Neural Control of Sign Languages.

 

Neurolinguists attempt to gain insight into the nature of language by determining which areas of the brain control various components of the language system. One way in which this is done is by studying the performance of people who have incurred brain damage, such as the localized damage resulting from strokes. In hearing, speaking people, numerous studies have found that damage to certain areas of the left cerebral hemisphere typically results in some type of language dysfunction, called aphasia. There are Different types of aphasia, in which damage to different areas of the brain results in Different types of language deficits. It is assumed that if a given function y is lost after damage to a certain area A, then that area is involved in the control or processing of that function. In this realm of investigation as well, similarities between signed and spoken languages have been found. Although damage to areas of the left hemisphere results in aphasia in hearing subjects, damage to areas of the right hemisphere typically does not. On the other hand, damage to the right hemisphere frequently results in the loss of various types of spatial functioning. For example, patients with damage to the right hemisphere may be unable to draw a complete picture of a clock (typically neglecting the left side), or they may get lost in places that were very familiar to them before the brain damage. It was shown in section 1 that the use of space plays an important role in the grammar of sign languages. For example, verb agreement and verbs of motion and location forms described there both involve spatial representation and manipulation. This situation, in which spatial cognition operations are directly related to linguistic cognition, prompts basic questions about brain organization with respect to spatial and linguistic functions in deaf signers. Several studies of deaf signers who have suffered brain damage have revealed patterns of language breakdown which emphasize the similarity rather than The difference between signed and spoken language in the mapping of both linguistic and spatial abilities within the brain.

Poizner et al. (1987) present case studies of six deaf patients with brain damage, and they show a striking difference between deaf patients with damage to the right vs. left hemispheres. Like hearing patients, deaf signers who have left hemisphere brain damage have aphasia – in this case, aphasia for sign language. Some patients have very slow, awkward signing, like the speech of a “Broca’s aphasic” (named after the French physician who first associated the linguistic breakdown with damage to a particular area of the brain). Others have signing which is more fluent, but which doesn’t make sense, like a “Wernicke’s aphasic” (a syndrome which results from damage to a different area of the left hemisphere). However, these patients have generally intact spatial cognitive abilities, such as drawing, face recognition, or localization of objects. In contrast, deaf signers who experience damage to the right hemisphere have severe spatial deficits. They show left neglect, get lost in the hospital, and lose the ability to draw or show spatial relations, just like hearing patients with right hemisphere damage. However, the most important point is this: their signing is not impaired. They sign fluently and meaningfully, even using the “spatial” grammatical devices, although they show some comprehension deficits, a point we will return to. This provides strong evidence that sign languages are controlled in the left hemisphere, where spoken language is controlled, rather than in the right hemisphere, where visuo-spatial abilities are controlled. These results imply that neural specialization for language is determined more by the type of cognitive operation involved – linguistic or spatial – than by the physical channel that mediates these operations. The evidence for a human language capacity that transcends the physical channel for its expression is by now diverse and compelling. There is something about human cognition that converges on a complex and rich language system with particular formal and even neurological characteristics, even when the evolutionarily dominant channel for its transmission is not available. Yet this is still not the whole story. Some recent findings and new interpretations of existing results offer tough challenges to certain received views, and point the way toward important research in the future.