Spatio-temporal tuning of VEPs: effect of mode of stimulation.

The spatial tuning function of the 8 Hz reversal Visual Evoked Potential (VEP) is frequently bimodal. This amplitude 'notch' largely disappears with 16 Hz pattern onset-offset (on-off) stimulation. To investigate the effect of temporal frequency on this phenomenon, we tested seven subjects using a range of rates centred on 8 Hz (reversal) and 16 Hz (on-off). With reversal, a deep notch was present at all temporal frequencies. A much shallower notch was evident with on-off. The on-off and reversal functions differed only in their low spatial frequency branches (below 4.5 c deg-1) where, as temporal frequency decreased, the reversal response was reduced by 50%, whilst the on-off responses doubled in amplitude. Thus, although they behave differently with on-off and reversal, multiple spatial frequency mechanisms are evident with both forms of stimulation.