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Early Category-Specific Semantic Processing In The Brain As Shown By Somatotopic Event-Related Responses To Auditorily Presented Action Words: ERP And L2 Minimum-Norm Source Analysis
NeuroImage 22, Suppl 1 (CD-ROM), #MO100, Budapest, Hungary, 2004
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A growing body of clinical and neuroimaging evidence suggests that not only the core language areas in the brain but also other cortical areas may participate in the processing of linguistic information. This evidence speaks in favor of the association learning theory which postulates that neurons that are active at the same time become strongly linked [1]. Thus, if a word is learned in conjunction with an object or action, a neuronal assembly will be formed comprising neurons in both language cortical areas and visual or sensorimotor cortices. Such neuronal network will function as a memory trace for the word in question and become active whenever the word is perceived [1]. A more fine-grain prediction would be that even sub-category differences may lead to differential cortical activation patterns. For action-related words, words referring to movements of different body parts should incorporate neurones in different parts of sensorimotor cortices whose topological organization of these cortices is well-known (Fig. 1). To test this hypothesis, we recorded mismatch negativity (MMN), a well-known index of experience-dependent memory traces in the brain [2], to investigate the processing of action-related words in the human brain. Responses to auditory presented movement-related English words were recorded in non-attend odd-ball paradigm using a high-density EEG set-up. MMN was calculated using responses to the same words presented as standard and deviant stimuli in different sessions to avoid contamination from phonetic-acoustic differences. The spatio-temporal patterns of the activation were analyzed by calculating ERPs as well as computing cortical source estimates (L2 minimum norm) for each subject; the data were then subjected to ANOVA. We found (Fig. 2) that: 1. The topography of the mismatch negativity to the action words showed unusual centro-posterior distribution of the responses suggesting that activity was at least in part generated posterior to usually observed frontal MMNs. 2. MMN responses to the hand-related word stimulus (pick) had a more spread-out lateral distribution, whereas the leg-related stimulus (kick) elicited a more focal dorsal negativity. These differences, remarkably reminiscent of the sensorimotor cortex somatotopy, were confirmed by the source analysis (L2 minimum-norm current estimates). 3. The latency of these word-specific MMN responses was in the range of 140-180 ms and could possibly be related to the recognition of the individual words. The present results are best explained in terms of distributed neuronal assemblies which function as category-specific memory traces for words and might involve sensorimotor cortical structures for encoding action words. The current data (see also [3]) are also confirmed by MEG [4] and fRMI [5] findings. The observed effects occurred early in time suggesting that semantic processing may commence in the brain as early as ~140ms after the word onset. References: 1.Pulvermuller F. Progr Neurobiol 67: 85-111 (2002) 2.Naatanen R et al. Nature 385:432-434 (1997) 3.Shtyrov Y et al. European J Neurosci (2004) 4.Pulvermuller F. et al. This volume. 5.Hauk O et al. Neuron 41: (2004)