IMPAIRMENTS OF SENTENCE COMPREHENSION

We examine two different forms of comprehension impairment, 'semantic dementia' and 'asyntactic comprehension', focusing on the assignment o f thematic roles: the determination of who did it to whom. We show, fi rst, that the loss of word meaning does not impede thematic assignment in semantic dementia, demonstrating that syntactic information, along with some knowledge of the verb, is sufficient for the assignment of thematic roles. Studies of normal subjects indicate, however, that thi s process is normally subject to semantic influences; asked to judge t he plausibility of sentences, subjects respond faster when thematic as signment is semantically constrained. The sentence plausibility judgme nts of 'asyntactic' comprehenders (aphasics with diminished syntactic control over thematic assignment) show increased effects of these sema ntic constraints. We discuss these results in relation to current issu es in sentence processing.