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Hector Rieiro, Susana Martinez-Conde, Jose Luis Pardo-Vazquez, Andrew Danielson, Stephen L. Macknik; Subject criterion can explain Bloch's law. Journal of Vision 2011;11(11):1235. doi: 10.1167/11.11.1235.
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The apparent contrast of a visual stimulus varies as a function of duration, a phenomenon known as temporal integration. There are two accepted principles to explain the role of stimulus duration in perceived contrast. Bloch's law states that below a critical duration, apparent contrast is a function of both stimulus intensity and duration, while above this critical duration, apparent contrast plateaus. Contrary to Bloch's predictions, Broca and Sulzer proposed that apparent contrast is maximized for specific stimulus durations, and that smaller or greater durations result in lesser apparent contrast. Despite the incompatibility between these two models, the literature on the topic contains studies supporting both of them. We hypothesize that the source of this discrepancy may be that previous studies were conducted on experienced subjects who knew the proposed hypotheses (i.e. the authors), and that no previous study was properly controlled for subject criterion. To address these concerns, we designed a 2-AFC task that counterbalanced stimulus dynamics and controlled for subject criterion. Five naive subjects were presented with Gabor patches of different contrasts and durations over a 50% grey background and were asked to report which of them had higher contrast. Our results support the model proposed by Broca and Sulzer: when the stimulus duration had a value between 50–100 ms, subjects experienced higher apparent contrast. When the same subjects repeated the same experiment without controlling for criterion, the peak in the perceived contrast tended to diminish or disappear, in a way similar to Bloch's predictions, suggesting that subject criterion could be causing the conflict in previous studies.
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