Visual evoked potentials elicited by chromatic motion onset

Visually Evoked Potentials (VEPs) were recorded in response to the onset of chromatic and luminance motion gratings of 1 cpd and luminance 40 cd m(-2) subtending a 7 degrees field. At slow speeds (< or =2 cycles s(-1)) the motion onset response exhibits a clear amplitude minimum at isoluminance. Over the Michelson contrast range tested (0.05-0.75) the chromatic response at 2 cycles s(-1) possesses a linear response function compared to the saturating function of the luminance response and the contrast dependency of the former is a factor of 5-6 times greater than for the latter. These differences are suggestive of different neural substrates for the chromatic and luminance motion VEPs at slow speeds. At 10 cycles s(-1) the chromatic motion onset VEP exhibits no amplitude minimum at isoluminance and becomes more like its luminance counterpart in terms of its saturating contrast response function. Furthermore, the contrast dependency of the chromatic and luminance responses differs by only a factor of 1.6 at this faster rate. These findings are consistent with the idea of separate motion mechanisms that operate at fast and slow speeds, the latter having separate channels for colour and luminance motion.