Amplitude modulation of gamma band oscillations at alpha frequency produced by photic driving.

Gamma band response to visual stimulation in humans has been observed to have both burst and resonance properties. Amplitude modulation of gamma activity at low frequencies has been seen in rat hippocampus and modeled in a number of forms. Significant amplitude modulation (p=0.05) of 33 Hz gamma frequency activity at the frequency of an 8 1/3 Hz photic driving stimulus, which also produced strong alpha entrainment, was observed in 67% of the channels in 42 human subjects. Similar amplitude modulation was found at a range of frequencies from greater than 50 Hz to about 28 Hz. The peak of the gamma amplitude modulation curve trailed the peak of the alpha signal by 25 to 30 ms, corresponding to a phase difference of 150 degrees to 180 degrees. The phase consistency of the gamma signal, measured across comparable times of the alpha signal, was least at the minimum amplitude modulation, and largest at the maximum. Although there was no consistent overall relation between the gamma amplitude and alpha amplitude, peak gamma amplitude values were consistently higher during post-target-stimulus alpha suppression, which occurs about 300-750 ms subsequent to stimulus presentation, than they were at the time of maximum alpha activity during the immediate post-stimulus period. It is hypothesized that there is an interaction between the alpha and gamma generating systems, in which gamma triggers alpha activity and is subsequently inhibited by it, thus producing the observed amplitude modulation. The transition from dark to light of the photic driving stimulus begins a phase resetting process in the gamma system and a concomitant burst of gamma activity; this produces an activation in the alpha system, similar to that found in the P1-N1 response in evoked potential experiments, and a subsequent inhibition of gamma production.