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Early semantic integration in the left temporal and inferior frontal cortex revealed by high-density MEG recordings.
Thirteenth Annual Meeting of the Cognitive Neuroscience Society, G42
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Mainstream view on semantically-related brain activity links lexico-semantic access to slow large-scale shifts occurring in evoked responses at 250-500 (typically 400) ms. To assess possible early dynamics of semantic processes, we recorded, in passive oddball paradigm, MEG responses to short spoken phrases where the second word was either congruent or incongruent with the first one. The acoustic, phonological and psycholinguistic features were strictly controlled for, and the contrasts were counterbalanced within and across subjects. Responses to the same critical words were also obtained outside of any context. We found that regardless of their acoustic features, semantically incongruent stimuli elicited an enhanced brain response in the form of a distinct peak already at ~120-160ms after the critical word onset. The same words did not produce such deflection in semantically legal contexts. The responses were maximal at left temporal and inferior-frontal sites which was also confirmed by distributed current source analysis. The temporal activation preceded the frontal one by ~16ms. As the observed responses were substantially earlier than the N400 context integration response, the latter appears to reflect secondary-stage processes. Such late semantic response disappeared in the current design when subjects focussed their attention away from the stimuli, suggesting that attention to language and/or task-related strategies may be necessary for its elicitation. The results suggest that the earliest brain processes of semantic context integration may occur in the left temporal and interio-frontal cortex at ~120ms after the acoustic input allows for identification of spoken words.