Semantic influences on thematic role assignment: evidence from normals and aphasics.

We report two studies that examine the role of semantic influences in the assignment of thematic roles. Semantic factors were manipulated by contrasting sentences in which one noun argument was a plausible filler of only one thematic role (e.g., the painting in The artist disliked the painting) with sentences in which both noun arguments were plausible fillers of both thematic roles (e.g., The robin ate the insect). Subjects were required to make plausibility judgments to sentences presented auditorily. Experiment 1 examined RTs of normal subjects on the plausibility judgment task. In Experiment 2, the same sentences were presented to aphasic patients identified as "asyntactic" comprehenders. In Experiment 1, RTs were speeded by semantic constraints on thematic assignment, particularly when the role-constrained NP occurred early in the sentence (as in The painting was disliked by the artist). The aphasic performance patterns in Experiment 2 paralleled those of normal subjects, but in greatly exaggerated fashion. The patients exhibited high error rates on sentences where semantic constraints conflicted with the syntactically based assignments, even on sentences with canonical (S-V-O) word order (e.g., #The deer shot the hunter).