Evidence for independent thalamic and cortical sources involved in the generation of the visual 40 Hz response in humans

Multichannel EEG was recorded to study the gamma-band (30-70 Hz) activity phase-locked to visual checkerboard stimulus onset, in grand-average data of 10 subjects and in three additional individuals. Two different approaches for source analysis were applied to reveal source locations and to determine there time course of activity. Two source regions were separated: one in the depth of the brain, suggested to reflect near thalamic activity, and a second at the visual cortex. Analysis of the source activity in time demonstrated significant different frequencies of the deep at 40 Hz and the cortical source at 37 Hz. There was no consistent phase relation between these source activities. These results contradict the thesis of recurrent thalamocortical activity causing gamma-band oscillations involved in the generation of phase-locked visual checkerboard evoked potentials. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.