Posterior alpha activity is not phase-reset by visual stimuli.

There is currently a debate as to whether event-related potentials and fields measured by using electroencephalography or magnetoencephalography are generated by ongoing oscillatory activity becoming phase-reset in response to a given stimulus. We performed a magnetoencephalography study measuring brain activity in response to visual stimuli. Using a measure termed the phase-preservation index we investigated the phase of oscillatory alpha activity (8-13 Hz) before and after the stimulus. We found that in single trials the alpha oscillations after visual stimuli preserve their phase relationship with respect to the phase before the stimuli. This finding argues against phase-resetting of ongoing oscillations as being responsible for visually evoked responses. The event-related field can be explained primarily by stimulus-locked activity in the band that is absent before the stimulus. These findings suggest that different neuronal events are responsible for generating the ongoing oscillations and the visually evoked responses.