Sorting of Hemifield Presented Temporal and Spatial Stimuli

Sorting of Hemifield Presented Temporal and Spatial Stimuli

SORTING OF HEMIFIELD PRESENTED TEMPORAL AND SPATIAL STIMULI 1 Rachel Brandeis and Harvey Babkoff (Department of Psychology, Bar-Han University, Ramat-...

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SORTING OF HEMIFIELD PRESENTED TEMPORAL AND SPATIAL STIMULI 1 Rachel Brandeis and Harvey Babkoff (Department of Psychology, Bar-Han University, Ramat-Gan, Israel)

One of the basic assumptions related to hemispheric asymmetry is that the left cerebral hemisphere (LH) subserves advanced verbal processing for most right-handed individuals, while the right hemisphere (RH) is involved mainly in spatial processing and in nonverbal, or only primitive verbal processing. Evidence for this conceptual organization of cerebral asymmetry arises from several sources: (1) Clinical studies of patients with RH or LH lesions (Bryden, 1967; Bogen, 1969; Sperry, Gazzaniga and Bogen, 1969; Zaidel and Sperry, 1973, De Renzi, 1978; Geschwind, 1979); (2) Clinical studies of split-brain patients (Bogen and Gazzaniga, 1965; Sperry 1969; Gazzaniga, 1970; Nebes, 1973; Zaidel and Sperry, 1973; Gazzaniga and Le Doux, 1978; Zaidel, 1978a, 1978b, 1979); (3) Normative studies, which are designed to allow the identification of the initial hemispheric processing of the stimuli, including dichotic listening, tachistoscopic and somatosensory paradigms (Kimura, 1966; Murphy and Venables, 1970; Studdert-Kennedy and Shankweiler, 1970; Geffen, Bradshaw and Wallace, 1971; Geffen, Bradshaw and Nettleton, 1972; Bryden, 1973; Kimura and Durnford, 1974; Bradshaw, Gates and Pat­ terson, 1976; Ornstein, Johnstone, Herron and Swencionis, 1980; Sidtis, 1980). Several studies have compared the performance of subjects on tasks involving verbal and spatial stimuli and have interpreted their results as evidence of hemispheric functional specificity. The difficulty with inter­ preting such studies is the problem of the confounding of stimulus qual­ ities with the cognitive demands of the task (Geffen et al., 1972; Segalo­ witz and Stewart, 1979). Several investigators have suggested that the superiority of the LH for processing verbal information may reflect a more primary specialization for processing rapidly changing temporal cues of which speech is perhaps the best example. Accordingly, certain nonverbal processing, requiring 1This study was partially supported by an Israel Foundations Trustees grant, David Rockefeller Fund for Doctoral students to the senior author.

Cortex (1984) 20, 179-192

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rapid and/or sequential analysis, may also occur preferentially in the left hemisphere of right-handed individuals (Efron, 1963a, 1963b; Tallal, 1980; Tallal and Schwartz, 1980). Several studies investigated the temporal aspect of this temporal/ spatial specialization dichotomy and found a LH superiority in a variety of tasks: Perception of simultaneity (Efron, 1963a), temporal-order dis­ crimination (Efron, 1963b; Carmon and Nachshon, 1971; Swisher and Hirsh, 1972; Halperin, Nachshon and Carmon, 1973; Tallal and Piercy, 1973; Carmon, 1978), critical flicker frequency (Goldman, Lodge, Ham­ mer, Semmes and Mishkin, 1968), temporal-interval discrimination (Van Allen, Benton ·and Gordon, 1966), voice-onset-time detection (Molfese, 1980), monaural and binaural thresholds for temporal order judgements of two tones (Mills and Rollman, 1980; Sherwin and Efron, 1980), and offsets of tones (Emmerich, Pitchford, Joyce and Koppell, 1981). How­ ever, no studies to date have used paradigms which allow a direci com­ parison of the spatial versus temporal dimensions of visual stimuli. Car­ mon (1978) compared subjects' performance using temporal and spatial stimuli but his paradigm did not differentiate between the two stimuli sufficiently to allow a clear evaluation of hemispheric functional specif­ icity (see below for a detailed argument). The present study was designed to investigate the hypothesized hem­ ispheric functional specificity with regard to a temporal/ spatial dichoto­ my, i.e., with regard to the processing of visual stimuli which differ along a temporal or a spatial dimension. Subjects were presented with either a temporal or a spatial visual stimulus (described below) to one of the two hemifields on each trial and required to identify the type of stimulus (either temporal or spatial) in a choice reaction-time (RT) paradigm. The task was, therefore, a sorting task with an accuracy and RT measure. The hypothesis tested with this paradigm is whether the correct decision to a temporal stimulus will be shorter if it is presented to the right visual field (with initial neural processing in the LH) than if it is presented to the left visual field. Conversely, will the correct decision to a spatial stimulus be shorter if it is presented to the left visual field (with initial neural pro­ cessing in the RH) than if it is presented to the right visual field.

MATERIALS AND METHOD

Subjects

Ten subjects, eight females and two males, participated in the study. All subjects were right-handed (scored + 100 on the Edinburgh Handedness Inven­ tory; Oldfield, 1971).

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Apparatus

Stimulus durations, the timing of the random sequences of the different conditions (Stimulus Type, Stimulus Size, Visual Field) within a trial, as well as the measurement of response accuracy and reaction-time, were controlled by a PDP-11E40 mini-computer. Stimuli

The two spatial and two temporal stimuli were produced by boring holes in the upper left and right quadrants of four black metal plates, which were then placed in the path of the four-fields of a Harvard Tachistoscope (Model T-4A). Illuminating these fields produced the spatial and temporal stimuli. The holes were bored in the diagonal plane on a 40° angle to avoid the right-left dilemma (Goldman, Lodge, Hammer, Semmes and Mishkin, 1968). The closest hole was bored approximately 8° to the right or left of center of the visual field. Each of the two spatial stimuli consisted of a pair of physically separate lights (two holes bored in the metal plates), subtending a 0.44° visual angle (3.5 mm). The distance between the centers oLthe two lights composing the "small" sti­ mulus pair was 1.31° ( 1.05 em). The.distance between the centers of the pairs of lights composing the "large" stimulus pair was 1.74° (1.40 em). The two light spots composing the spatial stimulus were illuminated simultaneously for 100 msec. (Efron, 1963a, 1963b). Each of the two "temporal" stimuli consisted of a single light, subtending 0.44° visual angle (3.5 mm), illuminated twice on each trial. The on-time duration for each of the members of a temporal pair of stimuli was 50 msec. The two stimulus pairs differed in terms of the length of the off-time separating the members of a pair, either 100 msec. (the "short" stimulus) or 150 msec, (the "long" stimulus). Light, intensity was 0.01 ml. Rise-time and fall-time of the stimuli were nominally 20 microsec. Procedure

The two stimuli, spatial and temporal, were presented to each subject, to the right visual field (RVF) and to the left visual field (LVF) of each eye, separately. In half of the sessions the subject viewed the stimulus with his right eye only, and in the other half, with his left eye only. In summary, each subject was presented with 16 different experimental conditiops: Eye X Visual Field X Stimulus ' (spatial/temporal) X Stimulus Size. Response accuracy and reaction times were measured by three horizontal electronic contact keys. A trial began with the subject's index finger placed on the central key. When a stimulus was presented, the subject was instructed to lift his right index finger from the central key, as quickly as possible, and to indicate the type of stimulus presented, by moving his finger as quickly as possible, to the right or left electronic contact key. 2 As noted above, this is essentially a sorting task. Subjects were assigned randomly to one of two groups (four females and one male to each group), differing in the direction of response to each of the two types

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of stimuli presented. One group responded by moving the index finger to the right to indicate a "spatial" stimulus and to the left to indicate a "temporal" stimulus; the other group responded in the opposite direction. The subject was seated facing the Tachistoscope with his head placed in the viewing hood. The subject was dark-adapted for ten minutes before the session began. The distance of the subject's eyes from the visual field was 46 em. The session began by instructing the subject to maintain his gaze on a fixation light, subtending a 0.44° visual angle (3.5 mm) placed in the center of the visual field, and not to move his eye to the right or to the left once a trial began. A trial began by warning the subject verbally of the onset of the trial. Each subject participated in eight sessions of approximately 280 trials each. The data for each of the 16 experimental conditions are based approximately on 125 trials for each subject. Only RTs within the range of 250 to 900 msec. were averaged for each subject for each condition. Excluded data ranged from 0 to 2% for all the conditions. All the subjects were trained for several sessions prior to data collection. RESULTS

The two dependent variables, error rate and choice reaction time (RT) were analyzed separately. Error Rate The error rate is very small, approximately between 2% and 5% over all the experimental conditions. Error rate data were analyzed by a 5-way ANOVA with 4 repeated variables (the type of stimulus (T), the size of stimulus (S), the viewing eye (E), the visual field (VF) and one nonrepeated variable, the group (G), i.e., the direction of response to the type of stimulus). The results indicate that none of the main effects are significant. · Despite the overall small error rate, one of the interactions is signif­ icant, i.e., the type of stimulus (T) (temporal/spatial ) X visual field (VF) (F = 7.20; d.f. = 1, 7; p < 0.03). This interaction accounts for 36% of the variance and is presented in Table I. TABLE I

Error Rate of the Spatial Stimulus and of the Temporal Stimulus Presented to the Right­ Visual Field and to the Left-Visual Field

Visual field Right visual field Left visual field

Type of stimulus Temporal

Spatial

2.32 3.30

4.74 3.49

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Table I indicates that the error rate of the temporal stimulus presented to the right visual field (RVF) is smaller than the error rate of the same stimulus presented to the left visual field (LVF). The error nite to the spatial stimulus presented to the LVF is smaller than the error rate of the same stimulus presented to the RVF (p < 0.01, by contrasts analysis, simple main effects; Winer, 1962). Choice Reaction-Time (RT) The choice RT data were analyzed by a 5-way ANOVA, with 4 repeated variables (the type of stimulus (T), the size of stimulus (S), the viewing eye (E), the visual field (VF) and one nomepeated variable, the group (G), i.e., the direction of response to the type of stimulus). The results indicate that none of the main effects are significant. Four of the interactions are significant, however: 1. The type of stimulus (T) X visual field(VF)(F = 99.22; d.f. = 1, 8; p < 0.0001); 2. The type of stimulus(T) X group (G) (direction of response to the type of stimulus) (F = 16.92; d.f. = 1, 8; p < 0.003); 3. Thevisualfield(VF) X group(G)(ditectionof response)(F = 7.81; d.f. = 1, 8; p < 0.02); 4. Thevisualfield(VF) X the sizeofstimulus(S) X group(G)(directionofresponse)(F = 20.84; d.f. = 1, 8; p < 0.01). The interaction between the type of stimulus (T) X visual field (VF) accounts for 34% of the variance and is illustrated in Figure 1, with group Fig. 1 -Reaction time (RT) is plotted, on the ordinate in msec. as a function of the visual hemifield (RVF, LVF). Data are plotted separately for the temporal and spatial stimuli.

(] 540
o---oTEMPORAL

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-SPATIAL

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iii ~

i=

z 520

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500 RVF

LVF VISUAL HEMIFIELD

Rachel Brandeis and Harvey Babkoff

184

average R T plotted on the ordinate as a function of the two visual fields (VF) on the abscissa, with the type of stimulus (T) (temporal/ spatial) plotted as parameter. Figure 1 indicates that RT to the temporal stimulus presented to the RVF is "V38 msec. shorter (505 msec) than the RT to the same stimulus presented to the LVF (542.9 msec.). RT to the spatial stimulus presented to the LVF is "V 21 msec. shorter ( 502.7 msec.) than the RT to the same stimulus presented to the RVF (523.9 msec.) (p < 0.01 by contrasts analysis, simple main effects; Winer, 1962). These data also indicate that the difference between the RT to both visual field. for the temporal stimulus is "V 16.5 msec. greater(38 msec.) than the difference between the RT to both visual fields for the spatial stimulus (21.5 msec.) (p < 0.01 by contrasts analysis, si~ple main effects; Winer, 1962). Viewed from a different perspective, RT to a temporal stimulus presented to the RVF is "V 19 msec. shorter than to a spatial stimulus presented to the same visual field; while RT to a spatial stimulus presented to the LVF is "V40 msec. shorter than to a temporal stimulus presented to the same visual field. The interaction between the type of stimulus {T) X group (G) (di­ rection of response) accounts for 20% of the variance and is showh in Table II. While both groups yield approximately the same RT to the temporal stimulus, the two groups differ significantly with respect to their RT to the spatial stimulus, "V46 msec. shorter for group 2 (response to the left for the spatial stimulus) than for group 1 (response to the right for the spatial stimulus). In addition, the RT for group 1 to the temporal stimulus is approximately 12 msec. shorter (response to the left) than to the spatial stimulus (response to the right); while for group 2, RT to the spatial stimulus (response to the left) is approximately 33 msec. shorter than to the temporal stimulus (response to the right). An analysis of contrasts (simple main effects; Winer, 1962) indicated that RT to the two types of stimuli differ significantly for each of the groups (p < 0.01).

TABLE II

RTs, in msec., of Groups 1* and 2** to the Temporal and Spatial Stimuli

Groups Group 1* Group 2** *Group 1: Spatial stimulus ­ right temporal stimulus ­ left **Group 2: Spatial stimulus ­ left temporal stimulus ­ right

Type of stimulus Temporal

Spatial

524.45 523.52

536.14 490.43

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The interaction between the visual fields (VF) X group (G) (direction of response to the type of stimulus) accounts for 4% of the variance. While for group 1 the overall RT to a stimulus appearing in either the left or the rightvisual fields is approximately the same, for group 2 the overall RT to a stimulus for the RVF is approximately 19 msec. shorter than to a stimulus appearing in the LVF. An analysis of contrasts (simple main effects; Winer, 1962) indicated that the difference is only significant for group 2 (p < 0.01). · The interaction of visual field (VF) X size of stimulus (S) X group (G) (direction of response to the type of stimulus) accounts for 7% of the variance. For group 1, RT to the smaller or shorter stimuli appearing in the RVF is "-' 19 msec. longer than to the larger or longer stimuli in the same visual field, whereas the R T to the smaller or shorter stimuli pre­ sented to the LVF is "-' 13 msec. shorter than to the larger or longer stimuli in the same visual field. For group 2 the tendency is the opposite: While R T to the smaller or shorter stimuli appearing in the R VF is 15 msec. shorter than to the larger or longer stimuli in the same visual fild, RT to the smaller or shorter stimuli presented to the LVF is "-' 5 msec. longer than to the larger or longer simuli in the same visual field. An analysis of contrasts (simple main effects; Winer, 1962) indicated that all of the differences are significant (0.025 < p < 0.005). Separate analyses for right-directed and left-directed movements

Separate analyses for each of the two directions of response were performed, because of the interaction between the direction of response X the type of stimulus, which was highly significant (see Table II). One analysis was performed for all right-directed movements across both groups and another for all left-directed movements across both groups. For both right- and left-directed movements, the error rate to the tem­ poral stimulus presented to the RVF is always lower and the RT shorter than when presented to the LVF, while the error rate to the spatial stimulus presented to the LVF is always lower and the R T shorter than when presented to the RVF. The differences between the error rate and the RT to temporal stimuli presented to the RVF and LVF are significant (p < 0.01). The difference in RT is quantitatively similar for both right­ and left-directed movements (42 and 34 msec., respectively). However, with regard to the spatial stimulus, the results for the two response directions differ. The differences in error rate and in RT to the spatial stimulus presented to the RVF and LVF are significant for the right­ directed movement. The difference in RT is approximately 37 msec. (p < 0.01). However, with regard to the left-directed movement, only the dif­

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ference in error rate to the spatial stimulus presented to the RVF and LVF is significant. The difference in RT to the spatial stimulus presented to the RVF and LVF, although in the appropriate direction (favoring the LVF), is only 6 msec. (p > 0.05). Several other interactions, explaining less than 5% of the R T variance, are also found, when the two response directions are analyzed separately, including the variables: Type of stimulus, visual field, and stimulus size. DISCUSSION

Several results of this study relate directly to the hypothesis raised in the Introduction and were, therefore, predicted. Other results, not pre­ dicted, relate to interactions involving the direction of response and certain stimulus and visual field parameters. Stimulus (Spatial! Temporal) X Visual Field

The major results of the present study support the basic hypothesis: i.e., more accurate and shorter decision times (RTs) to a temporal stimulus appearing in the right-visual-field (RVF) than when it appears in the left-visual-field (LVF). Conversely, more accurate and shorter RTs are found to a spatial stimulus appearing in the LVF than when it appears in the RVF. This interaction accounts for the major portion of the variance for both error rate (36%) and RT (34%). Although choice RT paradigms have been used in lexical or gramma­ tical decision-time tasks and in verbal versus face-recognition tasks, the present report is the first study using a choice RT measure with visual stimuli, which compares the performance of normal subjects, to both spatial and temporal stimuli. The manual choice RT measure yields larger hemispheric differences than the response accuracy measure, and, per­ haps, provides a more subtle measure of hemispheric differences (Day, 1977; Bradshaw and Gates, 1978; Babkoff, Ben-Uriah and Eliashar, 1980). The results of the present study which indicate preferential initial processing of spatial stimuli by the right hemisphere (LVF) and prefer­ ential initial processing of temporal stimuli by the left hemisphere (RVF) of normal right-handed individuals may, at first, seem to be in conflict with the results of studies by De Renzi, Faglioni and Previdi (1977) and Kim, Royer, Bonstelle and Boller (1980) on right hemisphere lesioned patients which indicate deficit in sequencing ("temporal processing") in these patients. Kim et al. (1980) even suggest that " ... the left hemisphere is

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responsible for verbal sequencing ability while the right hemisphere is responsible for nonverbal sequencing ability...". However, an examina­ tion of the tasks used by those researchers may suggest an explanation for this apparent discrepancy. The sequencing deficit reported in right-hem­ isphere lesioned patients in De Renzi et al.'s (1977) and Kim et al.'s (1980) studies was obtained for purely spatial stimuli (Swisher and Hirsh, 1972), cubes and blocks were to be arranged in a certain order in space. Fur­ thermore, the spatial component was not only dominant in the stimuli but also in the response (requiring the subjects to repeat the examiner's tapping or to point to the spaced cubes and blocks in exa~tly the same order), so that the demand characteristics of the task (Wolff, 1980) were designed to direct the subjects to adopt a "spatial" or an "holistic" cognitive strategy and therefore to show a right-hemisphere superiority. The present study is relatively free of this type of spatial component in the "temporal" stimulus and is able to encourage a "choice" strategy which could differentiate "spatial" from "temporal" dimensions. In addition, various other aspects of the stimulus and response var­ iables used in this study serve to differentiate it from other studies designed to compare right- and left-hemispheric functioning. Verbal versus nonverbal stimuli and tasks

The present study avoids the problems of unspecified cognitive demands (Geffen et al., 1972; Segalowitz and Stewart, 1979) which complicate the interpretation of studies using verbal versus nonverbel (e.g. facial, spatial) stimuli. Whatever "selfcueing" the subjects used (e.g. verbal or nonverbal) should be equally applicable to both types of stimuli, spatial and temporal. Complex versus simple stimuli and tasks

The use of unidimensional stimuli for discrimination in a relatively simple sorting task minimizes the problem of unspecified processes for correct performance which might be involved in other studies which used either complex stimuli (Carmon, 1978) or complex tasks (Carmon and Nachshon, 1971; Halperin, Carmon and Nachshon, 1973). Range of simuli and tasks

The present study avoids the introduction of an additional variable, short term memory, which may influence the results. Some of the previous studies (De Renzi et al., 1977; Carmon, 1978; Kim et al., 1980) used either

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relatively long (seconds) stimulus sequences and/or methodologies (De Renzi et al., 1977; Kim et al., 1980) requiring trial lengths of at least seconds. By using relatively short stimulus durations and trials of less than a second length, the present study emphasized more perceptual than memory processing. . The difference between RTs to the temporal stimulus presented to the RVF and LVF was significantly greater (38 msec.) than the comparable difference between the RTs to the spatial stimulus presented to the RVF and LVF (21.5 msec.). The "temporal specialization" of the left hemi­ sphere stands out much bolder than the "spatial .specialization" of the right hemisphere. These findings might be interpreted in a framework of results which show (Le Doux et al., 1977; Bradshaw and Nettleton, 1981) that the right hemisphere superiority in manipulo-spatial functions is not a "proper" specialization but rather one which occurs "by default" in consequence of a language invasion of the left hemisphere space. " ...the differences reflect the differential extent of specialization in the left-hemisphere for time­ dependent analysis ..." (Bradshaw and Nettleton, 1981). These findings are also congruent with Zaidel's (1979) interpretation of his data that the differences which are found for the left hemisphere are much clearer, more stable, substantial and resist outside changes than those found for the right hemisphere, which are more diffuse and are prone to "nonrelevarit" changes in the conditions of the experiment. " ... The right hemisphere, in contrast to the left hemisphere solves problems in a characteristically nonconstructive manner, i.e., without having access to a model of its own solution process, which can be subsequently updated." (Zaidel, 1979). Direction of response X stimulus X visual field

A second set of findings relates to group-stimulus interactions in which group represents response direction. The RTs for group 1 to a temporal stimulus are shorter than to a spatial stimulus, while the R Ts by group 2 to a spatial stimulus are shorter than to a temporal stimulus. The response by group I to the temporal stimulus and by group 2 to the spatial stimulus are left directed. This interaction accounts for 20% of the RT variance. There is no such group-stimulus interaction with error rate. An additional finding, accounting for only 7% of the variance, is a group­ size-field interaction, i.e., a shorter RT to smaller stimuli appearing in the left-visual-field than when they appear in the right-visual-field group 1; while the converse result is found for group 2. The RTs to the larger stimuli are shorter when they appear in the left-visual-field, for both groups.

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Perhaps these results could be interpreted as indicating an absolute preferred left direction by both groups. The results of the preliminary experiment, argue against this finding being caused by artifact due to key asymmetry or to a general response direction bias unrelated to the sti­ mulus2. Also the seemingly preferred direction is opposite to that which may have been predicted by Clark and Clark ( 1977). Those authors reported that right banders process "right" faster than they process "left". Their argument is that since most people are right-handed, right indicates "positivity", and is an "unmarked" stimulus. A stimulus "coded" posi­ tively should result in a faster movement than one coded negatively, i.e., "marked". This theory cannot explain our findings, which indicate the opposite trend, i.e., a faster response to the left direction than to the right. Clark and Clark's theory also cannot explain our findings that the RT is shorter for stimuli presented to the right-visual-field than for stimuli presented to the left-visual-field, for group 2 only but not for group 1. This interaction, however, does not seem to represent an important variable, accounting for only 4% of the variance. In conclusion, we have no really good explanation for the interactions involving direction of response X stimulus X visual field. Methodology and Interpretations

Several issues relating to the experimental paradigm and stimuli should be raised. First, despite the instructions, subjects might have used a different strategy to sort the stimuli into the temporal versus the spatial categories. This alternative method could have involved the sorting of "one" stimulus (the single flickering "temporal" light) as versus "two" stimuli (the "two" "spatial" lights lit simultaneously). Were this the actual sorting method used by the subjects, the results would have been radically different. No interaction of visual field and stimulus would have been found, since all stimulus sorts would then have been spatial (one versus two on the spatial dimensions). Such a result in audition was reported by Murphy and Venables (1970), i.e., a faster RT to a single auditory stimulus than to a double stimulus. However, Murphy and Venables also reported that the discri­ mination between both types of stimuli was performed better when 2A preliminary control experiment was performed to test response-movement direction (simple reaction-time) from the central response key to the two response keys. Five subjects were instructed to respond to a single light stimulus, appearing in the center of the visual field by moving the index finger to the left key on one half of the trials and to the right key on the other half of the trials. No significant differences were found between the response times to the two response keys or between the two response directions (right or left).

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the left ear was stimulated than when the right ear was stimulated. Second, there was a time-difference between the durations of the spatial stimulus and the temporal stimulus, the spatial stimuli were pre­ sented for 100 msec. while the temporal stimuli were longer and were presented for 200 or 250 msec. The temporal stimuli were longer because the subjects could not detect the on-off-on intervals of shorter stimuli. Despite the difference in overall duration, however, the RTs to the two types of stimuli do not reflect these differences. Third, there was also a size difference, between the two spatial stimuli as well as a duration difference between the two temporal stimuli. Thus, in effect, the four stimuli differed perceptually, from one another. If this were the basis for discrimination, however, size as a main effect, should have been significant. Stimulus size is not significant as a main effect and only one of the three-way interaction, visual field X stimulus size X group, reaches significance. However, this three-way interaction accounts for only 7% of the variance. In summary, the most reasonable explanation of the data is that the results are consistent with the hypothesis that initial processing of the temporal dimension of visual stimuli is performed better by the left hemisphere than by the right hemisphere and that the converse is true for the initial processing of the spatial dimension of visual stimuli. ABSTRACT

An experiment was performed, using a sorting task (choice reaction-time), to study the processing of stimuli, which differed along a temporal or a spatial dimension, presented to the right or to the left visual hemifield. The results indicate more accurate responses and shorter reaction times to a temporal sti­ mulus when it appears in the right-visual-field than when it appears in the left-visual-field. Conversely, more accurate responses and shorter reaction times are found to a spatial stimulus when it appears in the left-visual-field than when it appears in the right-visual-field. In addition to this major interaction, three more interactions are found, all of which involve response direction and one or two other stimulus variables. The results are consistent with the hypothesized hem­ ispheric functional specificity, i.e., that the initial processing of the temporal dimension of visual stimuli is performed better by the left hemisphere than by the right hemisphere and that the converse is true for the initial processing of the spatial dimension of visual stimuli. REFERENCES

BABKOFF, H., BEN-URIAH, Y., and ELIASHAR, S. Grammatical decision time and visual hemifield stimulation. Cortex, 16: 575-586, 1980. BOGEN, J£. The other side of the brain. Bull. Los Angeles Neurol. Soc., 34: 73-105, Part I (a), 135-161, Part II (b), 1969.

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