0028-3932/79/0701-0381$02.00/0

Neuropsychologia, Vol. 17, pp. 381 to 392. 6 Pergamon Press Ltd 1979.Printed in Great Britain.

VISUAL FIELD DIFFERENCES IN PERCEPTION OF THE VERTICAL WITH AND WITHOUT A VISIBLE FRAME OF REFERENCE COLIN PITBLADO Institute of Living, Hartford, CT 06106, U.S.A. (Received

Abstract-Reaction

28 November 1978)

times were obtained for judgments of tilt, relative to vertical, of slanted

line segments flashed in the right and left visual fields. With no visual reference axes present, RTs were faster in the right visual field: When a tilted square frame was added, simulating the rod-frame configuration, RTs increased for both fields, and by a significantly greater magnitude in the RVF. Accuracy measures were comparable to those obtained conventionally with unlimited exposure times. The “no-frame” result suggests involvement of a left-hemisphere mechanism in this class of spatial judgment, and a view of interhemispheric cooperation in rod-frame performance is presented.

ONE OF the major trends in the study of human brain function is the investigation of differential specialization of the two cerebral hemispheres. Among the important generalizations that have emerged from this line of research is the notion of a general right hemisphere superiority in visuospatial performance. A wide variety of experimental and clinical investigative techniques have contributed to this conclusion, including studies on brain injured patients [ 11, and commissurotomized patients [2]. Behavioral studies, employing, tachistoscopic stimulus presentations, have resulted in reports of greater right hemisphere accuracy for stereopsis [3], discrimination of faces [4], reproduction of dot patterns [5], and recognition of line slant [6]. More complete reviews can be found in KIMURA and DURNFORD [7], or WHITE [8]. In addition to accuracy measures, reaction time (RT) has also been used to explore hemispheric functions. RT differences with right and left visual field stimuli are often taken as evidence for localization of at least part of the information processing in the hemisphere associated with the shorter RT [9, lo]. Some findings based on use of this approach for visual discrimination include faster left visual field RTs for identification of faces [ll], and for matching of line orientation [12]. Few studies, however, have dealt with discriminations involving spatial orientation relative to gravity. Even studies using slanted line stimuli have not actually dealt with perceived gravitational orientations. Instead, the required responses have included recognition of the flashed line in a standard series of slanted line segments [6, 131 or same-different responses to the orientations of two line segments, presented either simultaneously or in sequence [12]. These kinds of responses may not necessarily involve the same family of perceptualcognitive operations as perception of the vertical or perception of orientation relative to the longitudinal body axis, for example. One study which did deal with a gravitional orienting task has been described by COHEN et al. [14]. Rod-frame scores of depressed female psychiatric patients were obtained before 381

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and after unilateral electroconvulsive shock therapy (ECT). Increased errors were found in patients who received ECT to the left hemisphere, while decreases were found in those receiving ECT to the right hemisphere. These findings clearly suggest the existence ol lateralized sub-processes in the rod-and-frame task. It is therefore desirable to determine whether these results will generalize to a normal subject population, and to further clarify the nature of the sub-processes they suggest.

METHOD Subjects

Twenty-four men and 24 women served as subjects in this study. Half of each group were right-handec and half were left-handed, as determined by uniform hand preference for writing, throwing, and eating (The mean age for men was 23.1 yr. The mean age for women was 19.5 yr.) Apparatus

Stimuli were presented in a Scientific Prototype (model GB) tachistoscope. A specially modified back pane in one field provided a straight black target line, about 4” in length and about 0” 10’ in width, whose orientation could be varied by means of a control knob on the outside of the panel. The line appeared against a plain white background with its midpoint in the center of the various frames. While photometric measurements of the stimulus array were not made, photometric properties of the stimuli were held constanl under all conditions by making all stimulus presentations in the same field of the tachistoscope at ful intensity. Cardboard cut-out panels controlled the shape of the field against which the line was seen. For trial: run without a usable visible frame of reference a circular aperture 5.8” in diameter was used. For the rod. and-frame configuration a 15” tilted square aperture was employed, with the diagonal of the square sub. tending 5.8”. The direction of frame tilt could be changed from right to left by simply removing the pane and reversing it. At all times, except during the 100 msec target flash, a luminous fixation point was presented in the seconc field of the tachistoscope, 2.2” to the right or left of the center line, as conditions required. When the subjec was fixating this point, the centrally positioned target line would appear in either the right or left visual field The stimuli were viewed from a pair of viewing apertures which were shielded by rubber eye cups. Whilt the subject’s face was positioned at the viewing apertures his head was in the vertical position. Room light! were extinguished during all trials, so that only the test target was visible. The subject’s task was to judge whether the target line appeared tilted to the left (counter-clockwise) 01 right (clockwise) of true vertical, and to indicate the judgment by pressing one of two telegraph keys, which were laterally separated to provide adequate stimulus-response compatibility for the task. Reaction times were obtained on a pair of Lafayette Instrument choice reaction timers, calibrated ir l/100 set units, and readable to the nearest 5 msec. A control switch initiated the flash in the tachistoscopc and started both reaction timers simultaneously. The left key stopped one timer, while the right key stoppec the other. Procedure

Subjects were brought into the laboratory and seated at the viewing apertures. The general procedure for making responses was then described to the subjects. Subjects were to begir each trial with the tips of the index and middle fingers on a designated “ready” position half way between the two response keys. At the same time, on signal from the experimenter, the subject was to look directly a the fixation point. The subject would then signal the experimenter, who after roughly 1 set, would operate the control switch. The subject was to make his discrimination and press the appropriate key as rapidly a! possible, and then close his eyes and return his hand to the ready position for the next trial. In all trials responses were made with the hand ipsilateral to the visual half-field receiving the stimulus Motor control of the responding hand thus originated in the hemisphere that received the visual input Any hemispherically-localized operation would require a two-way transmission of information across the commissures, and a consequent increase in RT, when the opposite hemisphere was receiving the stimulu: and initiating the motor response. When input and output both originate in the hemisphere containing the critical operation no cross-over is necessary in either direction. To control for inherent differences in tht simple RTs of the two hands, 20 simple RT trials were obtained for each hand, using a plain white disc disc stimulus, and alternating keys every trial. Means were calculated for each hand, and differences were used as correction factors in the results by adding them to the means of all conditions employing the fastel hand. Half the simple RT trials were run before the main experiment, and the other half were run immed. iately afterwards.

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Before the main experimental trials (choice RT trials) were begun the experimenter demonstrated the nature of the discrimination to the subject. Any questions from the subject were elicited and answered at this point, as it was necessary to make certain that the subject understood the working definitions of right and left tilt. To further ensure understanding of the instructions and to reduce the likelihood of long initial trial reaction times, three such practice trials with exaggerated degrees of tilt were given under each viewing condition of the experiment. There were two major stimulus presentation factors in this experiment, visual half-field (right or left), and visible frame of reference (circular, left-tilting square, or right-tilting square). The six possible combinations of these factors comprised the stimulus conditions of the experiment. To distribute any progressive errors (practice effects, etc.) over all conditions, a series of 12 different balanced orders was developed. One right-handed man, one left-handed man, one right-handed woman, and one left-handed woman were assigned to each of these orders. For each condition there were 18 trials, run in two blocks of nine trials each. Further balancing for progressive effects was achieved by separating these blocks and running the first blocks in the designated order for any given subject, and then running the second blocks in the reverse of that order. Each block of trials was run using a modified “staircase” method of stimulus presentation [15]. On the first trial the test line would be positioned with a tilt of 3” to either the right or left of the true vertical. If, for example, the subject correctly responded “right”, the line would be set 2” further left on the next trial, i.e. 1” to the right, objectively. If the subject again responded “right”, another 2” leftward step would be used on the following trial, and so on, until the subject finally changed his response, at which point the direction of line tilt on the subsequent trial would also be reversed. Thus, the subject would, in effect, “track” his own threshold for subjective vertical. For every condition of the experiment, one of the blocks of trials began with a 3” right tilt, and the other began with a 3” left tilt.

RESULTS Reaction time data

Group data were analyzed using a four-way ANOVA-visual hemifield, visual frame (none or rod-frame), sex and hand preference being the main factors. The results of this analysis are presented in Table 1. TABLE1. Analysis of variance on mean RTs as function of visual hemifield, visual frame of reference, sex and hand preference Source Visual frame condition (VF) iviual field (F) Hand preference (HP) VF x F Sex x VF Sex x F HP x VF HP x F Sex x HP Sex x VF x F HP x VF x F Sex x HP x VF Sex x HP x F Sex x HP x VF x F

F

31.3681 2.4456 0.4069 0.1751 5.3882 1.7381 1.435 0.1611 2.4181 1.3309 0.7065 1.8817 2.0422 0.3667 2.4498

P

0.0001 NS NS NS 0.025 NS NS NS NS NS NS NS NS NS NS

The ANOVA shows two significant effects. First, there is a highly significant difference between the no-frame and rod-frame stimulus conditions, the mean RT for rod-frame stimuli being greater than that for the no-frame trials. Secondly, there is a significant interaction between visual frame conditions and visual hemifield. Figure 1 demonstrates this interaction graphically. It is apparent that overall RTs for the “no frame” condition are

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faster with RVF presentations than with LVF presentations. For the rod-frame stimuli, the reverse is true. This suggests that while there was no overall visual field difference for the two tasks combined, there may very well be significant visual field differences for the two tasks considered separately, as the individual data suggested. Since the differences are opposite in direction, there is a nulling effect on visual hemifield as a main factor. In order to determine whether this was the case, separate one-sample t-tests were run on the LVFRVF scores in both the no-frame and rod-frame conditions.

0

Rod -frame

q No - frame

condltlon condition

LVF FIG. 1. Mean

reaction times for rod-frame and no-frame conditions, calculated separately for right and left visual fields.

The results of the t-tests were as follows. For the no-frame trials, the mean difference was 14.4 msec and SD = 31 .O, with df = 47, t = 3.22 and P < 0.005(two-tailed). The positive value indicates faster RTs for RVF (left hemisphere) presentation, For the rod-frame trials, MD = -2.8 msec, and SD = 42.3, with df = 47. t = -0.46, and is not significant. While no significant visual field differences appeared in the RTs for rod-frame trials, the overall increase in these RTs over those for the no-frame trials was significant. It was considered possible that this increase might represent at least one additional processing operation which is itself associated with visual field differences, opposite in direction from the difference observed in the no-frame trials. Accordingly, an additional step in data analysis was undertaken. For each subject mean RT differences between no-frame and. rod-frame trials (incremental RTs) were calculated separately for right and left visual field stimuli and subtracted (LVF-RVF). If the incremental RTs were not significantly different for the two visual fields, the group average should be close to zero. On the other hand, if the hypothesized LVF (right hemisphere) advantage were present, the group mean would be significantly negative. These alternatives were compared by a t-test. For the group, the mean difference between incremental scores for LVF and RVF was -17.2 msec, and SD = 51.4, t = -2.318, and P (one-tailed) < 0.005. Thus, the increments

VISUAL

FIELD

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in the RT between no-frame and rod-frame for RVF than for LVF presentations.

IN PERCEPTION

conditions

OF THE VERTICAL

are found to be significantly

385

greater

Accuracy data Measures of subjective vertical were abstracted from the data by taking the midpoints of the stimulus settings on every pair of consecutive trials in which a threshold reversal occurred. (This assumes that when a subject changes judgment on consecutive trials, a point halfway between the two trials settings would have appeared vertical.) For each condition of the experiment a mean subjective vertical was obtained for each subject by averaging all such midpoints in the condition. Algebraic conventions were necessary in order to represent the direction of a subject’s errors. In the no-frame condition errors to the right of true vertical were scored “+“, and errors to the left were scored “-“. The data for this condition were analyzed by a threeway analysis of variance, with visual field, sex and hand preference as the main factors. The model employed correlated measures on the visual field factor. The results of the ANOVA are presented in Table 2. Table 2. Three-way analysis of variance on mean constant errors in the no-frame condition. Factors are visual field, sex, and hand preference Source Visual field (F) Sex Hand preference (HP) F x Sex Sex x HP F x HP Sex x HP x F

F

P

9.6268 7.7010 0.0195 0.1416

0.0034 0.0081 0.8895 0.7086 0.1782 0.6004 0.2875

1.8725 0.2785 1.1596

The table shows significant main effects of visual field and sex. For RVF trials the overall mean error was -1.07” (SD = 1.47), and for LVF trials the mean error was -0.46” (SD = 1.40). Thus, there is a significantly greater leftward error in the RVF, although the magnitude of the difference is small in actual units of measure. Men made significantly greater leftward errors than did women. The mean for men was -1.31” (SD = 1.62), while for women the mean error was -0.46” (SD = 1.26). There was no significant effect of hand preference, and no significant interactions among any of the main factors. A different algebraic convention was used for representing errors in the rod-frame condition, In order to permit combining the data from right and left frame tilts, a “+” was used to represent an error in the direction of the frame tilt, and a “-” was used to represent errors opposite the direction of frame tilt. For each subject errors scored in this way were averaged separately for right and left visual fields. The means and standard deviations are presented in Table 3. The results for the rod-frame condition were evaluated with the same three-way ANOVA as that used on the no-frame data. The results appear in Table 4. The only significant effect was for sex, women making significantly greater errors than men. Relationships between accuracy and RT measures Two separate questions about RT-accuracy relationships need to be raised. First is whether trade-offs between speed and accuracy occurred. The possibility of such trade-offs

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Table 3. Mean and standard deviation of error scores in the rod-andframe condition LVF Males Right-handed Left-handed Females Right-handed Left-handed

RVF

X 2.87” 3.68”

SD 1.54 1.86

X 2.82” 3.52”

SD 2.07 2.01

4.16” 5.13”

3.06 2.32

4.35” 5.12”

2.20 3.27

Table 4. Three-way analysis of variance on mean constant errors in the rod-frame condition. Factors are visual field, sex, and hand preference Source Visual field (F) Sex Hand preference (HP) Sex x F Sex x HP F x HP Sex x HP x F

F

P

0.084 5.343 1.337 0.613 0 0.291 0.065

0.713 0.026 0.254 0.438 0.997 0.592 0.799

is inherent in any task involving choice reaction times. Second is the question of whether visual field dz#kences in RT and accuracy are related. A check on speed-accuracy trade-offs was made by finding the product-moment correlation between mean RTs and mean unsigned errors in the judgment of vertical. For both the no-frame and the rod-frame conditions, separate correlations were calculated for right and left visual field trials. In the no-frame condition, the correlations for LVF and RVF trials, respectively, are -0.273 and -0.310. Both of these are significant with P -C 0.05. In the rod-frame condition the corresponding values were 0.086, and -0.054. Neither of these is significant. To determine how visual field difirences in RT and accuracy relate, RVF-LVF scores on both measures were obtained for each subject in the no-frame condition. Such differences are taken as measures of the extent of lateralization in each variable. Product-moment correlations were then obtained. This was done separately for men and women, since these groups had been found to differ significantly on the accuracy variable. For men r = 0.011, and for women r = -0.109. Neither of these was significant. Thus, although significant visual field differences had been found for both RT and accuracy in this condition, the magnitudes of lateralization reflected in such measures are uncorrelated. No comparable computations were carried out on rod-frame data, since no significant visual field accuracy differences had been found in this condition. Estimation of inter-hemispheric transfer time The conditions of the experiment were arranged so that a two-way transfer of information between hemispheres would be required if any critical processing operating were localized in the hemisphere contralateral to the one receiving the primary stimulus input. All other things equal, the mean RT difference between RVF and LVF trials should reflect this twoway transfer. To determine whether the obtained RT differences are consistent with such an interpretation, an independent estimate of interhemispheric transfer time can be generated

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from the simple RT data that were gathered originally for purposes of equating hand speed. The rationale for this approach was developed by POFFENBERGER [16], and is as follows. Since motor control of a given hand originates in the contralateral hemisphere, a stimulus delivered to the ipsilateral hemisphere would require some form of information transfer to the opposite hemisphere before a motor response could be initiated. In a simple RT paradigm, RT differences between RVF and LVF stimulus presentations would reflect this one-way transfer of information. A conservative estimate of two-way transfer time could be made by simply doubling the one-way estimate. (The estimate would be conservative because it ignores possible additional time constants for organizing and transforming the information that is relayed back and forth in the two-way choice RT paradigm.) From the simple RT data in the present study the mean visual field difference in RT, averaged over subjects and hands, is 5.5 msec. This is comparable to values obtained by other investigators: POFFENBERGER [16], 6 msec; SMITH [17], 34 msec; BERLUCCHI et al. [18],3 msec. Doubling this yields a two-way estimate of 11.Omsec. Ninety-five per cent confidence limits for this estimate are obtained from the standard error of the one-way measures (3.24 msec). These limits thus define an interval between 4.65 and 17.35 msec. Although near the upper limit, the visual field differences in the no-frame condition and in the “incremental” RTs both lie within this interval. DISCUSSION The accuracy measures are in substantial agreement with the results of many previous studies. HOWARDand TEMPLETON[19] have reviewed a large body of literature on perception of the vertical in the absence of a visible reference framework. Various estimates from their review put the average unsigned error at about l”, with group averages ranging from as little as 3 to about y. The overall average of male and female subjects from the present study was 0.89” left of true vertical, thus falling within the range cited by HOWARD and TEMPLETON [19]. The rod-frame data are similarly comparable. The mean errors for men and women, respectively, were 3.22” and 4.74”. This compares to corresponding group averages of 3.09“ and 4.99” obtained by the author in a previously published study [20] in which similar RVF and LVF presentations were employed, with unlimited viewing time, and a conventional portable rod-and-frame device [21]. In both studies the sex differences were significant, a finding which occurs commonly in rod-and-frame studies (cf. MACCOBYand JACKLIN[22]). The comparability of error scores between the present study and other previously published results appears sufficient to support generalizations despite rather large operational differences. It also implies that the essential information processing operations can be accomplished within the brief time span of the reaction time paradigm without appreciable changes in outcome. The major thrust of this study was toward discovering evidence for cerebral asymmetries in visual orientation perception. Such asymmetries do indeed appear in the data, and their direction and nature depend upon whether the rod-and-frame configuration or a presentation without visual reference axes is used. The results of the “no-frame” condition will be disrussed first. Two different results appeared in the no-frame condition. The reaction time data showed a significant RVF advantage of 14.4 msec, suggesting significant left hemisphere involve-

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ment in this task. This represents a notable exception to current generalizations concerning right hemisphere superiority in visuospatial abilities. Accuracy, on the other hand, was found significantly greater in the LVF trials, theconstant error being 0.46” left, as compared to 1.0’7” left in the RVF trials. There was no significant correlation between the visual field differences in accuracy and RT, however, indicating that the opposing directions of differences are probably not related to differential tendencies toward speed and accuracy trading. A tentative interpretation can be made that the RT differences represent lateralization in the locus of some critical processing operation as opposed to a difference in the efficiency or mode with which the two hemispheres perform the function. The latter interpretation cannot be evaluated on the basis of the magnitudes of a RT differences alone, since there is no a priori basis for quantitatively predicting the magnitude of an efficiency difference. In the “localization” view, however, an RT difference is thought to represent a time constant for the organization and transfer of information across the commissures on those trials in which the stimulus input is delivered to the “wrong” hemisphere. Based on the independent confidence interval estimate of interhemispheric transfer time the RT difference in the noframe condition is consistent with predictions based on a hypothetical localized process. The accuracy difference seen in this study does not appear related to the cerebral lateralization of any important part of the orientation judging process. If RT differences are correctly interpreted as measures of lateralization, then the absence of any significant correlation with accuracy measures helps make this point. Also, the standard deviations of RVF and LVF measures are virtually identical, 1.47 and 1.40, respectively. This shows that there is no difference in uncertainty or variability as might be expected if differences in some higher order organizational process underlay these accuracy differences. Both the RVF and LVF means represent small constant errors in the estimation of vertical. A difference of similar magnitude was found in comparing the means of male and female subjects. This, too, appears independent of differentially lateralized operations, since there was neither a main effect of sex, nor any significant interaction tetween sex and visual field, in the RT measures. The mean constant errors for both sexes and for both visual fields fall within the general range of “no-frame” accuracy estimates [19]. Perhaps it may be simplest to look for small but systematic differences in the initial registration of visual inputs as the major contributor to accuracy differences of the present magnitude. Left hemisphere localization of some important part of a spatial orientation judgment runs counter to the conventional generalization concerning right hemisphere superiority in visuospatial functioning. In trying to determine just what this critical operation is, one might first consider a type of function which is popularly attributed to the left hemisphereverbal processing. If the experimental task involved verbal mediating processes the superiority of the lefthemisphere would be expected. It has been pointed out that when verbal labelling of supposedly nonverbal materials (geometric forms, for example) is possible, the verbal representations themselves become available for processing [23]. FONTENOT [24]has argued that uncontrolled verbal encodability may obscure possible right hemisphere contributions to shape discrimination by introducing a left hemisphere bias. UMILTA et al. [25] have shown that this sort of effect can occur in the judgment of line orientation. In their study, when two comparison groups of tilted lines were easily verbally represented, e.g. with labels like “vertical-left oblique vs horizontal-right oblique”, shorter RTs were found in RVF trials. As the encodability of the stimuli decreased this trend tended to reverse. The authors

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envisioned a process by which the subjects would create and store verbal representations of the comparison stimuli, generate a similar label for the flashed test stimulus, and then match the verbal representations. It is unlikely, however, that the present RVF effect involved the sort of verbal mediating operation hypothesized by UMILTA et al. [25]. In the present case the subject cannot attach the label “left” or “right” to the stimulus input until the comparison to his criterion for vertical has been made. Thus, verbal labelling is not a means by which the subject can mediate a comparison, but rather, is contingent upon his making the comparison. task that deserves attention. This is the imThere is another aspect of the “no-frame” portant role of the presence vs absence of visual reference axes, since it is this difference in frame cues that is most obviously related to the direction of visual field differences in RT. Gravitational orientation of a single line on the retina is inherently ambiguous-the orientation corresponding to vertical varies depending upon head and eye orientation when the stimulus is registered. Thus, in the absence of any visible reference axes, correct discrimination requires some non-visible discriminative stimulus, presumably an input from one or more postural senses. Thus, the necessity of joint use of visual and postural cues may be a key factor in determining the visual field asymmetry. Since the discrimination is no longer solely visual, the expectation of right hemisphere advantage may not hold. Other evidence to this effect was shown by POHL, BUTTERSand GOODGLASS[26], who found better left hemisphere performance in a dot-localizing task, as long as there were no visual reference features present. When they added visual reference markers, the hemispheric difference was no longer observed. Some additional researches into lateral asymmetries in spatial discriminations, with and without visual reference cues, seem highly worthwhile. In the rod and frame trials significant visual field differences in overall RTs did not occur. Still, a case can be made that there is an underlying tendency toward hemispheric specialization of one or more of the cognitive operations required in this condition. This argument is based on the idea that the postural cues employed under no-frame conditions continue to function as discriminative stimuli in rod-frame trials. In rod-frame tests, mean errors are seldom equal to the full magnitude of frame tilt (cf., for example, WITKIN, eta]. [27]). While the tilted frame clearly exerts significant stimulus control it may be inferred that postural variables also exert control, and that the overall judgment reflects this joint control. Therefore, the information processing operations in a rod-frame trial must include both the no-frame operations, plus extra steps required by the addition of discordant visual reference frame cues. Since a significant right visual field advantage is observed in the no-frame condition, it may be expected that this component of the overall rod-frame RTs will continue to be faster on right than on left visual field trials. If this is the case, then the lack of a visual hemifield difference in the overall RTs may be interpreted as a nulling effect due to a left visual field advantage in one or more of the “extra” processes in rod-frame trials. This interpretation yields the testable hypothesis that the differences between mean RTs on the no-frame trials and on rod-frame trials will show the predicted left visual field advantage, i.e. a smaller increment in RTs from LVF trials. The analysis of “incremental” RTs confirmed this hypothesis. The foregoing analysis suggests a view of rod-frame performance in which the conflicting postural and visual frame cues are mediated by mechanisms lateralized in opposite hemispheres-the left for postural cues, and the right for the visual frame cues. Since the final outcome almost always reflects joint control by both types of cues, some inter-

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hemispheric balancing or weighting is also implied. This general picture is consistent with results of COHEN et al. [14] who found reduction of frame-dependent errors in patients who received ECT to the right hemisphere, and increased errors following ECT to the left hemisphere. Since ECT is a highly interventive treatment, it is likely that the typical processes of the treated hemisphere would have been disrupted selectively for a short time after treatment. Thus, left hemisphere ECT would be expected to disrupt the postural cue-processing mechanisms, shifting the balance of stimulus control toward the right hemisphere, which presumably provides the frame information. The result is the increase in frame-dependent errors. Exactly the reverse would be expected after right hemisphere ECT: the balance of stimulus control should be shifted temporarily to the left hemisphere. Consequently, frame-dependent errors should diminish. COHEN et al. [14] expressed surprise at this latter result in their own study, but perhaps the present reaction time data contribute to an appropriate interpretation. The incremental RTs reflect the occurrence of additional cognitive operations required to deal with the additional information introduced by the tilted frame. These would include determining the relative orientations of the rod and the frame, detecting any conflict between frame cues and postural cues, and resolving any such conflict (presumably by the sort of weighting process discussed above) so that a single R or L response can be made. The present design does not offer any direct evidence to help choose among these possibilities or reveal others. However, further studies aimed at deriving separate reaction time constants for these various components of the incremental RTs are currently being designed and carried out. Their results should clarify this issue further. No significant effects of either sex or hand preference appeared in the RT data, nor did either factor interact significantly with visual field. This is somewhat surprising in that an abundant number of studies have suggested that differing degrees of lateralization are associated with these factors. For example, men show greater lateral asymmetry than women, as determined from dichotic listening tests [28], or in deficits following brain injury [29]. Similarly, right-handers have shown greater degrees of asymmetry than left-handers, most notably in tests for the lateralization of speech (cf. ANNETT [30]). LEVY [31] showed significantly lower WAIS performance I.Q.‘s in left-handed subjects than in right-handed subjects. As these latter scores are thought to reflect mainly right hemisphere function, a lesser degree of right hemisphere lateralization in left-handed subjects was inferred. Further experimental comparisons of processing requirements in the present tasks, where handedness does not show an effect, with requirements in other tasks where it does, should help delineate important aspects of cognitive organization in right- and left-handed persons. Similar comparisons should also contribute to better understanding of sex differences. Acknawledgemenrs-The author is greatly indebted to many members of the Psychology Department of the University of Hartford, most especially Dr. HARRY LEONHARDT and Dr. JAMES MATTHEW, for their generous sharing of equipment and space for use in this experiment and for much help in the recruitment of subjects. Special thanks are also extended to Mr. JON SHAPIRO and Mr. MICHAEL PETRIDES for their help in running the subjects and reducing the data.

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1969.

COLIN PITBLADO

392

On a mesurE les par rapport et gauche.

1 la verticale droie.

Lorsqu’un

blton-cadre,

significative

visuelle, cadre carr6

projetds

les TR 6taient inclind

Btait

L’exactitude

avec des expositions

etait

dans cette

de la coop&ration

plus rapides ajoutd

sans limitation

classe

de jugements

interh6misphGrique

spatiaux.

dans le

simulanc la confi-

comparable

avec absence de cadre suggPrent que les mkanismes

sent impliques

dans les h6michamps droit

les TR augmeneaiene pour les 2 champs visuels

dans le champ droit.

de faqon conveneionnellf tsts

(TR) pour le jugement d’inclinaison

de segments de ligne

Sans axe de Gfkence

champ visuel guration

temps de r&action

et de faGon

B celle

obtenue

de temps. Les r6sul-

de 1’hEmisphsre

gauche

On donne une incerpr6tation

101s de la performance

bfton-cadre.

Deutschsprachige Zusammenfassung: Reaktionszeiten wurden fiirBeurteilungen von vertikal gekippten oder schrlgen Liniensegmenten (in das rechte und linke Gesichtsfeld hineingeblitzt) registriert. Ohne visuelle Bezugsachsen waren die Reaktionszeiten schneller fib-das rechte Gesichtsfeld. Wenn ein gekippter viereckiger Rahmen hinzugegeben wurde, stiegen die Reaktionszeiten fiir beide Gesichtsfelder an und mit einem signifikant grSBeren Wert in der rechten Gesichtsfeldhalfte. Genauigkeitsmessungen waren vergleichbar jenen, die konventionell mit unbegrenzten Expositionszeiten erhalten wurden. Das Ergebnis bei Gesichtsfeld ohne Rabmen weist auf eine Beteiligung des linkshemispharischen Mechanismus in dieser Klasse von raumlichen Beurteilungen, und es wird die Auffassung einer interhemispharischen Kooperation bei der Verbindungsstab-Rahmen-Leistung dargelegt.