Aphasic and parkinsonian signing: differences in phonological disruption.

Since movements of the articulators in sign, unlike in speech, are directly observable, we can investigate signing not only as linguistic behavior but also as motor behavior and directly contrast linguistic-representational and motor-execution disorders of signing. We compared the temporal sequencing characteristics (duration of segments, pausing, periods of change in handshape posture), intactness of distinctive features, and correct use of prosodic templates in three pairs of signers--two Deaf aphasic signers with posterior damage in the left hemisphere, two signers with Parkinson's disease, and two gender- and age-matched control signers. With respect to distinctive features, the aphasic signers exhibited selection errors in the American Sign Language (ASL) distinctive features system, while the Parkinsonian signers showed an intact distinctive feature inventory, but with disturbances in executing these features. The Parkinsonian signers, unlike the aphasic and control signers, showed marked disturbances in the temporal organization and coordination of what we argue are the two subsystems of the ASL sign stream--handshape and movement. The findings demonstrate a phonetic deficit in Parkinsonian signers, in contrast with aphasic signers who showed a disruption in the underlying representation and syllabification processes in the language.