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Aurelio Bruno, Alan Johnston; Contrast gain not contrast change induces apparent temporal compression. Journal of Vision 2009;9(8):1085. doi: 10.1167/9.8.1085.
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The apparent duration of an interval containing a 50% luminance contrast grating drifting at 20Hz is compressed when preceded by a 90% contrast interval as compared to when preceded by a 10% contrast interval (Bruno & Johnston, 2007, Journal of Vision, 7(9): 376a). We linked this effect to contrast gain control mechanisms that are known to shorten the primate temporal impulse response in M cells, but not in P cells (Kaplan & Benardete, 2001, Prog Brain Res, 134:17–34). If true, we should not expect an effect of a static inducer on duration perception. Indeed, a 10% or 90% contrast static inducer had no significant effect on the perceived duration of a 50% contrast test drifting at either 5 or 20Hz. Also, we measured the perceived temporal frequency of a 50% contrast interval containing drifting motion when it followed either a 90% or a 10% contrast interval containing oscillating motion. We observed no significant difference in apparent speed for 20Hz drift, which induces temporal compression, as compared to 5Hz drift, which does not, indicating changes in temporal frequency are not critical for time compression. We also found a significant reduction in apparent contrast followed the 90% contrast interval, but no change after a 10% contrast inducer for both 20Hz and 5Hz drift. Thus changes in apparent contrast per se do not induce changes in duration. Finally, we asked the subjects to directly compare the duration of a 10% contrast interval with a 90% contrast interval containing drifting motion. No significant difference in apparent duration between 5 and 20Hz was observed. Our results show that contrast gain, but not contrast per se, has an effect on duration. This effect is limited to moving stimuli implicating the magnocellular pathway and is not mediated by changes in perceived temporal frequency or perceived contrast.
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