Temporal frequency response characteristic of motion channels measured with three different psychophysical techniques

The temporal frequency response characteristic of motion-sensitive channels in human vision was studied with three different psychophysical techniques: (a) measurements of threshold flicker sensitivity to drifting sinusoidal gratings; (b) measurements of the amount of contrast possessed by one sinusoidal component of a composite adapting stimulus when it just hulled the motion aftereffect produced by a second reference component drifting in the opposite direction; and (c) measurements of velocity difference thresholds. The threshold flicker measurements la) and null contrast measurements (b) corresponded exactly throughout the entire range of temporal frequencies studied as long as the velocity of the reference component in the null contrast method was not too fast. The correspondence means that the two types of measurements very likely have a common substrate comprised of directionally selective channels. The correspondence is also interesting in that one set of measurements (a) was obtained at threshold levels, whereas the second set of measurements (b) was obtained at suprathreshold levels. At high temporal frequencies, the function describing velocity dilierence thresholds departs from that which describes the other two types of measurements, suggesting that velocity discriminations at high temporal frequencies depend upon different cues or motion channels.