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A neuronal index of connection strength in lexical memory circuits: early electrophysiological responses reflect frequency of unattended spoken words
SHTYROV, Y., Kimppa, L., PULVERMULLER, F., Kujala, T.
Eighteenth Annual Meeting of the Cognitive Neuroscience Society, 219
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How are words represented in the brain and can these representations be qualitatively assessed? Recent research demonstrates that neurophysiological signatures of individual words can be measured when subjects do not focus their attention on speech. These automatic activations, manifest as negative deflections of ERPs, can appear surprisingly early (within ~200 ms) and are based on robust connections within neuronal circuits encoding individual words that ignite even when attentional resources are scarce. A critical prediction of this framework is that words with high frequency of occurrence have especially strongly connected underlying memory circuits and should thus yield more negative ERPs compared with rarer words. We tested this prediction by presenting our subjects, in passive non-attend conditions, with acoustically matched high- and low-frequency words and pseudo-word controls. Using factorial and correlation analyses, we found that already at 120 ms after the stimulus information was available response amplitude was modulated by the words’ lexical frequency. Topographic mapping and source analysis suggested that this early automatic frequency effect originates from the left inferior-frontal cortices. While, at ~120ms, lexical differences between words and pseudo-words were seen only for the most frequent words, later-on (~270 ms), a more global lexicality effect with bilateral perisylvian sources was found for all stimuli, suggesting faster access to more frequent lexical entries. Our results support the account of word memory traces as interconnected neuronal circuits, and suggest that speed and magnitude of their activation are determined by their internal connection strength, which, in turn, is determined by everyday language use.