OSCILLATORY NEUROMAGNETIC ACTIVITY-INDUCED BY LANGUAGE AND NON-LANGUAGE STIMULI

Event-related oscillatory brain activity during language perception di ffers from activity occurring during the processing of comparable non- language stimuli. This fact became apparent in the observation of chan ges in the normalized spectral power of magnetoencephalographic (MEG) signals during the subject's processing of these stimuli. MEG was reco rded over the left and right hemispheres of 12 right-handed subjects. During the experimental session, bisyllablic content words and physica lly similar non-language stimuli were presented with equal probability in a randomized order in either the visual or auditory modality. Appr oximately 15% of these stimuli were marked and the subject's task was to detect these marked stimuli. As a major characteristic of language vs. non-language processing, we obtained an enhancement of the normali zed spectral power around 240 ms in the 60-65-Hz band over the left he misphere for the language condition and over the right hemisphere for the non-language condition, independent of the modality of stimulus pr esentation. Starting at approximately the same latency but in lower-fr equency bands (15-45-Hz), an extended (250-600 ms) reduction of normal ized spectral power was observed, This reduction, although it generall y confirmed previous results, differed in that no hemisphere-specific reduction was found for the processing of words. A domain-specific enh ancement of normalized spectral power was also evident around 800-1200 ms in the 15-30-Hz band. In the auditory condition, this enhancement of the normalized spectral power was larger after the presentation of language stimuli whereas in the visual condition a larger enhancement of the normalized spectral power was obtained after presentation of no n-language stimuli. As this latter effect appears relatively late afte r the stimulus onset and differs in expression for both modalities of stimulus presentation, a simple relationship between language percepti on and oscillatory brain dynamics can be excluded for this enhancement . In contrast, the left hemispheric enhancement of the normalized spec tral power present around 240 ms in the 60-65-Hz band seems to reflect oscillatory pattern specific to the processing of words.