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Para Kang, Steven Shevell; The flash-lag effect for two features changing simultaneously: a test of alternative hypotheses. Journal of Vision 2012;12(9):1062. doi: 10.1167/12.9.1062.
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In the flash-lag effect, a feature of a continuously-changing stimulus is perceived to be ahead along its feature continuum compared to the same feature of a pulsed stimulus (Sheth et al., 2000). This "lead time" is different for color and orientation; when both features change continuously, observers report a combination of features at the time of the pulse that was never actually presented (Kang & Shevell, JOSA A in press). Two hypotheses have been proposed to explain the flash-lag effect. The extrapolation hypothesis assumes that the visual system corrects for neural latencies by extrapolating the trajectory of the continuously-changing stimulus into the future. The postdiction hypothesis assumes that the pulse sets the start of a temporal integration window for the continuously-changing stimulus. These hypotheses were tested by changing features either (i) only before the pulse (PRE condition, to test extrapolation) or (ii) only after the pulse (POST condition, to test postdiction).
A circular window (diameter 2.6deg) with a 1.3cpd square-wave grating appeared on one side of fixation. In the PRE condition, both the color and orientation of the grating changed continuously for 720msec before a pulsed stimulus appeared on the other side of fixation. During the 80msec pulse, the color and orientation of the pulsed and continuously-changing stimulus were the same. Following the pulse, the features of the continuously-changing stimulus did not change. The POST condition was the opposite: the continuously-changing stimulus changed its features for 720msec only after the pulse. Observers compared the pulsed to the continuously-changing stimulus in both color and orientation. In the POST [PRE] condition, the lead time was 182msec [53msec] for color and 63msec [-1msec] for orientation. The lead-time differences primarily support the postdiction hypothesis, though the short lead time for color in the PRE condition suggests the integration window begins prior to pulse onset.
Meeting abstract presented at VSS 2012
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