Dissociations of Lexical Function: Semantics, Syntax, and Morphology

Aphasic patients who exhibit “asyntactic comprehension” show poor performance o n a sentence-picture matching task with semantically unconstrained sentences, such as “The cow bit the horse,” but good performance when the sentences are semantically constrained, such as “The boy threw the ball.” The assumption has been that such patients are able to interpret these latter sentences by relying solely on the meanings of the individual words, and that their intact lexical semantics can support some amount of sentence processing. We test this claim by investigating, in detail, the lexical semantics of an aphasic patient (JG), whose speech production is severely agrammatic and whose sentence-picture matching is asyntactic. We explore the semantics of JG’s lexicon for both morphologically simple and complex words. We find that word meanings are represented normally in his mental lexicon, and he is able to use this information to integrate words into phrases. In contrast, lexical syntactic and morphological processes are severely impaired. This pattern confirms that lexical semantics can support some limited amount of sentence processing in patients with asyntactic comprehension, and that different lexically based processes can be impaired differentially following brain damage.