luminance could not be completely crossed with the other independent variables and could not be made uniform across the field, a preliminary multiple regression/ partial correlation analysis was performed to assess the effects of perceived brightness independent of the position factors.4 Any significant effect of luminance on accuracy could be examined and, if necessary, eliminated by correction with the /3 weights. Correction was unnecessary, however, because luminance showed such a small (and not significantly different from zero) partial correlation with accuracy, in contrast with the other within- and between-subjects factors. Brightness accounted for no more than .0009 of the variance. We therefore proceeded with the primary analysis of variance. All Subjects Mixed-design completely-crossed analysis of variance was performed using the withinsubjects factors position up-down (UPDN) and position left-right (LFRT) and the between-subjects factors sex of subject and 3 Data were eliminated from 23 subjects who exceeded the maximum performance criterion and 4 subjects whose performance was too poor. The minimum criterion (56.1% = 157 out of 280 possible) is different from 50% (chance) at the .OS level; the maximum criterion (93.9% — 263 out of 280 possible) was chosen to be symmetrical with the minimum. The 32 remaining subjects were tested at the following exposure durations in msec, with the number of subjects in each group indicated in parentheses: 1 (7), 2 (7), 3 (3), 4 (6), 5 (2), 6 (1), 7 ( 1 ) , 8 (1), 12 (3), 20 (1). * The STATJOB REGAN2 package of the Madison Academic Computing Center was used for the multiple regression and partial correlation analyses. Also included in the model were position factors left-right, up-down, and distance-fromcenter, plus sex, experimenter, average log latency, average accuracy, and log latency.

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experimenter, with accuracy as the dependent measure.5 UPDN and LFRT each produced significant main effects for accuracy, P(4, 112) = 32.2, p < .01 and F(6, 168)= 5.4, p< .01, respectively. Sex and experimenter did not. No interactions were significant with the exception of the UPDN X LFRT X Sex interaction, F(24, 672) = 1.5, p < .05. Trend analysis (Grant, 1956; Myers, 1972) within the significant main effects revealed (a) quadratic components for both UPDN, F(l, 28) =55.0, p < .01, and LFRT, F(l, 28)= 23.0, p < .01, and (b) a linear component in the UPDN effect, F(l, 38)= 34.4, p < .01. That is, first, accuracy dropped off with the square of distance from the center in both the UPDN and LFRT dimensions (as might be expected because of the correlated decreasing density of receptors, Polyak, 1957, and the increasing ratio of cones to ganglion cells, Brindley, 1970). Second, there was a significant asymmetry in accuracy scores in the UPDN dimension (the top was superior). There was no evidence for a left or right superiority. The three-way interaction between UPDN, LFRT, and sex was the result of a linear UPDN X Cubic LFRT X Sex interaction. (Females did not show as much decrement in the upper corners as did males.) At this point the results indicated (a) a general decrease in accuracy with distance from center and (b) a top superiority, but (c) there was no evidence for any left-toright asymmetry. The conclusion from Point c might be that the procedure and stimuli had eliminated the factors (such as order of report, directionality of stimuli) responsible for the left or right superiorities found in earlier studies. This may be the correct conclusion, but further examination of the data indicates the case is complicated by differences between individuals. s The STATJOB NWAY1 programs of MACC were used for the analysis of variance. Trend analyses were as described in Grant (1956) and Myers (1972).

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M. JOSEPH SCHALLER AND GREGORY M. DZIADOSZ

Individual Subjects Examination of the data for individual subjects by means of a more sensitive matched-pair procedure (comparing accuracy for each position on the left or top of center with its corresponding position on the right or bottom of center) revealed two things: (a) accuracy for all subjects as a group was superior on the left, £(448) =2.47, p = .007; but (b) the distribution of performance was bimodal with 21 (11 females) of the 32 subjects making fewer errors on the left and 11 (5 females) making fewer on the right. When each subject's accuracy score was expressed as a matched-pair t value,8 then only two scores fell within the interval t = ± .5, whereas 13 would be expected to do so if the scores were really distributed as t (Fisher & Yates, 1953). This distribution is significantly different from a t distribution with a mean of t = .436, X 2 (24)= 57.05, p < .001.7 Rather than a normal distribution, the LFRT performance scores split into two groups, each clustering around a t value of +1.5 for the left-superior group and —1.5 for the right-superior group. The UPDN scores, on the other hand, when treated in a similar manner are grouped unimodally and approximately normally around t = +1.5. This analysis indicates that the subject population is composed of two distinct groups, each with its own mode of LFRT superiority. The two modes taken together (although the left-superior group predominates in the present sample) probably cancel the effect of each other in the less sensitive group analysis of variance. This rinding should be replicated, but for now it appears 6

This measure again takes each left (or top) position minus its corresponding right (or bottom) position, sums the differences, and divides each subject's score by his standard deviation. Thus a t score of zero signifies equal performance left and right; a positive score signifies left superiority, and so on. These scores should ordinarily be distributed about the mean as t. 7 The x2 computation was based on one-quarter wide intervals on the t distribution (Fisher & Yates, 1953) from -3 to +3. The distribution of the matched pair ts was also computed using a pooled variance, but this did not change the outcome, x2 (24) = 66.82, p < .001.

that separate analyses of the two groups are justified. Left-Superior and Right-Superior Groups Analyses of variance for accuracy, followed by trend analyses, were performed separately for the 21 left-superior subjects and for the 11 right-superior subjects.8 Each group showed significant UPDN and LFRT linear trends: Both groups were top superior and either significantly left or right superior. Both groups also showed a decrease in accuracy with the square of distance from center as before (quadratic UPDN and LFRT effects). The shapes of the accuracy-by-position functions for the left- and right-superior groups are shown in Figures 2 and 3, respectively. The plots of equal-accuracy contours (taken from equations from the trend analysis), represent the shift of the most accurate region away from fixation toward the top and toward the left or right, respectively. The decline in accuracy with square of distance from center is also evident. The significant left- and right-superior effects following the splitting of the group may not be surprising, but neither are they trivial; the separation of the two groups on the basis of left-right performance seems fully justified by the preceding x2 analysis showing bimodality. There are two distinct modes of significantly asymmetrical performance shown here. This fact indicates that the conventional explanations of the asymmetry phenomenon may be in8 The STATJOB programs cannot adequately handle mixed designs if all factors are not balanced and crossed. Therefore, sex and experimenter (which would not have been quite balanced) were not included as factors. Examination of the distribution on both the LFRT and UPDN dimensions, using the matched-pairs procedure again, did not reveal any effects of sex or experimenter, or, for that matter, of exposure duration. Significant F tests for the trend analyses were, for left superior, linear UPDN, F(l, 20) =15.5, quadratic UPDN, F(l, 20)= 35.9, linear LFRT, F(l, 20)= 68.8, quadratic LFRT, F(l, 20)= 18.6; and for right superior, linear UPDN, F(l, 10) = 26.9, quadratic UPDN, F(l, 10) = 26.9, linear LFRT, F(l, 10) = 60.4, quadratic LFRT, F(l, 10)= 5.8, p s < .01.

INDIVIDUAL DIFFERENCES IN ADULT FOVEAL VISUAL ASYMMETRIES til

359

top

: .bo \ •? left

- .25

.00

.25

1 .25

.Ml

1 .50

right Degrees from Fixation

FIGURE 2. Equal-accuracy (percent correct) contours projected onto the] visual field for the significant effects shown by the left-superior subjects. (Each contour represents a drop in accuracy of 2% of perfect. The fixation point, marked by the cross, was | at 0° horizontal and 0° vertical. Measurements on each axis are in terms of degrees from fixation.) adequate, since they have not examined individual performance. Finally, to recheck the possibility that unintended effects of the nonuniform luminance of the stimulus field biased these results, a further regression analysis was run, now that significant higher-order effects had been identified. Table 1 shows the final results and model for this analysis. Again, as expected, the nonuniform field had no influence on accuracy. DISCUSSION The primary finding of the present study is that when the possible causes of artifactual asymmetries which we have identified earlier are removed, differences persist in accuracy of recognition of an array as a function of target position within the fovea. The quad-

ratic decreases in accuracy with increasing distance from the center are not surprising in view of the falloff in acuity away from the TABLE jl REGRESSION ANALYSIS Foi LEFT AND RIGHT SUPERIOR SUBJECTS

Independent variable Log luminance LFRT (linear) UPDN (linear) LFRT (quadratic) UPDN (quadratic) Log latency Presentation time Sex Experimenter Average log latency Average accuracy

Left superior Partial correlation coefficient P -.022 -.154 -.136 -.105 -.191 -.171 .000 -.000 .000 .122 .378

J4090 ,0000

,;oooo .0001

,0000 ,0000 .9946 .9949 J999 1

.booo .pooo

Right superior Partial correlation coefficient t -.014 .177 -.159 -.089 -.195 -.108 -.000 .000 .000 .072 .305

.7091 .0000 .0000 .0144 .0000 .0028 .9998 .9986 .9985 .0477 .0000

Note. Accuracy of response is the dependent variable. LFRT indicates left-right position in the! array; UPDN indicates position along the up-down dimension.

M. JOSEPH SCHALLER AND GREGORY M. DZIADOSZ

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RIGHT SUPERIOR flOULT SUBJECTS

-.75

left

-.50

-.25

.00

.25

.75

1.00

1 .25

1

.50

right Degrees from Fixation

FIGURE 3. Equal-accuracy (percent correct) contours projected onto the visual field for the significant effects shown by the right-superior subjects. (Each contour represents a drop in accuracy of 2% of perfect. The fixation point, marked by the cross, was at 0° horizontal and 0° vertical. Measurements on each axis are in terms of degrees from fixation.)

center of the fovea (cf. Brindley, 1970; Jones & Higgins, 1947; Polyak, 1957; Riggs, 1965). The linear trends, on the other hand, show asymmetries that are independent of this quadratic effect and probably cannot be explained in terms of known acuity gradients. Performance is in general better for the top portion of the array, but no general statement can be made about the left-right dimensions: Two thirds of our group performed better for targets on the left of the array, while the other third did better on the right. The existence of two modes of performance may explain some of the discrepant results concerning the existence of left-right asymmetries in the previous literature (e.g., Ayres, 1966; Harcum et al., 1963; Smith & Ramunas, 1971) and indicates that reevaluation of these studies

in terms of individual differences might be profitable. Although the left superiority shown by two thirds of the subjects in our experiment is in the same direction as that found in many previous studies, the shape of the present accuracy-by-position function differs from those in previous studies. The most striking difference in shape is the absence of higher accuracy regions at the ends of the curve. Most previous studies show Uor W-shaped accuracy-by-position functions (e.g., Crovitz & Schiffman, 1965; Miller, 1972; Smith & Ramunas, 1971; Winnick & Bruder, 1968; however, cf. Hershenson, 1969, and White, 1970). The upturned ends of the previous curves seem to indicate the presence of rehearsal and reporting artifacts (analogous to primacy-recency effects)

INDIVIDUAL DIFFERENCES IN ADULT FOVEAL VISUAL ASYMMETRIES

or of lateral "unmasking" (Bouma, 1970, 1973). Explanation of the Asymmetries The most widely held explanation of the origin of end asymmetries is that they are the result of sequential processing of visual information (Braine, 1972; Bryden, 1967; Harcum, 1970). That is, the elements of a stimulus are said to be processed serially in a given order and those elements processed first have a perceptual advantage over those processed later. In addition, this "scan" is hypothesized to obtain its direction from left-to-right and top-to-bottom readingscan habits (Heron, 1957; Kimura, 1959). The present study was designed to test for recognition asymmetries in the absence of confounds and not as a specific test of the sequential-processing hypothesis; however, the "reading-scan" hypothesis is difficult to reconcile both with the absence of a strong left superiority over all these college subjects and with the uniformity and strength of the top superiority. Left-to-right scanning habits predominate over top-to-bottom scanning in reading (cf. Alpern, 1971) even for very poor readers, it would seem, and thus there should be more consistency in the left-right performance. It is also evident from Figures 2 and 3 that the asymmetry does not represent a simple advantage for only the uppermost left corner members of the array (which in fact were not the most accurately reported) as would seem to be predicted from this hypothesis. In addition, we have more recently tested prereading and reading school children (Schaller & Dziadosz, Note 3) and find unimodal right superiorities that are even more difficult to reconcile with the reading-scan sequential-processing hypothesis. (In addition to showing quadratic effects of position on accuracy similar to those shown here, our study indicated that children's performance became increasingly top superior and became right superior by 9 yr. There were no systematic individual differences like those in the current study.) A second factor which may contribute to end asymmetries is the presence of

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asymmetries in the peripheral visual system. Greater acuity in one eye, for example, could interact with the greater number of fibers in the nasal retinal projections to produce an advantage for one side of the visual field, for example, the left hemifield for left-eyeacute persons (Hayashi & Bryden, 1967). Crovitz (1961) reported approximately 42% left- versus 31% right-acuity-superior individuals in the adult population, a proportion that fits reasonably well with the distribution of left- and right-performancesuperior adults described here, x2 (1) = .9814, p = .329. We are currently checking this possibility. However, the peripheral asymmetries hypothesis does not offer a ready explanation for the strong top superiority found here. To fit with the peripheral hypothesis, projections from the lower hemiretina would have to be more dense than those from the upper hemiretina, and/or the image projected on the lower hemiretina more accurately focused. We know of no indication in the literature that this is the case, although the projections from the retina to the cortex are separate for the upper and lower hemiretinae as well as for the left and right hemiretinae (Teuber, Battersby, & Bender, 1960). Third, while the end-asymmetry phenomenon has been seen as distinct from hemifield differences due to cerebral hemispheric specialization for function (see White, 1969b, 1972, 1973; Dziadosz & Schaller, Note 1), other researchers have asserted that such specialization is a major cause of end asymmetries (McKeever, 1974; McKeever & Huling, 1970). In past studies, hemispheric specialization probably did play a role in producing the results of studies involving detection or partial report of verbal stimuli (Miller, 1971, 1972; Smith & Ramunas, 1971). In the present study, the array elements were circles and bars, for which there is no clear evidence of hemispheric specialization in processing (White, 1972). White (1971), using verbal report, a possible source of confounding errors, has reported that adults showed a right-field superiority

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for labeling (with nftmbers) the orientation of a single line; in the present study two thirds of the subjects were superior in the left field, however. To account for the asymmetries in terms of known hemispheric asymmetries, it is necessary to maintain that right-superior adults viewed the stimuli as verbal in nature or used verbal modes of processing (cf. Kinsbourne, 1973; Bruce & Kinsbourne, Note 4), while the left-superior adults (the majority) treated the task (inappropriately) as a spatial localization problem (cf. Kimura, 1969), or used visuospatial means of processing. This possibility may be unlikely, since targets were never called letters and subjects were given explicit instructions to report only the orientation of the target irrespective of its position, and also since the procedures allowed only such information to be reported, using a bilaterally controlled gross motor response. Finally, hemispheric specialization cannot explain UPDN asymmetries. There is no evidence for top-bottom critical differences in function in any way analogous to the putative left-right hemispheric specialization. A fourth possible explanation for end asymmetries is selective attention. Kaufman and Richards (1969) reported that when presented with forms subtending less than 5°, subjects' "scanning" of the figures appeared to be done through volitional shifts in attention rather than through shifts in fixation. The center of fixation and the focus of attention could differ by as much as 2°. If subjects were predisposed innately, or through experience with the environment, to focus attention to a point other than the center of fixation, then a more accurate initial perception of elements in that part of the field might result. Physiological evidence indicates that attention can "sharpen" sensory input by reducing the "signal" to "noise" ratio in ongoing neural activity (Horn, 1960; Spinelli & Pribram, 1967; Young, Elison, & Feeney, 1971) and is under cortical control (Gerbrandt, Spinelli, & Pribram, 1970; Segundo, Arana, & French, 1955), presumably making

attention susceptible to learning and allowing directional biases to form.9 If important or new information generally enters the visual field from above in the normal environment (cf. J. Gibson, 1966, p. 175), then one would expect an organism habitually to attend first to that part of the visual field, and top superiority would be a result. However, the possible cause of attentional biases for either the left or right fields and especially for individual differences are less obvious. We are now working to test some of these ideas. 9 This is a different notion of selective attention than that discussed by Shiffrin, for example (Shiffrin & Gardner, 1972; Shiffrin & Grantham, 1974). Physiologically, attention does not seem to represent a complete or partial turning on or off of channels, but rather an enhancement of a neural signal through inhibition of the background activity in an incoming sensory channel, Shiffrin's experiments did not directly test for enhancement of signal detection with a noisy background, since noise values in his experiments were minimal. Further, we are speaking here of a preexposure attention set, that is, a propensity of the subject and his nervous system to expect an input from a given area of the 'visual field. Kaufman and Richards (1969) indicate that this "focus" of attention is not confined to the point of actual fixation. This type of attention shift seems quite different from the extremely short latency shift discussed by Shiffrin.

REFERENCE NOTES 1. Dziadosz, G. M., & Schaller, M. J. Asymmetries in foveal visual perception: a revieiv. Unpublished manuscript, 1974. (Available from the authors, Department of Psychology, University of Wisconsin, W. J. Brogden Psychology Building, Charter at Johnson Streets, Madison, Wisconsin 53706.) 2. Zaidel, E. Language, dichotic listening, and the disconnected hemispheres. Paper presented at the Conference on Human Brain Function, University of California, Los Angeles, September 27, 1974. 3. Schaller, M. J., & Dziadosz, G. M. Top- and right-superiorities in foveal vision increase between 44 and 9 years of age. Paper presented at the meeting of the Society of Research in Child Development, Denver, April 1975. 4. Bruce, R., & Kinsbourne, M. Orientation model of perceptual asymmetry. Paper presented at the Psychonomic Society Meetings, Boston, November 1974.

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