DISCRIMINATION AND STUDY OF RHYTHMICAL BRAIN ACTIVITIES IN THE ALPHA-BAND - A NEUROMAGNETIC FREQUENCY RESPONSIVENESS TEST

In this paper we propose a fast procedure - stimulation paradigm, data analysis, data presentation - that permits the study of frequency sel ective enhancements or suppressions of the cortical responses in selec ted frequency windows. These responses can be used to classify the und erlying activity. This novel procedure combines, in one single test, m ultichannel detection, finely frequency scanned stimulation, a paradig m that permits to study the effect of the stimulation on the underlyin g activity (during the non-stimulated periods intermingled with the bu rst of stimuli) and a compact presentation to appreciate the whole rat e/frequency/topographical dependency of the spectral responses. In par ticular we applied this procedure to non-invasively study for the firs t time the frequency responsiveness within the extended alpha band (6 Hz to 14 Hz) under separate visual and somatosensory stimulation. The test proved robust and the responses-quite stationary. We discriminate d the lower and higher cu band through their different topographical a nd frequency responsiveness features and suggested them to be the same sub-bands recently discriminated via cognitive experiments on the bas is of their functional correlates. We measured a rate dependent alpha suppression (both in the somatosensory and visual modality) that is su ggested to be linked to the decoding of the stimulus rate. A slight de crease in the frequency of the spontaneous activity following stimulat ions at any rate is suggested to be connected to attentional load. We stressed the importance of our simple sensorial discrimination of thos e activities already evidenced on the basis of their functional correl ates, as well as the possible clinical uses of the test: on epilepsy, Alzheimer's disease (recently described to have effects on the respons iveness of the cortex to sensory stimuli. at different frequencies) as well as other disabling pathologies.