Two temporal channels or three? A re-evaluation

The initial filtering of the image by the human visual system involves only a small number of temporal filters. Several studies suggest there are in fact only two, but some suggest that a third filter, sensitive to high frequencies, exists, at least at low spatial frequencies. This conclusion is derived in part from the observation that temporal frequency discrimination performance is better at very high (30-40 Hz) than at medium (20 Hz) temporal frequencies. We show that this apparent improvement at high frequencies is not real but is an artifact of differences in the rate of perceptual fading as a function of temporal frequency. Using suprathreshold counterphase gratings and a stimulus duration of 1.5 sec we replicated the finding of an improvement at high frequencies at a low (0.5 c/deg) spatial frequency. But when duration was reduced to 300 msec, to minimize fading cues, this improvement disappeared. Similarly, at 4 c/deg, the improvement was present at 3 sec duration but absent at 1.5 sec or less. Direct evidence that this effect of duration reflects differences in the ability to use fading cues was obtained in an experiment in which naive subjects were instructed to discriminate on the basis of fading: at high temporal frequencies and long durations performance was as good or better than for subjects instructed to use frequency; at short durations performance on this task was poor. Thus, the claim that a third temporal channel exists may need to be re-evaluated.