Abstract
Recent adaptation studies provide evidence for early visual areas playing a role in duration perception. One explanation for the pronounced duration compression commonly found with adaptation is that it reflects adaptation-driven stimulus-specific reduction in neural activity in early visual areas. If this level of stimulus-associated neural activity does drive duration, then we would expect a strong effect of contrast on perceived duration as electrophysiological studies shows neural activity in early visual areas to be strongly related to contrast. We employed a spatially isotropic noise stimulus where the luminance of each noise element was independently sinusoidally modulated at 4 Hz. Participants matched the perceived duration of a high (0.9) or low (0.1) contrast stimulus to a previously presented standard stimulus (600 ms, contrast = 0.3). To achieve perceptually equivalent durations, the low contrast stimulus had to be presented for longer than the high contrast stimulus. This occurred when we controlled for stimulus size and when we adjusted for individual differences in perceived temporal frequency. Further, we show that the effect cannot be explained by shifts in perceived onset and offset and is not explained by a simple contrast-driven response bias. The direction of our results is clearly consistent with the idea that level of neural activity drives duration. However, the magnitude of the effect (~10% duration difference over a 0.9–0.1 contrast reduction) is in marked contrast to the larger duration distortions that can be found with repetition suppression and the oddball effect; particularly when these may be associated with smaller differences in neural activity than that expected from our contrast difference. Taken together, these results indicate that level of stimulus-related neural activity in early visual areas is unlikely to provide a general mechanism for explaining differences in perceived duration.
Highlights
Given that we can judge how long a stimulus or interval lasts, it is clear that we have some sense of duration
The inclusion of the circular window stimulus, which is not affected by this issue of size driven by contrast threshold, allows us to control for this factor and provides an additional set of measures to test for any effect of contrast on perceived duration
Where differences are calculated (Figure 3C) we take the difference in perceived duration between the 0.1 contrast and 0.9 contrast match stimuli
Summary
Given that we can judge how long a stimulus or interval lasts, it is clear that we have some sense of duration. The idea is that the longer apparent duration of the oddball reflects duration compression of the repeated stimulus, the latter being driven by the suppression in neural activity. A clear and simple prediction, derived from the idea that perceived duration is driven by the level of stimulus-associated neural activity, is that a high contrast stimulus should appear to last longer than a low contrast stimulus. The study showed that perceived duration increased as a function of the difference between the static stimulus (a uniform 4.9◦ square) and its background, irrespective of whether that difference was a luminance increase or a luminance decrease Whilst this is certainly consonant with an effect of contrast, it could instead be driven by the evident change in luminous flux. The nature of this relationship constrains models and adds to our wider understanding of duration perception
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