The pre-stimulus oscillatory alpha phase affects neural correlates of early visual perception.

A growing number of studies suggest the phase of ongoing alpha oscillations in the brain influences visual perception. However, it remained largely unconsidered if this is associated with a phase dependence of neurophysiological processes especially in the visual cortex. Therefore, this study investigated the link between the pre-stimulus oscillatory alpha phase and neural correlates of early visual perception. In 64 subjects a 64-channel EEG system was used to examine the phase dependence of pattern-reversal visual evoked potentials (VEP) in a visual perception experiment. The pre-stimulus oscillatory phase over the primary visual cortex was determined for the individual alpha peak frequency (iAPF) as well as the frequency of maximal phase locking (PLFfmax). The phase dependence of VEP latency was determined using single-trial phase sorting. The results indicate a significantly shorter latency for the N75 and P100 components of the VEP between 40°-100° (p < 0.05) and 90°-120° (p < 0.05), respectively when trials were phase-sorted based on the iAPF. In contrast, the PLFfmax phase did not affect the N75 or P100 latency. The study indicates a link between the pre-stimulus alpha phase and neural correlates of early visual perception. These results extend previous behavioral findings to the neurophysiological level and support current models suggesting visual perception is modulated by ongoing alpha oscillations.