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Daniel Gajewski, John Philbeck; Perception of Distance in the Most Fleeting of Glimpses. Journal of Vision 2010;10(7):62. doi: https://doi.org/10.1167/10.7.62.
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© ARVO (1962-2015); The Authors (2016-present)
Humans can walk without vision to previewed targets (floor-level, 3–5 m distant) without large systematic error and with near perfect sensitivity to target distance, even when targets are glimpsed for as little as 9 ms. To determine whether performance at brief viewing durations is controlled by perceived distance, versus nonperceptual strategies (e.g., inferential reasoning), we compared blindwalking to verbal distance estimates and gestured size estimates (9- and 113-ms viewing durations). In Experiment 1, blindwalking and verbal reports showed equivalent sensitivity to distance. While there was greater underestimation in verbal reports (−27%) than blindwalking (−17%), the pattern remained constant across viewing durations, despite very different functional requirements across tasks. This suggests that both responses are controlled by the same variable, ostensibly perceived distance. In Experiment 2, targets varied in size and distance, and the required response (blindwalking or size gesture) was not revealed until after the glimpse. We assumed that participants would thus be unlikely to use inferential strategies for both responses on each trial. There were large differences in bias: distance was underestimated (−18%) and size was overestimated (47%). The magnitude of bias was unaffected by viewing duration in the blindwalking task, but the bias towards overestimation in size judgments was reduced with extended viewing. Nevertheless, participants were highly sensitive to changes in target size, even though the visual angle remained constant across distances. Furthermore, sensitivity to size changes was statistically equivalent to sensitivity to distance changes, suggesting a constant ratio of indicated size to indicated distance when visual angle remained fixed–in accordance with Emmert's law. This linkage strongly suggests that perceived distance indeed varies during brief glimpses and likely controls the responses tested here. The overall pattern of results provides converging evidence for the idea that distance is perceived even in the most fleeting of glimpses.
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