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Spatio-Temporal Pattern Of Word-Elicited Activity In The Human Neocortex.
NeuroImage 22, Suppl 1 (CD-ROM), #MO101, Budapest, Hungary, 2004
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Identical acoustic contrasts are known to produce larger mismatch negativity (MMN) response when incorporated in word than in non-word context [1-4]. This word-related enhancement of the MMN was linked to activation of long-term memory traces for words [1,3]. Word-elicited MMN responses were consecutively suggested as a tool for investigating spatio-temporal patterns of neural memory traces for words, words' individual ìsignaturesî in the brain. In our previous study, we used this tool to demonstrate for the first time the pattern of language-related activation spreading in the left hemisphere [5]. Acoustically presented words activated superior temporal cortex at about 140ms after the relevant information was present in the auditory input; at ~20ms later, the activation could be registered in the inferior-frontal areas. It remained unclear, however, whether such activation patterns are characteristic of any phonological stimulus processing or are elicited only by meaningful members of the lexicon. To further detail spatio-temporal profile of language-related cortical activity, we performed another study. Using state-of the-art high-density MEG (Neuromag Vectorview, 306 channels) and passive odd-ball paradigm (visual attention task, no attention to the auditory stimulation), we recorded MMN responses elicited by identical acoustic change (presence/absence of final inflectional affix [t]) in Finnish verb, noun and a pseudoword. All stimuli were closely matched for their physical, acoustic and phonological properties; the MMN was computed as a difference between responses to the same words presented as deviant (i.e., rare unexpected) and standard (frequent) stimuli to avoid any contamination from phonetic-acoustic differences. We found (Figs. 1-2) that: 1. The MMN responses to both words were larger then that to the matched pseudoword. This difference was most pronounced in the left hemisphere. 2. The MMNs to words were larger in the left than in the right hemisphere. The MMN to the matched pseudoword stimulus was less lateralized than those to words. 3. In the left hemisphere, the MMN responses to words first peaked in the superior temporal lobe. This was followed, with a delay of ~20ms, by left inferior frontal activation. 4. In the right hemisphere, no latency differences between the activation of temporal and frontal areas could be seen. 5. The pseudoword did not demonstrate a similar activation pattern; the smaller pseudoword response peaked near-simultaneously in both temporal and frontal channels. These data confirm the previously reported MMN enhancement for word stimuli. They suggest that MMN can be used to document specifically spatio-temporal patterns of cortical activity related to word processing: the spread of activation from temporal to inferior-frontal cortices could only be seen for words but not for matched pseudoword stimulus. The current results also indicate early lexical and/or semantic processing in the brain which commences before ~150ms after the relevant information is available in the auditory input and is largely independent of focused attention. References: 1. Pulvermuller F et al. NeuroImage 14:607-616 (2001) 2. Korpilahti P et al. Brain Lang 76:332-339 (2001) 3. Shtyrov Y, Pulvermuller F. NeuroReport 13:521-525 (2002) 4. Endrass T et al. This volume. 5. Pulvermuller F, Shtyrov Y et al. NeuroImage 20:2 1020-1025 (2003)