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Early contextual semantic integration in the left temporal and inferior-frontal cortex as revealed by high-density magnetoencephalography.
Neuroimage 31, S116
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Introduction Semantic integration processes are traditionally linked to the so-called N400 response, maximal at centro-parietal sites at 250-500 (typically 400) ms after a word mismatching the preceding sentence context is presented (Kutas, Hillyard, 1980). This has substantially influenced psycho- and neuro-linguistic theories of language processing, e.g. led to a belief that semantic processing commences markedly later than and is therefore subsequent to the early syntactic parsing (e.g. Friederici, 2002). A number of behavioural studies however suggested a substantially earlier access to contextual semantics which may be also parallel to other aspects of linguistic processing (e.g. Marslen-Wilson, 1975, 1985). The aim of this study was to assess any possible earlier dynamics of semantic processes using high-density spatio-temporal neuroimaging tools. Methods 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. Results 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 (Figure 1). 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 (L1 minimum-norm current estimates). The temporal activation preceded the frontal one by ~16ms (cf. Pulvermuller, Shtyrov, Ilmoniemi, 2003). Discussion and Conclusions The results suggest that the earliest brain processes of semantic context integration may occur in the left temporal and inferior-frontal cortex at ~120ms after the acoustic input allows for identification of spoken words. They also seem to take place at about the same time as the early phonological, lexical and syntactic access suggested previously (Pulvermuller et al, 2001, 2003; Shtyrov et al, 2000, 2002, 2003) strongly advocating parallel access to various types of linguistic information. Structures of the left temporal and inferior frontal cortices seem to mediate these putative early semantic integration processes and are activated by spoken language with a minimal delay thus revealing dynamics of neural activation spread in perisylvian areas. 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. References Friederici A. Trends Cogn Sci, 6:78-84 (2002). Kutas, M., Hillyard, S. Science, 207:203-205 (1980). Marslen-Wilson W. Science, 189:226-228 (1975). Marslen-Wilson, W. Speech Communication, 4:55-73 1985. Pulvermuller F et al. NeuroImage 14:607-616 (2001) Pulvermuller F, Shtyrov Y, Ilmoniemi R. NeuroImage 20:1020-5 (2003) Shtyrov Y et al. NeuroImage 12:657-663 (2000) Shtyrov Y, Pulvermuller F. Eur J Neurosci 15:1085-1091 (2002) Shtyrov Y, Pulvermuller F. NeuroReport 13:521-525 (2002) Shtyrov Y. et al. J Cogn Neurosci, 15:1195-206 (2003)