Anatomy And Behavior

As the resurgence of interest in the results of callosot-omy helped to elucidate the functional capacities of each hemisphere, questions arose regarding the specificity of callosal function. In an early review of the literature on callosal organization, Georgio Innocenti offered some general principles regarding the topography of callosal fibers. He considered the organization of the callosum in humans to be an interesting question because of the demonstration by Sidtis, Gazzaniga, and colleagues in the human split-brain subject that semantic and sensory aspects of visual stimuli were transferred in different parts of the callosum. The Gazzaniga laboratory and others have continued to report very specific limitations on transfer after partial lesions to the callosum.

Nonetheless, Francisco Abolitz and Eran Zaidel argue that there is great equipotentiality across the callosum. There is other anatomical support for this view. The de Lacoste work demonstrates substantial overlap between temporal lobe and superior frontal lobe fibers in the body of the callosum. The superior parietal lobe, temporal parietal junction, and occipital lobe all have some fibers passing in the splenium. Behaviorally, inconsistencies reported later suggest minimally that there may be notable individual differences.

One factor that can make the specific role of the callosum difficult to isolate is the myriad of additional interhemispheric tracts and other interactions that occur in the midbrain and brain stem. There are numerous other commissures not routinely part of split-brain surgery in humans, including the posterior commissure, the habenular commissure, the commissures of the inferior and superior colliculi and the massa intermedia, and the thalamic commissure (which is not present in all brains). As fibers descend into the brain stem, there is much less segregation and many pathways may share information across the midline. These commissures are so deep in the brain that they are never severed during split-brain surgery in humans because of the devastating effects this would have on life-sustaining behavior. Hence, when an unexpected transfer of information occurs, the possibility that there is some subcortical communication that accounts for it is often raised.

Nonetheless, there are consistent deficits after complete section of the corpus callosum. Joseph Bogen outlined the principal deficits associated with lesions of the corpus callosum. He enumerated 10 symptoms that should be tested to confirm cases of callosal disconnection in normally lateralized right-handed subjects. In all cases, information is isolated in one hemisphere due to the loss of callosal fibers, resulting in the following symptoms:

1. Unilateral "verbal anosia'': If an odor is presented unilaterally to the right nostril, it cannot be named, although the left hand can pick out the item associated with the odor.

2. Double hemianopsia: If responses are permitted from one hand at the time, the patient will appear to have a homonomous hemianopsia (i.e., an apparent blindness for one-half of space) in the field ipsilateral to the response hand. When responses are required from the opposite hand, the side of the "blind" field will change.

3. Hemialexia: If words are presented rapidly in one visual field, the ipsilateral hemisphere cannot give any sign that it has read or even seen the words.

4. Auditory suppression: Information presented from one ear is suppressed or extinguished by the ipsilateral hemisphere.

5. Unilateral (left) ideomotor apraxia: Because the left hemisphere is dominant not just for language but also generally for motor planning, the left hand is unable to carry out actions to command. It is essential to demonstrate that the deficit is not due to weakness or problems with coordination or ataxia.

6. Unilateral (left) agraphia: Most right-handed people have some ability to write with their left hand, although the letters may be less fluent and well formed than those made by their dominant hand. After callosal disconnection, the left hand loses this ability.

7. Unilateral (left) tactile anomia: Objects placed in the left hand but not seen cannot be named. Often, the function ofthe object can be demonstrated or a related object can be selected.

8. Unilateral (right) constructional apraxia: Because the right hemisphere is better at visual-spatial problems, the right hand will have difficulty executing complex drawings or manipulating three-dimensional objects or puzzles. However, it should be noted that the Gazzaniga laboratory has observed bilateral constructional apraxia in several patients. Gazzaniga has argued that these skills depend on interhemispheric integration in some patients, and therefore can be observed to decline for either hand after callosal section in these patients.

9. Spatial acalculia: The degree of visual-spatial impairment may be sufficiently great that patients are more successful at solving verbal arithmetic problems mentally than at writing them down with paper and pencil due to the distortion introduced by the act of writing. However, observation of this symptom may vary in the same way that it does for constructional apraxia.

10. Inability to transfer somesthetic information: As discussed previously, sensory and position information cannot be passed from hand to hand after callosotomy.

These distinctive sequelae of the disruption of cortical fibers had been noticed in part by the great 19th-century neurologists who laid the groundwork for our modern understanding of brain function. Sadly, many of their insights were lost or obscured by later experiments that failed due to shortcomings in observational techniques.

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