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Spatio-temporal pattern of language processing in the brain: minimum-norm current estimates of magnetic MMN to spoken word. (Poster)
In: MMN03, Third International Workshop on Mismatch Negativity and Auditory Functions and Dysfunctions, Lyon, France, p. 104, 2003.
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The inferior frontal and superior temporal areas in the left hemisphere are well-known to be crucial for language processing in most individuals. This was established by classical neurological investigations and recent neuropsychological studies and metabolic brain imaging have revealed converging evidence. Here, we use fast neurophysiological brain imaging, magnetoencephalography, and L1 Minimum-Norm Current Estimates (MCE) to scrutinise the time course of cortical activation during the processing of spoken language. Subjects repeatedly heard the same recording of a spoken Finnish word. An oddball task was applied in which the critical word occurred as the infrequent deviant stimulus in the context of a standard stimulus that only minimally differed from the deviant towards its end. The delivery of the deviant spoken word led to a magnetic Mismatch Negativity brain response (MMN) earlier proposed as a unique index of memory traces for language elements [1-4]. The MMN was maximal over the left hemisphere and MCE revealed two pronounced left-hemispheric sources, one in superior temporal areas and the other in inferior frontal areas. The temporal source was significantly stronger than the one in inferior frontal cortex (p<0.01). The evoked activity localised in the interior frontal cortex was delayed compared to that in the superior temporal cortex, the time difference of 22 ms being statistically significant (p<0.02). Relative to the earliest point in time when the acoustic input included the information necessary for unambiguously identifying the critical word, the average delays of peak activation in superior temporal and inferior frontal areas were 136ms and 158ms, respectively.

In conclusion, the processing of spoken language coincided with near-simultaneous activation in superior temporal and inferior frontal areas in the left, language-dominant hemisphere. The minimal delay of ~20ms between superior temporal and inferior frontal excitations may be caused by signal conduction through the most common myelinated cortico-cortical fibres [5-6], possibly in the Fasciculus arcuatus, and matches well with estimates of cortico-cortical conduction delays on the basis of combined use of transcranial magnetic stimulation and neurophysiological recordings [7]. The use of MCE for determining the cortical sources of multi-channel MEG recordings can lead to important insights into the time course of cortical activation that constitutes the basis of cognitive processing.

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