The Visual Evoked Potential is independent of surface alpha rhythm phase

A Visual Evoked Potential (VEP) is an electrical signal picked up by a surface electrode in response to the activation of visual cortex by a visual stimulus. Because the VEP is typically much smaller in magnitude than the ongoing spontaneous EEG signal, the VEP is derived by averaging a large number of responses time-locked to stimulus presentation. Standard theory has it that the VEP is independent of the ongoing EEG, however, there has long been a competing view that the VEP is caused by a partial phase reset of the spontaneous alpha rhythm. We calculated the VEP where stimuli were presented at four different phases of the ongoing alpha rhythm, and subtracted away the responses to null trials synchronized to the same alpha rhythm phases, creating estimates of the VEP as a function of ongoing alpha rhythm phase. For some subjects there was evidence of an interaction between the VEP and the phase of the ongoing alpha rhythm, but this was idiosyncratic between subjects and conditions, and mostly evident in a later period when the VEP magnitude was very small. However, in general the VEP is independent of the phase of the ongoing alpha rhythm, and hence cannot be primarily caused by a partial phase resetting of the spontaneous EEG. It is possible that the VEP is either a phase-reset of an ongoing oscillation, or an oscillation induced by the sudden onset of a stimulus, but it cannot be the same oscillation as the surface alpha.