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David M. Eagleman, Terrence J. Sejnowski; The flash-lag illusion: distinguishing a spatial from a temporal effect, and why that matters for interpreting visual physiology. Journal of Vision 2001;1(3):16. doi: 10.1167/1.3.16.
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In the flash-lag illusion (FLI), a flash aligned with a moving object appears to be offset. Previously, we presented evidence that visual information from ~80 ms after an event can be used by the visual system to attribute a perceptual decision to the time of the event (Eagleman & Sejnowski, 2000, Science, 287: 2036). The attributed position of the moving object is an interpolation of its previous positions, which suggests that the FLI is a spatial illusion. Our framework contrasts with the latency difference (LD) hypothesis, which assumes that a flash takes a longer time to reach awareness than a continuously moving object; this would make the FLI a temporal illusion. A consequence of the LD framework is that a flash and a moving object presented simultaneously will be perceived with illusory temporal order. We tested this prediction and found that, in contrast to the LD hypothesis, subjects do not perceive mis-ordered stimuli, even under the same experimental conditions that produce the FLI. Our framework of spatial interpolation after a perceptual delay explains other, seemingly-unrelated illusions, such as the Frohlich effect. Being clear on whether the visual system suffers temporal vs. spatial difficulties is critical when interpreting the known physiology. Specifically, if the system does not generally exhibit temporal illusions, and yet different stimuli cause neural responses with different latencies, there must be some mechanism for correctly “time-stamping” the onset of different stimuli. We suggest that in order to compensate for the different latencies, the cerebral cortex waits for slower information to arrive before committing to a perceptual decision. In this way, widely varying stimulus-evoked latencies can be reconciled with precise psychophysical temporal detection.
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