Thematic roles in Alzheimer's disease: Hierarchy violations in psychological predicates

The nature of the verb deficit in Alzheimer's disease (AD) was investigated with a special focus on thematic-role assignment, employing verbs whose argument realization follows canonical thematic hierarchy, with Agent and Theme as main roles (e.g., The hunter killed the deer), and verbs whose argument realization deviates from canonical hierarchy, such as psych verbs (e.g., fear, frighten). Subject-Experiencer verbs (e.g., fear) do not assign an Agent role to the subject position of the sentence, but instead assign the role of Experiencer to that position. Object-Experiencer verbs (e.g., frighten) deviate from canonical thematic hierarchy in two ways. Like fear verbs, the frighten verbs do not assign the role of Agent. Moreover, they assign the role of Theme to the subject position and the role of Experiencer to the object position, thus resulting in the non-canonical Theme < Experiencer argument realization. Ten AD patients, 11 matched elderly controls, and 49 young controls performed a sentence completion task in which they had to choose a verb that would render the sentence grammatical and meaningful. AD patients showed no problems with canonical structures, but performed worse than controls in psych verb sentences, demonstrating greater difficulty with object-Experiencer sentences. Results suggest that AD patients may have an impairment in more fine-grained aspects of verb-semantic representation, such as thematic roles.