With the use of the simple motor reaction time (RT), we examined the contribution of the hemiretinas to the timing of the pattern reversal visual evoked potential (PVEP). RTs were obtained to the reversal of a progressively dimmer check falling either on the central, nasal, temporal, superior or inferior retina. Results were correlated with the peak time of the P1 wave evoked to the reversal of the entire checkerboard stimulus to the same luminance conditions as that used to generate the RT measurements. Our results clearly indicate that the time taken to trigger a manual response depends not only on the luminance of the stimulus, but also on where it falls on the retina. Comparing the above data with the peak time of the PVEP obtained to the reversal of the entire screen indicates that at brighter luminance the peak time of the P1 wave correlates with that measured from the most central retina, while at dimmer intensities it correlates with the mathematical average of all the retinal locations tested.