Biomedical subjects

Paul M Corballis

Publications and source records attributed to Paul M Corballis.

15 recordsLinked to original sources

PubMed2006

Attending to depth: electrophysiological evidence for a viewer-centered asymmetry.

It has been proposed that the configuration of visuospatial attention in depth is viewer-centered such that an attentional gradient is concentrated between an observer and attended depth, trailing off steeply beyond. To investigate this asymmetry, event-related potentials were recorded while participants attended to far or near depths in a pictorial scene. The attention-sensitive visual components, P1 (100-160 ms) and N1 (160-220 ms), were assessed for amplitude differences. Amplitude enhancement of the P1 component was present when participants attended far but not near depths. Reaction time facilitation also followed this pattern. Results are consistent with a viewer-centered asymmetry because such a configuration predicts the gradient of attention to distribute differentially to far depths but to remain constant for near depths.

Adolescent↗

PubMed2005

Now you see it, now you don't: variable hemineglect in a commissurotomized man.

We describe the case of a callosotomized man, D.D.V., who shows unusual neglect of stimuli in the left visual field (LVF). This is manifest in simple reaction time (RT) to stimuli flashed in the LVF and in judging whether pairs of filled circles in the LVF are of the same or different color. It may reflect strong left-hemispheric control and consequent attention restricted to the right side of space. It is not evident in simple RT when there are continuous markers in the visual fields to indicate the locations of the stimuli. In this condition, his RTs are actually faster to LVF than to right visual field (RVF) stimuli, suggesting a switch to right-hemispheric control that eliminates the hemineglect. Neglect is also not evident when D.D.V. responds by pointing to or touching the locations of the stimuli, perhaps because these responses are controlled by the dorsal rather than the ventral visual system. Despite his atypical manifestations of hemineglect, D.D.V. showed evidence of functional disconnection typical of split-brained subjects, including prolonged crossed-uncrossed different in simple reaction time, inability to match colors between visual fields, and enhanced redundancy gain in simple RT to bilateral stimuli even when the stimulus in the LVF was neglected.

Attention↗

PubMed2005

Brain mechanisms underlying perceptual causality.

Functional magnetic resonance imaging (fMRI) was used to examine the neural correlates of perceptual causality. Participants were imaged while viewing alternating blocks of causal events in which a ball collides with, and causes movement of another ball, versus non-causal events in which a spatial or a temporal gap precedes the movement of a second ball. There were significantly higher levels of relative activation in the right middle frontal gyrus and the right inferior parietal lobule for causal relative to non-causal events. Furthermore, when the differential effects of spatial and temporal incontiguities were subtracted from the contiguous stimuli, we observed both common (right prefrontal) and unique (right parietal and right temporal) regions of activation as a function of spatial and temporal processing of contiguity, respectively. Taken together, these data provide a means to help determine how the visual system extracts causality from dynamic visual information in the environment using spatial and temporal cues.

Adult↗

PubMed2005

Hemispheric asymmetry in a dissociation between the visuomotor and visuoperceptual streams.

We have explored the level of interaction between the putative visuoperceptual (VP) and visuomotor (VM) processing streams within each cerebral hemisphere. Six patients with unilateral brain lesions and 26 intact-brain participants were tested. We presented three-dimensional versions of the Mueller-Lyer illusion separately to the left and to the right hemifields of each participant. In one condition, the participant grasped the central shaft of the illusion object between the thumb and forefinger (visuomotor task). In the other, the participant estimated the size of the shaft using the same fingers (visuoperceptual task). We calculated the magnitude of the illusion in each task for each hemifield. Intact-brain participants had a significantly larger illusion in estimation than in grasping in both hemifields. The two patients with right-hemisphere damage showed a large dissociation between grasping and estimation illusions, whereas the four patients with left-hemisphere damage showed no detectable dissociation. These results are consistent with the idea that the visuomotor and visuoperceptual streams are more dissociated in the left hemisphere than in the right.

Aged↗

PubMed2005

Visual grouping on binocular rivalry in a split-brain observer.

We studied the effects of visual grouping on binocular rivalry in the left and right hemispheres of a split-brain observer, JW. In Experiments 1 and 2, we compared responses to traditional rivalry stimuli (e.g., a red vertical grating presented to the left eye and a green horizontal grating presented to the right eye) with responses to Diaz-Caneja stimuli (i.e., half of each grating was presented to one eye and the other half to the other eye). As found for intact-brain observers, JW reported episodes of exclusive visibility of coherent stimuli (e.g., of a red vertical grating alternating with a green horizontal grating) with Diaz-Caneja stimuli that were fewer and briefer than with traditional stimuli. This occurred in both hemispheres, demonstrating that during binocular rivalry, contours from one eye can be grouped with those of the opposite eye to create a coherent percept, even in the isolated hemispheres of the split-brain observer. In Experiment 3, we studied the tendency of rivalry in adjacent patches to synchronize. When both patches were in one of JW's hemifields, rivalry synchronized for similarly oriented stimuli, the same as happened for intact-brain observers. When the patches were in JW's opposite hemifields, there was no synchronizing of rivalry, unlike what happened for intact-brain observers. This suggests that rivalry processed in JW's two hemispheres is independent. We conclude that rivalry is processed fully within each hemisphere.

Adult↗

PubMed2005

Dissociating processes supporting causal perception and causal inference in the brain.

An understanding of relations between causes and effects is essential for making sense of the dynamic physical world. It has been argued that this understanding of causality depends on both perceptual and inferential components. To investigate whether causal perception and causal inference rely on common or on distinct processes, the authors tested 2 callosotomy (split-brain) patients and a group of neurologically intact participants. The authors show that the direct perception of causality and the ability to infer causality depend on different hemispheres of the divided brain. This finding implies that understanding causality is not a unitary process and that causal perception and causal inference can proceed independently.

Chi-Square Distribution↗

PubMed2004

Hemispheric integration and differences in perception of a line-motion illusion in the divided brain.

Five people lacking the corpus callosum (two callosotomized, three with agenesis of the corpus callosum) and neurologically normal subjects were shown vertical lines that appeared instantaneously between pairs of rectangles in one or other visual field. When one of the rectangles flashed prior to the presentation of the line, and the line was in the same visual field, all subjects perceived the line as spreading from the flashed rectangle to the other. Normal subjects and one of the callosotomized subjects showed a slight but significant right visual-field advantage, perhaps reflecting a left-hemispheric superiority in processing rapid temporal events. The illusion was also induced when the line and the flash were in opposite visual fields in one of the callosotomized, one of the acallosal subjects, and about half of the normal subjects, implying interhemispheric integration even in the absence of the corpus callosum.

Adult↗

PubMed2004

Redundancy gain in simple reaction time following partial and complete callosotomy.

Four subjects with partial or complete section of the corpus callosum were tested on simple reaction time (RT) to visual stimuli presented either singly in one or other visual field, or simultaneously in both visual fields. The subject with posterior callosal section showed evidence of redundancy gain with bilateral stimuli beyond that attributable to probability summation ("enhanced" redundancy gain), and prolonged interhemispheric transfer. One of the two subjects with anterior section, like normals, showed little evidence of enhanced redundancy gain, and no evidence of prolonged interhemispheric transfer. The other did show some enhanced redundancy gain at the fast end of the RT distribution. These and other results suggest that the posterior corpus callosum provides the principal route or routes of interhemispheric transfer of the information required for simple visuomotor responses, and is also responsible for the much reduced redundancy gain in normal subjects relative to that in split-brained subjects. The subject with complete callosal section was unusual in that he responded only very rarely to stimuli in the left visual field (LVF), yet he showed markedly reduced RTs to bilateral relative to right visual field (RVF) stimuli.

Adult↗

PubMed2003

Binocular rivalry in split-brain observers.

During binocular rivalry, visual perception switches between a stimulus viewed by one eye and a different stimulus viewed by the other. We studied rivalry in split-brain observers to test two explanations. Rivalry could reflect switching of activity between the cerebral hemispheres, or switching by a structure in the right frontoparietal cortex. From these two theories, we predict no rivalry when stimuli are presented to a split-brain observer's left hemisphere. Yet we found similar rivalry from the left and right hemispheres of the split-brain observers, consistent with switchings being mediated by low-level processes within each hemisphere.

Consciousness↗

PubMed2003

Visuospatial processing and the right-hemisphere interpreter.

Popular views of hemispheric asymmetry hold that the left hemisphere is specialized for linguistic and cognitive processes and fine motor control, whereas the right is specialized for visuospatial processing. Although this dichotomy contains more than a grain of truth, it is an oversimplification. Experiments with split-brain patients have demonstrated that the left hemisphere retains relatively sophisticated visuospatial abilities, and that the asymmetries that favor the right hemisphere are subtler than those that favor the left. A consideration of the constructive nature of visual perception, and the organization of the visual system in the two hemispheres suggests that asymmetries are likely to arise relatively late in visual processing in areas that represent both sides of visual space. I present evidence in favor of the view that the right hemisphere can be considered more "visually intelligent" than the left, and postulate the existence of a "right-hemisphere interpreter" dedicated to constructing a representation of the visual world.

Brain↗

PubMed2003

Temporal discrimination in the split brain.

Divided visual field studies of neurologically normal adults indicate that the left hemisphere is superior to the right in making temporal judgments. Some neuroimaging and neuropsychological studies, however, have suggested a role for the right hemisphere in temporal processing. We tested the divided hemispheres of a split-brain patient in two tasks requiring temporal judgments about visually presented stimuli. In one task, the patient judged whether two circles presented to one visual field appeared for the same or different durations. In the second task, the patient judged whether the temporal gaps in two circles occurred simultaneously or sequentially. In both tasks, the performance of the right hemisphere was superior to that of the left. This suggests that the right hemisphere plays an important role in making temporal judgments about visually presented stimuli.

Brain↗

PubMed2003

Independent control of processing strategies for different locations in the visual field.

We investigated how the selection of information-processing strategies is organized. We contrasted three alternative ways in which strategies may be controlled: a centralized mechanism, a hemisphere-specific mechanism or a local mechanism. We built on the previous observation that the size of the noise-compatibility effect (flanker effect) is influenced by observers' expectancies for compatible or incompatible arrays. In three experiments, we varied the probabilities of compatible or incompatible noise in different locations of the visual field. We found that observers can adapt their processing strategies, as indicated by changes in the size of the noise-compatibility effect, independently in the left and right hemifields. However, processing of midline stimuli is determined by the expectancies existing in the lateral locations. These data support the notion of a hemisphere- or location-specific selection of processing strategies and suggest that the strategy selection process is not unitary.

Adult↗

PubMed2002

Paradoxical interhemispheric summation in the split brain.

We measured simple reaction time (RT) to light flashes, presented either singly or in pairs, in two people who had undergone callosotomy, one person with agenesis of the corpus callosum, and 17 normal subjects. The three split-brained subjects' RTs were decreased to bilateral pairs beyond predictions based on a simple race between independent unilateral processes, while those of the normal subjects were actually longer than predicted by the race model. This effect was present whether the bilateral pairs were in mirror-image locations or not, but was not present when the pairs were presented unilaterally. Since summation does not depend on close spatial correspondence, and also occurs when inputs are staggered in time, we suggest that it is due to cortical projection to a subcortical arousal system, and is normally inhibited by the corpus callosum.

Adult↗

PubMed2002

Hemispheric asymmetries for simple visual judgments in the split brain.

While it is commonly noted that the right cerebral hemisphere is specialized for visuospatial processing, the scope and nature of this specialization remain somewhat ill defined. Our previous research with callosotomy ('split-brain') patients has suggested that the asymmetry may be limited to conditions that have an explicit spatial component. To investigate this we compared the performance of the divided hemispheres of two callosotomy patients on four simple visual-matching tasks. These tasks were orientation discrimination, vernier offset discrimination, size discrimination, and luminance discrimination. In each task, two stimuli were presented briefly to one visual hemifield and the patient was asked to discriminate whether they were the same or different. The first three tasks (orientation, vernier, and size) were all spatial in nature and were performed better by the right hemisphere. The luminance discrimination task, which is non-spatial, was performed equivalently by the two hemispheres. These results support the view that the fundamental difference in visual function between the hemispheres is in the ability to perform spatial discriminations.

Cognition↗

PubMedDate not supplied

An investigation of the line motion effect in a callosotomy patient.

When a line is flashed instantaneously between two markers it can appear to propagate from one marker to the other. This illusion is known as the line motion effect. We investigated this effect in the two hemispheres of a callosotomy ("split-brain") patient. We found that both hemispheres perceived the line motion effect, and that flashing one of the markers biased the direction of motion away from that marker regardless of which hemisphere received the stimulus. In contrast, matching the width of the line to the width of one of the markers biased the direction of motion away from the marker only when it appeared in the left visual hemifield. This suggests that multiple mechanisms can contribute to the line motion effect, and that some of these mechanisms rely on different neural structures.

Corpus Callosum↗