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BOLD evidence for word-elicited MMN enhancement.
In: Fourth Conference on Mismatch Negativity (MMN) and its Clinical and Scientific Applications, p.121
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We demonstrated that the MMN response is enhanced for word stimuli as compared with acoustically and phonologically matched pseudowords. This enhancement is taken as a signature of activation of long-term memory traces for language elements in the brain. The phenomenon was here tacked using fMRI to complement the currently available electrophysiological data. Word stimuli were randomly presented as deviants among acoustically matched pseudowords; in the other set of conditions, the reverse was used with infrequent pseudowords among standard word stimuli. The subjects were instructed to ignore the auditory stimulation and concentrate on watching a film, while echo planar images were obtained. Random-effect analysis (SPM2000) indicated clearly distinct patterns in the word and pseudoword blocks. Whereas MMN was found as being significant for the word deviants, it did not reach significance for the pseudoword conditions. This effect was more pronounced in the left than in the right hemisphere. To further analyse this, we extracted average parameter estimates in voxels within 15mm sphere around the centre of Heschl's gyrus in each hemisphere, and submitted these to ANOVA. This confirmed differential activation and produced significant hemisphere-by-stimulus type interaction indicating stronger BOLD MMN responses to words than pseudowords in the left but not in the right hemisphere's auditory cortex. Additionally, results from scans obtained at different delays after the deviant presentation indicated possibly diverging temporal dynamics of word- and pseudoword-related activity. Our results confirm the existence of automatic pre-attentive word-elicited MMN enhancement, putatively reflecting the presence of a memory trace for a spoken word. For the first time, this enhancement is documented using fMRI, which indicated strong involvement of auditory structures in the vicinity of left-hemisphere's Heschl's gyrus in sustaining such word-specific cortical networks.