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Egelman, David A flashed stimulus perceptually lags a moving one due to rewriting of the past, not extrapolation into the future To explain the
flash-lag effect, wherein coinciding flashed and moving objects appear
to be displaced, it has been proposed that the visual system accounts
for neural delays by extrapolating the trajectory of moving stimuli.
We here present evidence that motion extrapolation cannot be the explanation
underlying the flash-lag effect. Subjects observed a ring moving in
a circular trajectory on a computer screen (Fig 1), and indicated
whether a flashed disc appeared to be directly in the middle of that
ring. We assayed the perceived displacement between the flashed and
moving objects by varying their physical displacement. The initial
trajectory of the ring was identical in all cases; however, immediately
after the flash, the ring either continued, stopped, or reversed.
If motion extrapolation were occurring, the extrapolated trajectory
should be the same in all cases, since the initial trajectory was
identical. Contrary to that hypothesis, for subjects to report spatial
coincidence, the flash needed to be differentially displaced depending
on the trajectory succeeding the flash (see figure). We found identical
perceptual results when the ring did not begin to move until after
the flash appeared, further demonstrating that the flash-lag effect
is independent of initial trajectory. Moreover, only ~50 msec of movement
after the flash is sufficient to determine the direction of flash-lag.
Our results are not consistent with motion extrapolation. Instead,
the perception attributed to an event at time t=to seems
to depend on what happens in to < t < to+50
msec.
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