Time perception: the surprising effects of surprising stimuli.

The effects of stimulus repetition often increase when repetitions are more common (i.e., when repeats become more predictable), consistent with the idea that repetition effects reflect expectations about the recurrence of recent items. In contrast, the present experiments found a surprising pattern in which the compressed subjective duration of repeated items was reduced, eliminated, and even reversed when the frequency of repetitions was increased. Experiments 1-4b found that this pattern generalized across tasks, durations, and stimulus types; Experiments 5-9 investigated the mechanisms underlying these effects and suggest that recent exposure produces a short-lived contraction of subjective time consistent with a low-level process, such as neural fatigue, whereas elevating the predictability of a repeat produces a subjective time expansion that may result from more efficient perceptual processing. These findings (a) establish the important point that first-order repetition and second-order repetition expectations can have opposing functional effects, a possibility that has received little attention in general treatments of repetition effects, (b) run contrary to existing accounts of repetition effects in time perception, and suggest that there may be no simple mapping between apparent duration and the overall magnitude of the neural response, and (c) suggest a framework in which subjective time depends on the interplay between bottom-up signal strength and top-down gain control.