MRI assessment of spared fibers following callosotomy: a second look.
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Corpus Callosum↗
Biomedical subjects
M G Funnell
Publications and source records attributed to M G Funnell.
Cortical and subcortical interhemispheric interactions following partial and complete callosotomy.
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Cerebral Cortex↗
Insights into the functional specificity of the human corpus callosum.
Patient VP underwent complete callosotomy for the control of intractable epilepsy at the age of 27 years. Subsequent MRI, however, revealed spared callosal fibres in the rostral and splenial ends of the corpus callosum. We report a series of experiments designed to determine whether these fibres support functional transfer of information between the two cerebral hemispheres. Although we found no evidence for transfer of colour, shape or size information, there is good evidence for transfer of word information. This suggests that the spared splenial fibres in VP's corpus callosum are material-specific. The results of these experiments illustrate the remarkable degree of functional specificity within the corpus callosum
Brain↗
A dissociation between spatial and identity matching in callosotomy patients.
Although they are structurally similar, the two hemispheres of the human brain have many functional asymmetries. Some of these, such as language and motor control, have been well characterized. Others, such as visuospatial asymmetries, are less well understood. Many researchers have noted that the right hemisphere appears to be specialized for visuospatial processing. We investigated the abilities of the divided cerebral hemispheres of two callosotomy patients to perform discriminations based on spatial or identity information. The data revealed a robust right-hemisphere superiority for spatial judgments. In contrast, the left hemisphere was somewhat better than the right at making identity judgments. These results suggest that the right hemisphere is specialized for spatial processing, and the left is specialized for pattern recognition.
Brain Mapping↗
A deficit in perceptual matching in the left hemisphere of a callosotomy patient.
Decades of research have demonstrated dramatic differences between the hemispheres of the brain. While the most obvious asymmetries are in the areas of language and motor control, the visuospatial abilities of the left hemisphere are also known to differ from those of the right hemisphere. This hemispheric difference has been demonstrated empirically but its basis is thus far unclear. In this paper, we investigate the hypothesis that the left hemisphere is capable of sophisticated visual processing, but represents spatial information relatively crudely compared to the right hemisphere. The implication of this hypothesis is that pattern recognition is a function of both hemispheres but the right hemisphere is further specialized for processing spatial information. In a series of seven experiments we examined perceptual matching of mirror-reversed stimuli by the divided cerebral hemispheres of a callosotomy patient. In each experiment the left hemisphere's performance was impaired relative to the right hemisphere. This finding was independent of stimulus type, response bias and stimulus duration. These results are consistent with the idea that visual processing in the left hemisphere is directed towards pattern recognition at the expense of spatial information.
Adult↗
Hemispheric contributions to the integration of visual and auditory information in speech perception.
Differential hemispheric contributions to the perceptual phenomenon known as the McGurk effect were examined in normal subjects, 1 callosotomy patient, and 4 patients with intractable epilepsy. Twenty-five right-handed subjects were more likely to demonstrate an influence of a mouthed word on identification of a dubbed acoustic word when the speaker's face was lateralized to the LVF as compared with the RVF. In contrast, display of printed response alternatives in the RVF elicited a greater percentage of McGurk responses than display in the LVF. Visual field differences were absent in a group of 15 left-handed subjects. These results suggest that in right-handers, the two hemispheres may make distinct contributions to the McGurk effect. The callosotomy patient demonstrated reliable McGurk effects, but at a lower rate than the normal subjects and the epileptic control subjects. These data support the view that both the right and left hemisphere can make significant contributions to the McGurk effect.
Adult↗
Audiovisual integration in perception of real words.
Three experiments follow up on Easton and Basala's (1982) report that the "McGurk effect" (an influence of a visibly mouthed utterance on a dubbed acoustic one) does not occur when utterances are real words rather than nonsense syllables. In contrast, with real-word stimuli, Easton and Basala report a strong reverse effect whereby a dubbed soundtrack strongly affects identification of lipread words. In Experiment 1, we showed that a strong McGurk effect does obtain when dubbed real words are discrepant with observed words in consonantal place of articulation. A second experiment obtained only a weak reverse effect of dubbed words on judgments of lipread words. A final experiment was designed to provide a sensitive test of effects of lipread words on judgments of heard words and of heard words on judgments of lipread words. The findings reinforced those of the first two experiments that both effects occur, but, with place-of-articulation information discrepant across the modalities, the McGurk effect is strong and the reverse effect weak.
Attention↗
Hemispheric processing asymmetries: implications for memory.
Recent research has demonstrated that memory for words elicits left hemisphere activation, faces right hemisphere activation, and nameable objects bilateral activation. This pattern of results was attributed to dual coding of information, with the left hemisphere employing a verbal code and the right a nonverbal code. Nameable objects can be encoded either verbally or nonverbally and this accounts for their bilateral activation. We investigated this hypothesis in a callosotomy patient. Consistent with dual coding, the left hemisphere was superior to the right in memory for words, whereas the right was superior for faces. Contrary to prediction, performance on nameable pictures was not equivalent in the two hemispheres, but rather resulted in a right hemisphere superiority. In addition, memory for pictures was significantly better than for either words or faces. These findings suggest that the dual code hypothesis is an oversimplification of the processing capabilities of the two hemispheres.
Brain↗
An evolutionary perspective on hemispheric asymmetries.
The evolution of hemispheric specialization of function has obvious benefits in terms of overall processing capacity, but it may also have associated costs to each hemisphere in isolation. These costs are not apparent in an intact brain since information can be readily transferred between the hemispheres via the cortical commissures. We suggest that the evolution of language in the left hemisphere may have occurred at the expense of some visuospatial functions. Because the right hemisphere was still capable of performing those functions, the relative left-hemisphere impairment for visuospatial processing would be invisible. If the cortical commissures are severed, as in callosotomy patients, the costs of specialization may become more apparent. We report data supporting the idea that the left hemisphere may have lost visuospatial abilities that it once possessed, and suggest that this process may also result in right hemisphere deficits in abilities related to linguistic processing.
Animals↗