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Early and late brain reflections of what makes sense: attention effects in a neuronal model of the language cortex.
Proceedings of the Experimental Psychology Society Annual Meeting, Edinburgh - July 2007. p.122
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Meaningful material (e.g., words) and senseless unknown stimuli (e.g., pseudowords) lead to different patterns of brain activation. The major neurophysiological response indexing "sense" has been the N400, a late response peaking around 400ms, larger for pseudowords than for matched words. Recently, early (100-250ms) MEG and EEG differences have also been recorded – for example, in the Mismatch Negativity (MMN), elicited even when subjects are distracted and is larger for words than for pseudowords, thus exhibiting the reverse pattern seen for the N400. We present a neuroanatomically-grounded neural-network model of the left-perisylvian language cortex that offers a unifying explanatory account for these seemingly contradictory data. The network was trained using biologically-plausible Hebbian learning mechanisms and then stimulated with "words" and "pseudowords". We found that low levels of global inhibition feedback, simulating abundant attentional resources, produced late activation differences, with a stronger response to pseudowords than to words (mirroring the N400 pattern). In contrast, high levels of global inhibition, simulating lack of attentional resources, lead to early differences, with a stronger response to words (as in the MMN data). The model's predictions of the modulation effects of attention on latency and magnitude of event-related brain responses are confirmed by recent experimental evidence.