Influence of velocity, temporal frequency and initial phase position of grating patterns on motion VEP.

The cortical potential visually evoked by motion of a periodic grating (motion VEP) is composed of a transient component which decays within 500 ms of stimulus-onset (motion-on VEP) and a sustained component. Amplitude and peak latency of wave N2 of the motion-on VEP are functions of grating velocity. Both remain constant at spatial frequencies between 0.6 and 4.3 c/deg and at temporal frequencies within the equivalent intervals. The transient component of the motion VEP is independent of the spatial phase position of the grating before motion onset. The sustained component can only be seen in the averaged motion VEP at constant phase position of the grating before motion onset. This potential consists of periodical fluctuations with a main frequency equal to the temporal frequency of the moving grating. As a result of psychophysical investigations some authors suggest pattern velocity is the relevant variable of velocity perception, others temporal frequency. The motion VEP is dependent on both velocity and temporal frequency, the transient component is a function of velocity, the sustained component of temporal frequency.