Callosal Transfer

Many research groups have been predominantly concerned with the speed and direction of callosal transfer or interhemispheric transfer time (IHTT). Perhaps the oldest method of estimation was developed by Albert Poffenberger in the early 19th century. He used lateralized visual displays to compare reaction time to material in the fields ipsilateral and contralateral to each response hand. Material in the contralateral field should be processed in the same hemisphere that initiates the manual response without need for callosal transfer. In contrast, ipsilateral displays must be processed in the opposite hemisphere and the signal must be transferred into the responding hemisphere. He subtracted crossed from uncrossed reaction times to yield a crossed-uncrossed difference as an estimate of how much time was required for this transfer. His results yielded an estimate of 2 or 3 msec for healthy adults. Such short transfer times are thought to implicate the large myelinated fibers of the callosum.

The advent of averaged evoked potentials to measure electrophysiological responses of populations of neurons provided another way of measuring IHTT. By comparing the time course of ipsilateral and contralateral peaks to lateralized visual or somatosensory stimulation, another estimate of IHTT has been obtained. Clifford Saron and Richard Davidson found about a 12-msec IHTT to visual stimuli. In contrast, somatosensory stimulation yielded estimates from 8 to 26 msec. The differences in even these simple stimulus-response IHTTs suggest to some that there is more than one route of callosal transfer, which would be expected if different sources of information indeed cross in different sections of the callosum. However, anatomical evidence of widespread callosal distribution of fibers from particular cortical loci also presents the possibility of multiple routes of crossing for specific types of information.

There have been many studies of callosal transfer that suggest that this transfer of information may not be symmetric. Carlo Marzi performed a meta-analysis that indicated that for right-handed people, responses to displays in the left visual field with the right hand are faster than the converse. One implication of this finding is that the right hemisphere transfers information to the left hemisphere more rapidly than the left hemisphere transfers it to the right hemisphere. Because this advantage does not appear in left-handed people (although determination of hemispheric dominance is more problematic in this population), it may not mark a consistent or important principle of brain organization. It is nonetheless interesting to note that the nondominant hemisphere appears to be more adept at "reporting" to the dominant hemisphere, at least in simple reaction time tasks.

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Breaking Bulimia

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