Combined EEG and MEG recordings of visual 40 Hz responses to illusory triangles in human.

EEG and MEG were simultaneously recorded to study the visual gamma-band (30-70 Hz) responses. The electrical gamma-band response phase-locked to stimulus onset can be subdivided into a central component at 39 Hz and an occipital component at 36 Hz. A new high-frequency magnetic phase-locked response recorded over the occipital lobe is described. Its topography is complex and probably reflects the activity of multiple sources. Both electrical and magnetic high-frequency responses differ in topography from the low-frequency responses in the same latency range, suggesting that at least partially distinct sources are involved. The existence of a non-phase-locked 40 Hz component around 280 ms is confirmed in EEG data but is not detectable in MEG data.