PARADOXICALLY GREATER INTERHEMISPHERIC-TRANSFER DEFICITS IN PARTIAL THAN COMPLETE CALLOSAL AGENESIS

Symptoms of interhemispheric disconnection are typically much less sev ere in callosal agenesis than after surgical section of the corpus cal losum. Sperry [Sperry, R. W., Plasticity of neural maturation. Develop mental Biology, 1968, 2 (Suppl.), 306-327.] has attributed this differ ence to two interconnected factors: (1) the callosal section is usuall y performed after the brain has lost the maximal degree of functional plasticity associated with the early stages of development and (2) the removal of an already formed structure is more disruptive for functio nal brain organization than the failure of the same structure ta devel op. It has been suggested that functional compensation is less efficie nt if callosal agenesis is partial rather than complete [Dennis, M., I mpaired sensory and motor differentiation with corpus callosum agenesi s: A lack of callosal inhibition during ontogeny? Neuropsychologia, 19 76, 14, 455-469.]. This suggestion is supported by the present finding s of partial left-hand anemia, partial left-field alexia and poor tact ile cross-localization in a subject with a congenital absence of the p osterior part of the corpus callosum due to an arteriovenous malformat ion. In agreement with many previous studies, similar, though more sev ere, symptoms of interhemispheric disconnection were found in a subjec t with a complete section of the corpus callosum, but not in a subject with,complete callosal agenesis. Praxic control of the left hand on v erbal commands was severely deficient in the callosotomy subject, but it was normal in the subject with callosal hypogenesis. The lesser deg ree of compensation in partial compared to complete callosal agenesis may be explained by a reduced pressure to develop extracallosal means of interhemispheric communication, contingent on the partial existence of callosal connections, as well as by the later occurrence in develo pment of the causes of callosal hypogenesis compared to those of total callosal agenesis. (C) 1998 Elsevier Science Ltd. All rights reserved .