Learning to divide the labor between syntax and semantics: a connectionist account of deficits in light and heavy verb production.

In aphasic subjects who demonstrate difficulty producing verbs, a double dissociation has been observed between light verbs (e.g., GO) and heavy verbs (e.g., FLY). A simple connectionist model of sentence production, lesioned to simulate agrammatic- and anomic-like deficits, suggests that this dissociation arises from light verbs learning to rely more on syntactic cues and heavy verbs learning to rely more on semantic cues. Systematic manipulations of the variables which distinguish light and heavy verbs reveal that it is primarily the greater frequency of occurrence of light verbs, and the fact that they are specified by fewer semantic features, which cause them to depend more on syntactic information. Implications are discussed for models of lexical access in both normal and aphasic populations.