EXPERIMENT 3
To determine how much information after the flash the brain collects
for its decision, we designed stimuli analogous to those in Fig 1b,
but which include a direction reversal: immediately after the flash,
the ring moves in one direction before reversing direction after a
variable number of frames (Fig. 2 in manuscript). If the visual
system only employs information in the next 10 - 20 msec after the
flash (as might be implied from Fig. 1a and 1b, and from a latency
difference hypothesis), then the trajectory of the ring after that
time window should not affect the percept. Contrary to that
hypothesis, movement up to 80 msec after the flash influences the
percept. We find that 67 - 80 msec of unidirectional movement is
necessary to approach the illusory displacement measured in Fig. 1a
and 1b. As the amount of time before the reversal is reduced, the
illusory displacement is lessened, until with only 26 msec before
reversal, the flash lag effect is effectively canceled out (as though
the ring were stopped). With only one frame before reversal, the
illusion turns the other direction. These data are consistent with a
temporally-weighted spatial averaging that takes place over ~80 msec
after the flash. The results are the same when the ring has a
lifetime of only 6 frames after the appearance of the flash (as
opposed to remaining on screen until the end of the trial; n=2 of the
6 subjects). Physiological mechanisms for the spatiotemporal
integration may involve a form of temporal recruitment, the process by
which motion signals in the neural tissue are combined over time.
However, most of the available literature implicitly assumes that
motion integration would occur over the time before the flash, that
is, the visual system would collect information until the time of the
stimulus, with perceptual processing following on-line. Our data
indicate instead that visual awareness employs information after the
flash. The direction reversal experiment indicates that the position
of the moving object is interpolated as a point within the integrated
path, and given the results of Fig. 1a and 1b, our interpretation is
that the flash serves to reset the motion integration.
Click here for an MPEG demonstration.
Note on the movie: for the purposes of demonstration, the flash
appears exactly in the middle of the ring each time. In the real
experiments, the flash appeared in different positions for quantification of the
illusory displacement. Also, the size of the presentation is much reduced for
the movie, and the frame rate will play differently on different
browsers.
For more information, please see our manuscript: D. M. Eagleman and
T. J. Sejnowski, "Motion Integration and Postdiction in Visual
Awareness", Science, 287(5460), 2000.
Updated 2/2000, DME