The practice of surgical sectioning of the corpus callosum for the control of epilepsy greatly stimulated interest in the special functions of the right cerebral hemisphere when isolated from the left. It is in the sphere of visuospatial perception that right hemispheral dominance is most convincing. Lesions of the right posterior cerebral region result in an inability to utilize information about spatial relationships in making perceptual judgments and in responding to objects in a spatial framework. This is manifest in constructing figures (constructional apraxia), in the spatial orientation of the patient in relation to the environment (topographic agnosia), in identifying faces (prosopagnosia), and in relating a scattering of visual stimuli to one another (simultanagnosia). Also, there are claims that the right hemisphere is more important than the left in visual imagery, attention, emotion (both in feeling and in the perception of emotion in others), and manual drawing (but not writing); in respect to these functions, however, the evidence is less firm. The idea that attention is a function of the right hemisphere derives from the neglect of left visual space and of somatic sensation in the anosognosic syndrome and also from the apathy that characterizes such patients. Certainly, the popular notion of the right hemisphere as “emotional” in contrast to the left one as “logical” has no basis in fact and represents a gross oversimplification of brain function and localization.
Similar issues arise, of course, in relation to handedness and language dominance in the left hemisphere as discussed in the following chapter. Here we comment only on how intriguing it is that praxis and linguistic skill are aligned on the same side of the brain, suggesting that an essential property of the dominant hemisphere is its ability to comprehend and manipulate symbolic representations of all types. At the same time, the colocalization of gnosis and visuospatial ability in the nondominant hemisphere has salience in that the two are so often interdependent in normal functioning.
Following the insightful clinical observations and anatomic studies of Wernicke, Dejerine, and Liepmann, the concept of disconnection of parts of one or both cerebral hemispheres as a cause of neurologic difficulty was introduced to neurologic thinking. In recent years, these ideas were resurrected and modernized by Geschwind (1965) and greatly extended by Sperry and Gazzaniga. Geschwind called attention to several clinical syndromes resulting from interruption of the connections between the two cerebral hemispheres in the corpus callosum or between different parts of one hemisphere. Some of these are illustrated in Fig. 21-6.
When the entire corpus callosum is destroyed by tumor or surgical section, the language and perception areas of the left hemisphere are isolated from the right hemisphere. Patients with such lesions, if blindfolded, are unable to match an object held in one hand with that in the other. Objects placed in the right hand are named correctly, but not those in the left. Furthermore, if rapid presentation is used to avoid bilateral visual scanning, such patients cannot match an object seen in the right half of the visual field with one in the left half. They are also alexic in the left visual field, because the verbal symbols that are seen there and are projected to regions of the right hemisphere have no access to the language areas of the left hemisphere. If given a verbal command, such patients will execute it correctly with the right hand but not with the left; if asked to write from dictation with the left hand, they will produce only an illegible scrawl. Many remarkable conclusions regarding the nature of behavior and the special roles of each cerebral hemisphere have been drawn from clever observations of patients with callosal section. Extensive discussion of these neuropsychologic abnormalities cannot be undertaken here; suffice it to say that these are not features seen in patients with the usual neurologic diseases, but they are nonetheless of interest to neurologists and are discussed in the writings of Gazzaniga.