Cerebral specialization for speech perception and movement organization in adults with Down's syndrome.

Adults with Down's syndrome and a group of undifferentiated mentally handicapped persons were examined using a free recall dichotic listening procedure to determine a laterality index for the perception of speech sounds. Subjects also performed both the visual and verbal portions of a standard apraxia battery. As in previous research, subjects with Down's syndrome tended to display a left ear advantage on the dichotic listening test. As well, they performed better on the apraxia battery when movements were cued visually rather than verbally. This verbal-motor disadvantage increased as the left ear dichotic listening advantage became more pronounced. It is argued that the verbal-motor difficulties experienced by persons with Down's syndrome stem from a dissociation of the functional systems responsible for speech perception and movement organization (Elliott and Weeks, 1990).