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Abstract:
The N400, an event-related potential (ERP) component
extensively studied in language processing tasks, can be elicited
by violations of semantic expectancies at the end of sentences. A
later positive component (LPC) from 600-900 msec may index
spontaneous categorization of anomalous sentences or a
post-sentence error monitoring process. Intracranial recordings
have suggested that anterior fusiform and parahippocampal gyri are
generators of the N400 (McCarthy et al., 1995), but N400-like
potentials have also been recorded in hippocampus, rhinal cortex,
superior and middle temporal gyri, lateral orbitofrontal cortex,
and near Broca's area (Halgren et al., 1994; Guillem et al., 1995).
Since neuroimaging studies have implicated left inferior frontal
gyrus (LIFG) in semantic processing, we recorded ERPs from patients
with focal damage to lateral prefrontal cortex during sentence
reading. Frontal patients (including anomic aphasics with LIFG
lesions) generated N400s (300-600 msec) that were comparable to
controls. However, frontal lesions (particularly on the left)
eliminated the LPC to incongruous sentence completions. We suggest
that lateral prefrontal cortex does not contribute to the
contextual integration processes reflected in the scalp-recorded
N400. Instead, left frontal regions may participate in later
categorization or evaluative processes.
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