In most lesions confined to the posterior portion of the corpus callosum (splenium), only the visual part of the disconnection syndrome occurs. Cases of occlusion of the left posterior cerebral artery provide the best examples. Because infarction of the left occipital lobe causes a right homonymous hemianopia, all visual information needed for activating the speech areas of the left hemisphere must thereafter come from the right occipital lobe. The patient with a lesion of the splenium of the corpus callosum or the adjacent white matter cannot read or name colors because the visual information cannot reach the left language areas. There is, however, no difficulty in copying words; presumably, the visual information for activating the left motor area crosses the corpus callosum more anteriorly. Spontaneous writing and writing to dictation are also intact because the language areas, including the angular gyrus, Wernicke and Broca areas, and the left motor cortex, are intact and interconnected, but after a delay, the patient is unable to read what he has previously written (unless it was memorized). This is the syndrome of alexia without agraphia mentioned earlier.
Surprisingly, a lesion that is limited to the anterior third of the corpus callosum (or a surgical section of this part, as in patients with intractable epilepsy) does not result in an apraxia of the left hand. A section of the entire corpus callosum does result in such an apraxia, that is, a failure of only the left hand to obey spoken commands, the right one performing normally, indicating that the fiber systems that connect the left to the right motor areas cross in the corpus callosum posterior to the genu (but anterior to the splenium). Object naming and matching of colors without naming them are also done without error. However, when blinded, the patient cannot name a finger touched on the left hand or use it to touch a designated part of the body.
Of interest to the authors is the fact that one sometimes encounters patients with a lesion in all or some part of the corpus callosum without being able to demonstrate any aspect of the aforementioned disconnection syndromes. Notable is the observation that in some patients with a congenital agenesis of the corpus callosum (a developmental abnormality), none of the interhemispheric disconnection syndromes can be found. One must suppose that in such patients, information is transferred by another route—perhaps the anterior or posterior commissure—or that dual dominance for language and praxis was established during early development. (See a review of this subject by Lassonde and Jeeves.)
In addition to alexia without agraphia, the following intrahemispheric disconnections have received the most attention. They are mentioned here only briefly and are considered in more detail in the following chapter.
1. Conduction (also called “central”) aphasia. The patient has severely impaired repetition, but fluent and paraphasic speech and writing and relatively intact comprehension of spoken and written language. The Wernicke area in the temporal lobe is putatively separated from the Broca area, presumably by a lesion in the arcuate fasciculus or external capsule or subcortical white matter. However, most often the lesion is in the supramarginal gyrus, as discussed in Chap. 22.
2. Sympathetic apraxia in Broca aphasia. By destroying the origin of the fibers that connect the left and right motor association cortices, a lesion in the more anterior parts of the corpus callosum or the subcortical white matter underlying Broca area and contiguous frontal cortex causes an apraxia of commanded movements of the left hand (see Chap. 3 and earlier discussion).