Information Processing Speed during Functional Neuroimaging of Sentence Comprehension

Abstract:
Abstract: This study continues previous work investigating the neural basis for grammatical and short-term memory components of sentence comprehension. We hypothesized that greater demands placed upon a third component, information processing speed in the form of a faster stimulus presentation rate, is shouldered by the caudate nucleus. To test this hypothesis, neural activity was measured with fMRI in 15 healthy, right-handed English-speakers as they determined the agent of the action in sentences with subject-relative or object-relative center-embedded clauses that included "short" (three-word) and "long" (seven-word) spans between the antecedent noun phrase (NP) and the "gap" where the NP is interpreted. These 13-word, written sentences were presented in a word-by-word fashion at 500 msec/word and 750 msec/word, the 500-msec rate requiring rapid information processing but the 750-msec rate requiring greater short-term memory. Caudate was activated when words were presented at a rapid rate, particularly when short-term memory was minimized in the short antecedent-gap sentences. Long antecedent-gap sentences presented slowly to maximize short-term memory load revealed no caudate activity. Caudate activation persisted in the context of challenging grammar, as seen during object-relative sentences presented rapidly. These observations support the hypothesis that the caudate contributes to sentence processing in the form of negotiating rapid information processing, not short-term memory demand, during sentence comprehension.