Low- and high-frequency evoked responses following pattern reversal stimuli: a MEG study supported by fMRI constraint.

We investigated the neural generators of N1 and P1 components of visual magnetic responses through the concomitant study of low (1-15 Hz)- and high (15-30 Hz)-frequency brain activities phase-locked to stimulus and elicited by pattern reversal visual stimuli. Whole helmet magnetic recordings and dipole modeling technique with support of functional magnetic resonance imaging (fMRI) were used to characterize locations and orientations of N1 and P1 sources as a function of four stimulated visual field quadrants. A comparison between low- and high-frequency activities revealed fundamental differences among orientations of the quadrants dipoles thus suggesting partly distinct neural populations underlying low- and high-frequency responses to transient contrast visual stimuli. Moreover, for both low- and high-frequency bands the specific study of locations and orientations of N1 and P1 sources indicated V1/V2 cortex as the neural substrate generating the two components. In summary, we provided strong support for a cortical genesis of human oscillatory mass activity following transient contrast stimuli with specific neural districts active in the low- and high-frequency bands. The converging results obtained from the concomitant investigation of probably different brain activities provided new evidences for a striate genesis of N1 and P1 components of the broadband visual-evoked responses following pattern reversal.