It is understood that damage to the left cerebral hemisphere in adulthood may result in syndromes of language disturbances called the aphasias. The study of these syndromes sheds light on normal language processes, the relationship between language behavior and the brain, and how best to treat aphasic individuals. Aphasia, for some, is a central communication disorder affecting all symbolic behavior in all modalities (i.e., speech, writing, and gesture). Difficulty producing symbolic gestures on command is called apraxia. Others view aphasia as a manifestation of a motor-sequencing disorder affecting all gestural systems including those required for speech movements. These divergent theories of the underlying nature of aphasia can be tested through examination of deaf individuals who use sign language before onset of aphasia. Poizner et al. [Am. J. Physiol. 246 (Regulatory Integrative Comp. Physiol. 15): R868-R883, 1984] studied three such patients with different aphasia syndromes: one patient had a nonsymbolic, motor-sequencing disorder; one had a gestural apraxia; and one had neither. These findings force the conclusion that neither the symbolic nor motor-sequencing theory of aphasia can account for the many varieties of that disorder.
The effects of context, meaning frequency, and associative strength on semantic selection: Distinct contributions from each cerebral hemisphere
