Neural and Computational Bases of Language Symposium

Presenter: David Caplan, Ph.D., M.D.

Functional Brain Imaging Studies of Syntactic Processing

This paper reports a series of studies of syntactic processing using positron emission tomography (PET) and event related functional magnetic resonance imaging (er-fMRI). In all experiments, we used a plausibility judgment task and contrasted hemodynamic responses to sentences with more complex sentence structures with responses to sentences with less complex syntactic structures. Using PET, we found that regional cerebral blood flow (rCBF) increased in Broca's area in young adults when the syntactic differences pertained to the complexity of relative clause structures (object vs. subject relativized stuctures). This brain region remained active in young adults when subjects made these judgments with concurrent articulation to impede rehearsal. A second syntactic contrast--passive vs. active structure--did not result in reliable increases in rCBF. In elderly subjects, the contrast involving relative clauses led to activation in the inferior parietal lobe. In many studies, midline frontal structures also increased their blood flow. Using er-fMRI, we presented sentences in slow RSVP form. We found that blood oxygenation level dependent signal (BOLD signal) increased in the temporo-parietal junction in young adults for plausible object- compared to subject-relativized sentences at a time point that corresponds to their viewing the relative clause. The results across all experiments suggest that task demands and subject factors make for differences in the regions that show hemodynamic responses to processing more complex syntactic structures.

Biography

Dr. David Caplan graduated from M.I.T. (1968) and received his Ph.D. degree in Linguistics at M.I.T. (1971) and his M.D. at McGill (1976). He has held appointments in Neurology at the University of Ottawa, Temple University, McGill University (Montreal Neurological Institute), and Harvard University (Massachusetts General Hospital). He is currently Professor of Neurology at Harvard Medical School, and holds several positions in the Neurology Service at the Massachusetts General Hospital (Medical Director of the Psychology Assessment Center; Medical Director of the Reading Disabilities Unit; Director of the Behavioral/Cognitive Neurology Clinic). He has adjunct appointments at M.I.T. and Boston University. Dr. Caplan's research interests lie in the area of disorders of syntactic processing in sentence comprehension and the neural basis for this process. He is studying disorders of syntactic processing using off-line and on-line measures. He is studying the nature of the processing resource (or working memory) system involved in syntactic processing in sentence comprehension by examining the effect of concurrent memory load on syntactic processing and by studying the consequences of lowered WM capacity on syntactic processing, using normal subjects and patients with low WM, such as AD and PD patients. He is approaching the neural basis for syntactic processing by correlation of deficits in syntactic processing in stroke patients with MR and FDG PET studies of their lesion sites, and by activation studies using PET and fMRI in normal subjects.

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