Patterns of discourse cohesion and coherence in Alzheimer's disease.

Patterns of discourse--as reflected in the use of propositions, cohesion devices, and in judgments of coherence--in the speech of 6 well elderly and 6 patients with senile dementia of the Alzheimer type (SDAT) during topic-centered interviews were compared. Compared to the well elderly, SDAT patients used significantly more words and conversational turns. The interviewer also used more words and turns when interviewing SDAT patients, making those interviews more lengthy and more interactive than those of the elderly comparison group. Although significant differences in propositional form and cohesion devices were not found, a pattern of cohesion disruptions in SDAT patients was identified that appears consistent with previously noted patterns of language dissolution. Coherence judgements by four listeners showed significant differences between the two groups. The breakdowns in coherence were related to one subtype of cohesion disruption, missing element. This suggests that incoherence may result in part from a loss of the ability to take the listener's perspective in developing thematic structure during conversation. It is argued that SDAT results not only in the impairment of linguistic abilities but also in the impairment of discourse abilities that contribute to Alzheimer patients' incoherent speech.