Abstract

How is lexical knowledge organized in the brain? Does grammatical information play a role in how this knowledge is organized? The patterns of lexical processing deficits in aphasia may help answer these questions. There are patients who have disproportionate difficulty with words from certain grammatical categories but not others. What do such deficits reveal about the representation of grammatical category information during the course of language processing? Two classes of explanations have been offered for the noun-verb dissociations in aphasia: "lexical-grammatical" explanations, which trace dissociations between nouns and verbs to differences in form class and their role in morphosyntactic processing; and "semantic-conceptual" explanations, which reduce differences between the two categories to effects of dimensions of lexical meaning that are correlated with grammatical class (e.g., concreteness). Although these are often discussed as mutually exclusive explanations, the neuropsychological and neuroimaging evidence supports the claim that both dimensions play a role in the organization of lexical knowledge in the brain. Here we propose that independent neural circuits are involved in processing verbs and nouns - a fronto-parietal circuit for verbs and a fronto-temporal circuit for nouns. Furthermore, we will argue that discrete components of the left frontal cortex are dedicated to prosecuting verb-specific and noun-specific grammatical operations, such as assigning the inflections that mark tense and number agreement.