Here, we examine evidence from partial split-brain surgery, agenesis of the corpus callosum, and callosal lesions caused by stroke to better understand if different portions of the corpus callosum can be associated with different symptoms. After complete callosotomy, the whole range of symptoms discussed by Bogen should be found, with exceptions due to individual differences in lateralization. However, partial lesions may result in a subset of symptoms occurring, and these natural lesions provide another window into callosal function.

The discoveries of the Sperry, Gazzaniga, and Bogen group led to the identification of a new syndrome, alien (or anarchic) hand syndrome, by the French neurologists Serge Brion and C. P. Jedynak. They recognized that the unusual behaviors of a series of patients were related to naturally occurring callosal lesions. These lesions were the result of cerebrovascu-lar disease or tumor. The patients shared a partial lesion of the corpus callosum and the experience of a left or nondominant hand that appeared to be completing complex motor behaviors outside of the control of the dominant left hemisphere. Although Gary Goldberg subsequently showed that medial frontal lobe damage can result in anarchic hand sign that has both a dominant and a nondominant form, patients with no evidence of frontal damage represent what Todd Feinberg terms the callosal type of anarchic hand syndrome. Comparison of the symptoms and lesion location of patients with callosal anarchic hand sign provides another window into the location of the information that transfers in the callosum.

First, we review the observations made based on partial callosotomies, and then we use reports from patients with natural lesions of the callosum to refine these observations regarding specific transfer of information. We know from the differences observed in spilt-brain patients who had either anterior or posterior section of the callosum first that the splenium or posterior callosum is important in the transfer of visual information. This is expected given the function of the occipital lobes and the preservation of the anterior/ posterior cortical organization of the cortex in the position of the callosal fibers.

Gail Risse and colleagues examined the transfer of visual, somatosensory, kinesthetic, auditory, and motor information in seven patients with section of the anterior callosum sparing the splenium and varying amounts of the posterior section of the body. By comparing transfer ability across these patients, they were able to confirm that intact splenial fibers permitted transfer of visual information for both naming and same/different judgments. However, patients that had only splenial fibers intact were unable to name objects palpated by the left hand. This suggests that the information transferred in the splenium is specifically visual and does not include higher order information synthesized from sensory information in another modality (i.e., it is generally observed that the right hemisphere is able to identify objects palpated by the left hand). If palpated objects cannot be identified by name in subjects with an intact splenium, it suggests that the tactile information necessary to build a semantic representation cannot cross to the left hemisphere in the splenium and neither can higher order information representing the semantic identification. In contrast, patients with the posterior one-third to one-fifth of the body of the callosum intact were able to name these objects relatively well. Generally, this patient group could also match limb position across hemispheres and could complete tests of apraxia with both the right and the left hand. Some difficulty with intermanual point localization was observed. A suppression of left ear auditory stimuli under dichotic conditions was noted for all but one subject. Although prior work indicates a role for the anterior callosum in praxis (due most likely to an interruption in the pathway necessary to carry the left hemisphere to "translation" of verbal commands for the right hemisphere to perform), there was little evidence of limb apraxia in this study except in the patients with sections that extended to the splenium. Another point made by Risse is that the left suppression on dichotic listening tasks in patients with good somatosensory function suggests that auditory fibers may be crossing anterior to somatosensory fibers, contrary to the results of Deepak Pandya and Benjamin Seltzer.

Although many sources confirm the importance of the splenium in the transfer of visual information, there may be even more specific function within the splenium. One of the patients investigated by the Gazzaniga laboratory, V.P., has shown some very specific transfer abilities. Margaret Funnell, Paul Corballis, and Michael Gazzaniga demonstrated that although V.P. is unable to transfer information about color, size, or shape across these spared fibers, she does show evidence of some access to words displayed to either hemisphere in the other, despite MRI confirmation of her status as a "complete" split. This remarkable specificity is supported by the work of Kyoko Suzuki and colleagues, who studied a young man with a small ventroposterior lesion to the callosum who could not read words in the left visual field. He could name pictures, however, suggesting that the anterior to middle section transfers picture information and the ventroposterior region is specific to letter transfer. Such precise lesions are rare but are revealing when carefully investigated. Of course, further work to confirm these observations is required, but they suggest that we are moving toward a much more specific understanding of the nature and location of the information transmitted in the callosum.

There remains disagreement regarding the relation between somatosensory and auditory fibers. A patient with a very discrete callosal lesion following a head injury showed increased suppression of left ear stimuli in dichotic testing when investigated by Michael Alexander and colleagues. His lesion appears to coincide with the posterior one-third to one-fourth of the body observed to be intact in some of Risse's patients. However, Risse found no auditory suppression in patients with good somatosensory function, suggesting auditory fibers were crossing anteriorly.

This was not true of Alexander's patient, who showed mildly impaired praxis and somatosensory function. (Unfortunately, transfer could not be tested because there were too many errors within a single hand to make it possible to observe decline in transfer.)

Early dichotic studies were in agreement that the portion of the callosum that caused greater left ear suppression was anterior to the splenium and was made up of the posterior one-half or one-third of the body of the callosum or was in the area of the isthmus. This was consistent with anatomical observations that indicated superior temporal lobe fibers crossed the callosum in this position. However, Sugushita and colleagues' careful examination of six patients with varied abnormalities of the corpus callosum suggested that damage to the splenium and posterior trunk leads to chronic left ear suppression. The patients in this study often had tumors that resulted in partial callosal resections, so there may have been some reorganization of function in response to tumor growth.

Given the individual differences in gyri, it may be that splenial fibers might better be defined by a common point of origin or by functional criteria rather than by a mathematically defined proportion. One paradox is that although some very precise disruptions in callosal transfer have been documented, there is significant variation in the areas where this transfer occurs. If this represents individual differences in the organization of the callosum, mathematical precision in separating the sections may not be helpful. In the meantime, the Risse observation remains difficult to reconcile with our current understanding ofanatomy, and further observations will be necessary to understand the nature of the differences.

Another important function of the callosum is the transfer of information about an item identified either visually or tactilely in the right hemisphere to the left for oral naming. In a systematic investigation of the clinical signs associated with callosal transfer during a 12-month review of 282 new cases of cerebral infarction, Giroud and Dumas noted only 1 case of tactile anomia, and this patient had the most posterior callosal lesion, including the anterior one-third of the splenium. A patient investigated by Kathleen Baynes, Mark Tramo, and Michael Gazzaniga corroborates the observation that the anterior one-third of the splenium may be crucial for naming of items palpated by the left hand. This may represent transfer of word information as in the patient V.P., discussed previously.

However, all these observations await a better method for defining the regions of the callosum and tracing the origin and destination of fiber tracts than is currently available in order to be adequately confirmed or denied. It is also necessary to consider differences between right-to-left and left-to-right transmission because there are consistent behavioral advantages associated with direction of transfer.

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