Cerebral dominance and lateral differences in perception and memory

The present investigation was designed to replicate a previous finding that tachistoscopically presented complex forms of low verbal association value would be more accurately recognized when presented to the left visual field than to the right, thus implying dominance of the right hemisphere for the perception of this type of stimulus material. The successful replication of this finding was important because of previous unsuccessful attempts to show such an effect. This finding, in conjunction with the well established right visual field (i.e. left hemisphere) superiority for the perception of verbal material, strongly supports the hypothesis that asymmetry in human perceptual performance reflects hemispheric asymmetry of function rather than peripheral factors.The second question investigated concerned the role of memory in producing perceptual asymmetry. Complex figures were presented for 15–25 msec in either the left or right visual field; after a delay of 0–20 sec, the subject was required to indicate whether or not a form presented in central vision was the target form. The results indicate that the observed left visual field superiority for these complex forms appears to arise from hemispheric differences in memory rather than from purely perceptual processes.     

Hemispheric asymmetry for spatially filtered stimuli belonging to different semantic categories

The hemispheric specialization for processing visual stimuli as a function of spatial-frequency content and semantic category was investigated. Spatially filtered pictures of animals and tools were displayed to both hemifields at nine levels of spatial-frequency filtering following a coarse-to-fine design. Results showed a differential hemispheric specialization in relation to the semantic category of stimuli. Animals might be identified at low spatial frequencies (coarse information) in a similar extent in both hemifield conditions, while a hemispheric dissociation was found for tools (the increasing of levels of high-spatial frequency information was especially needed when such stimuli were presented to the left visual field /right hemisphere (RH) relative to that for right visual field/left hemisphere presentations).     

Transient interference of right hemispheric function due to automatic emotional processing

We examined the effects of emotional stimuli on right and left hemisphere detection performance in a hemifield visual discrimination task. A group of 18 healthy subjects were asked to discriminate between upright and inverted triangles (target). Targets were randomly presented in the left or right visual hemifield (150 ms target duration). A brief emotional picture (pleasant or unpleasant; 150 ms stimulus duration) or neutral picture selected from the International Affective Picture System was randomly presented either in the same (47%) or the opposite (47%) spatial location to the subsequent target. Emotional or neutral stimuli offset 150 ms prior to the subsequent target. Subjects were instructed to ignore the pictures and respond to the targets as quickly and accurately as possible. Independent of field of presentation, emotional stimuli prolonged reaction times (P < 0.01) to LVF targets, with unpleasant stimuli showing a greater effect than pleasant stimuli. The current study shows that brief emotional stimuli selectively impair right hemispheric visual discrimination capacity. The findings suggest automatic processing of emotional stimuli captures right hemispheric processing resources and transiently interferes with other right hemispheric functions.     

Hemispheric asymmetries in the perception of emotional and neutral faces

This study was designed to investigate hemispheric asymmetries in the perception of both positive and negative emotion, while minimizing extraneous factors known to favour right hemisphere processes. Pairs of faces (happy-neutral or sad-neutral) were presented, one to each visual field, and subjects responded to the face that made them feel either better or worse. Performance was superior when the emotional faces were presented to the left visual field-right hemisphere. This occurred in both the better and the worse conditions, and for both sexes. Indeed, the data indicate that all emotional stimuli were being processed by the right hemisphere, and that the effect for emotional faces is due to the expressions and not merely the faces themselves.     

Differences in hemispheric asymmetry between dyslexic and normal children on a lateralised lexical decision task

The present experiment was conducted in order to further investigate the relationship between deficits in left hemisphere processing and phonetic decoding in dyslexic children. We administered a lateralised lexical decision task that manipulated wordness, length, and word regularity of grapheme-phoneme conversion. Right-handed male dyslexic children and normal control children were presented with words and pronounceable nonwords. Although there were no overall differences in hemispheric asymmetry between the groups, they did differ in laterality effects in accuracy when responding to nonwords and to phonetically regular words, with the normal children showing the right visual field advantage/ sensitivity (left hemisphere dominance/sensitivity), while the dyslexics failed to show any visual field advantage or sensitivity for these stimuli. Further, group differences were observed in left but not right hemisphere functioning. The results suggest that deficits in left hemisphere processing are apparent only when the dyslexics are attempting to utilise the rules of phonics. Support for the use of this paradigm for use with dyslexic children is also discussed.     

Hemispheric differences for visual evoked potentials from checkerboard stimuli

Visual evoked potentials (VEPs) were elicited in right-handed male subjects with an alternating checkerboard pattern stimulus presented to either the left or right visual half-field. The sizes of the individual checks in different conditions were 0.25, 0.5, 2.0, or 4.0 cycles/degree of visual angle. The amplitude of the P100 VEP decreased while peak latency increased as check size decreased across both visual field conditions. Left hemisphere responses demonstrated significantly larger P100 amplitudes compared to the right hemisphere responses, although the interaction between hemisphere and stimulus size was not significant. No hemispheric effects of P100 latency were observed. The results suggest that the left hemisphere is engaged more than the right hemisphere for the sensory analysis of visual stimuli composed of straight edges over a wide range of spatial frequencies.     

Hemispheric differences in dot detection

Two experiments are reported using normal subjects which show a superior detection for dots in the left visual field. This is shown to be more pronounced for stimuli of reduced contrast and for male subjects. The cerebral activation hypothesis of hemispheric asymmetries is tested and found to contribute towards a visual field advantage to some extent but a structural account is also suggested. It is considered that the right hemisphere is prepotent for dealing with simple perceptual stimuli even when there is no spatial component for the task.     

Interhemispheric differences in the planning and execution of sequences of skilled finger movements

An index-middle finger (double) tapping task was used to examine hemispheric differences in the planning and execution of skilled finger movements. In two experiments, subjects responded to a simple cue presented tachistoscopically in the left or right visual field, by performing a predetermined number of double taps, (between one and eight inclusive), with either the left or right hand. Reaction times (RT) increased linearly as a function of increasing number of taps, when response sequences were controlled by the left hemisphere. In contrast, an inverse quadratic trend was obtained with right hemisphere control. When both hemispheres were involved in the stimulus-response sequence, the latency function incorporated elements of both trends, suggesting interaction between the hemispheres. The RT trends reflect differences in motor planning between the hemispheres. The conditions engaging only the right or left hemispheres did not differ in motor execution, as measured by tapping duration, variability or errors. However, when both hemispheres were involved there was evidence of interaction, which was evident as interference when the right visual field or left hemisphere was cued but the motor response was under the control of the right hemisphere (left hand). Overall, the results indicate that hand differences in fine motor skill may be determined by hemispheric differences associated with motor preparation rather than response execution.     

Parallel-serial processing and hemispheric function.

In one experiment 14 subjects made same-different responses to geometric forms varying in number from two to four, and presented randomly in the left or right visual hemifields. In a second experiment two different groups of subjects (N₁ = N₂ = 6) responded to letters, varying in number from two to four which could be matched on either nominal or physical identity. In both experiments reaction times were faster for stimuli directed to the right hemisphere and in no condition did reaction times increase with increasing number of stimuli. Implications for a relationship between mode of processing and hemispheric function were discussed.     

Hemispheric differences in global and local processing with orientation classification tasks

Hemispheric differences in global and local processing were examined in one experiment with hierarchical stimuli. The figures consisted of large squares with the right or left side missing made up of small squares with the right or left side missing. The subjects were asked to decide the opening (left/right) of the square either at the global level or at the local level. The findings showed that with the task and stimuli used here global judgements were as fast and accurate as local judgements, the interference was bidirectional and symmetrical and, finally, that the right hemisphere and the left hemisphere had the same ability to manage with global and local information. So, the experiment does not provide evidence for hemispheric specialisation in global and local processing.     

Relationship between changes in hemispheric advantage during familiarization to faces and proficiency in facial recognition

Female college students were given a task involving the recognition of initially unfamiliar faces which were tachistoscopically presented to one or the other visual field. Subjects who showed a left visual field advantage made fewer errors than those who showed a right visual field advantage both at the very beginning of the task, when the faces were totally unfamiliar, and at the end of the task, when the faces were relatively familiar; however, during intermediate phases of familiarization, there was no difference in number of errors between subjects with right and left visual field advantages. The results support our previously proposed view that there are shifts in the processing of facial information, beginning with a relatively undifferentiated holistic type of right hemisphere processing, progressing to an analytic mode of left hemisphere processing and culminating in a mode of right hemisphere processing in which distinctive features are incorporated into an articulated whole. The results further suggest that the direction of hemispheric advantage shown at different periods in the course of familiarization is related to level of proficiency.     

A clear left hemisphere advantage for visuo-spatially based verbal categorization

For a range of tachistoscopically presented stimuli that varied from modified complex Shepard and Metzler figures to simple lines, subjects had to judge whether or not the top or bottom of the stimuli contained the target feature. A marked and consistent right visual field (left hemisphere) advantage was observed for all such tasks. In addition, a task that required subjects to judge whether a solid black rectangle was in front of or behind a white rectangle also yielded a clear left hemisphere advantage. The results are explained in terms of a left hemisphere specialization for spatially based verbal categorizations in general support of (Psychol. Rev. 94, 148–175, 1984) model.     

Processing of spatial locations: hemispace effects during encoding but not recall

Traditionally, functional differences in the visual modality between the two hemispheres are investigated by tachistoscopic procedures. In these experiments, the stimuli reach the contralateral hemisphere first, and results are commonly interpreted on the basis of neuroanatomical access models. However, numerous studies demonstrated that the hemispace where the stimulus is perceived also plays a critical role in producing laterality effects ("hemispace effects"). In the present experiment, subjects were instructed to memorize the relative spatial positions of six figures horizontally aligned on a presentation board. The presentation board was located either to the left, to the right or in front of the subjects (left, right and central learning positions). During a recall phase, each figure was presented in the center of a computer screen and subjects were required to indicate by keypress whether a figure had been located in the left or right half of the presentation board. As in the learning phase, the computer screen was located to the left, the right or in front of the subjects (left, right and central recall positions). We found that the positions of the figures initially memorized in the left hemispace were recalled faster than figures initially memorized in the right hemispace. Hemispatial position during recall had no effect on performance. These results are discussed with respect to hemispheric specialization and theories of hemispace effects.     

Hemispheric differences in evoked potentials to pictures of faces in the left and right visual fields

Fifteen right-handed women evaluated the similarity of two faces presented to them either in the left or in the right visual field. The subjects' task was to decide whether the faces were the same or different and accordingly to press a button or abstain from doing so. Errors made by the subjects and the visual evoked potentials (VEPs) from points O1 and O2 referred to point CZ were recorded. Behavioural and electrophysiological results demonstrated the superiority of the right hemisphere in the perception of faces. The subjects made fewer errors when faces were presented in the left visual field. The number of errors also decreased when the presented faces were different from each other. Analysis of the VEPs indicates the existence of hemispheric asymmetry as early as 100 msec after the first stimulus disappeared. The asymmetry is larger in response to the second stimulus. On the basis of these results we conclude that differentiation in hemispheric functions takes place in the phase of short-term memory. The smaller number of errors made when the presented faces were different we explain by the strategy applied by the subjects.     

Hemispheric specialization in the assessment of female physical attractiveness

Female physical attractiveness has been widely related to waist-to-hip ratio (WHR) and body weight (BW). The present study was conducted to examine the role of hemispheric specialization in the perception of physical attractiveness. Drawings of female figures that differed in BW (underweight, normal and overweight) and WHR (0.7 and 1.0) were presented to both male and female subjects using the visual half-field technique. The stimuli were presented for an unusually short duration (180 msec). Under these conditions, male but not female subjects rated the various female figures as differing in attractiveness. Thus, male judgements of female attractiveness depended on weight and WHR. Reaction time and accuracy scores obtained from male subjects suggested that the left hemisphere (LH) was slower but more accurate than the right hemisphere in detecting differences in the attractiveness of the figures. Additionally, the most attractive figure was detected significantly more accurately than the least attractive figure when the figures were presented to the LH. The findings were discussed in terms of evolutionary views of sex differences in mate selection.     

Hemispheric preponderance in categorical and coordinate visual processes.

Two experiments were conducted to address whether a left hemispheric bias would be observed for categorical processing in both 'what' and 'where' systems (experiment 1) while a reverse bias would characterize coordinate processing whatever the systems (experiment 2). Young normal subjects were tested using divided visual field tasks. The results of experiment 1 indicated that subjects made categorical judgments in both what and where systems faster when stimuli are presented to the left hemisphere. The results of experiment 2 showed a significant interaction between visual field and difficulty of processing coordinate relationships. Indeed, a left-hemisphere advantage was observed when the task required easy processing whereas a right-hemisphere advantage was noted for difficult distinctions either in location (where system) or in lightness (what system). The left-hemisphere advantage we observed for categorization in both systems confirms the Kosslyn's hypothesis (1989) for the where system and suggests that the same left-hemisphere advantage also exists for the what system. Concerning coordinate processing, our findings highlight the influence of processing difficulty on the hemispheric lateralization and evidence a right hemispheric advantage for difficult coordinate processing and a left hemispheric advantage for easy coordinate processing. The results are discussed in terms of possible link between on the one hand difficulty and coordinate processing, and easiness and categorization on the other hand.     

Sorting of hemifield presented temporal and spatial stimuli

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 stimulus 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 hemispheric 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.     

Hemispheric asymmetries in categorical perception of orientation in infants and adults

Orientation CP is the faster or more accurate discrimination of two orientations from different categories (e.g., oblique1 and vertical1) compared to two orientations from the same category (e.g., oblique1 and oblique2), even when the degree of difference is equated across conditions. Here, we assess whether there are hemispheric asymmetries in this effect for adults and 5-month-old infants. Experiment 1 identified the location of the vertical-oblique category boundary. Experiment 2, using a visual search task with oriented lines found that adult search was more accurate when the target and distractors were from different orientation categories, compared to targets and distractors of an equivalent physical difference taken from the same category. This effect was stronger for targets lateralized to the left visual field (LVF) than the right visual field (RVF), indicating a right hemisphere (RH) bias in adult orientation CP. Experiment 3, replicated the RH bias using different stimuli and also investigated the impact of visual and verbal interference on the category effect. Experiment 4, using the same visual search task, found that infant search was also faster when the target and distractors were from different orientation categories than the same, yet this category effect was stronger for RVF than LVF lateralized targets, indicating a LH bias in orientation CP at 5 months. These findings are contrasted to equivalent studies on the lateralization of color CP (e.g., Gilbert, Regier, Kay, & Ivry, 2005). The implications for theories on the contribution of the left and right hemispheres of the infant and adult brain to categorical computations are discussed.     

Hemispheric disconnection syndrome with a "crossed avoiding" reaction in a case of Marchiafava-Bignami disease.

A clinicopathological study is presented of a case of Marchiafava-Bignami disease with a hemispheric disconnection syndrome, an association that does not appear to have been reported previously. Gross and microscopic examination of the brain revealed necrosis of the corpus callosum (sparing a small area in front of the splenium) and of the anterior commissure, cortical and subcortical infarction of the right lingual gyrus, diffuse cortical lesions of the laminar sclerosis type, and lacunae in the basal ganglia and the pons. The patient was unable to grasp objects presented to the right visual half-field with the left hand, or to respond to contralateral somaesthetic stimuli with either of the upper limbs. This motor inhibition, with the associated extended posture, is described as a "crossed avoiding reaction", and attributed to the inability of one hemisphere to respond to visual or somaesthetic stimuli projected to the other hemisphere. Clinicopathological correlations and visuomotor coordination mechanisms are discussed in the light of previous clinical and experimental studies. Anomia to pictures projected tachistoscopically to the left visual field, disturbances in the transfer of somaesthetic information, left sided ideomotor apraxia with agraphia, right sided dyscopia, and ideational apraxia especially marked in the right visual field were observed.     

Lateralised repetition priming for featurally and configurally manipulated familiar faces: evidence for differentially lateralised processing mechanisms

Although early research suggested that the right hemisphere was dominant for processing faces, more recent studies have provided evidence for both hemispheres being involved, at least to some extent. In this experiment we examined hemispheric specialisations by using a lateralised repetition-priming paradigm with selectively degraded faces. Configurally degraded prime faces produced negative priming when presented to the left visual field (right hemisphere) and positive priming (facilitation) when presented to the right visual field (left hemisphere). Featurally degraded prime faces produced the opposite pattern of effects: positive priming when presented to the left visual field (right hemisphere) and negative priming when presented to the right visual field (left hemisphere). These results support the proposal that each hemisphere is differentially specialised for processing distinct forms of facial information: the right hemisphere for configural information and the left hemisphere for featural information.