Perceiving the timing of visual events is an important visual function. In a dynamic visual environment, changes in neural activity in response to both event onsets and offsets may provide reference signals for perceiving temporal events. Recent evidence indicates that neural latencies to event offsets are shorter, less dependent on stimulus parameters and at times less variable than latencies to event onsets (Bair et al. (2002). J. Neurosci. 22, 3189). Does this result - observed on the single cell level - affect temporal precision at the perceptual level?

We measured the temporal precision for discriminating two brief events differing in either onset or offset asynchrony (on a 200Hz monitor). In the onset asynchrony condition, a Gabor patch appeared slightly before another Gabor, and then both disappeared synchronously 750 ± 100ms later. Observers identified which Gabor appeared first. In the offset asynchrony task, observers identified which Gabor disappeared first. At a 3.3deg separation between Gabors, event offsets were perceived much more accurately than event onsets (threshold = 7.5 vs. 16.3 ms). Indeed, at all separations tested (up to 20deg), temporal precision for perceiving offsets was better by a factor of two than that for perceiving onsets.

In an analogous experiment, we measured temporal precision for discriminating motion onset and offset asynchronies of drifting (6Hz) Gabor patches. Temporal precision for perceiving motion offsets was better than that for perceiving motion onsets (20 vs. 40ms). These effects are robust and generalize well over a range of stimulus parameters. Collectively, our results suggest that event offsets are encoded with high temporal precision. This agrees with known neurophysiology and suggests that vision may rely on response offsets as reference signals for perceiving temporal events.