EXPERIMENT 1
To directly pit extrapolation into the future against interpolation of the past, we designed this two-alternative forced choice task. Subjects were instructed to indicate whether a flash of light (white disk) occurred above or below the center of a moving ring (Fig 1a in manuscript; ring speed 360 deg/sec). Beginning with the frame following the flash, the ring took one of 3 randomly interleaved trajectories: continuing, stopping*, or reversing direction. The initial trajectory of the ring (up to and including the frame with the flash) was identical in all three conditions; thus, if motion extrapolation were occurring, the predicted trajectory should be the same. Instead, the perceived position of the flash relative to the ring was independent of the initial trajectory. In the case of the continuous trajectory, subjects perceived the ring to be about 6 degrees ahead of the flash (as would be expected from previous studies of the flash-lag effect). However, in the presentations wherein the moving ring stopped, there was no illusion of displacement, indicating that the pre-flash movement was not sufficient to yield the flash-lag illusion. When the ring reversed direction immediately after the flash, participants perceived the ring about 6 degrees above the flash, which is the same size, but opposite direction, of the continuous case. In other words, what participants report to have seen at the time of the flash depends on the events after the flash.

If visual awareness were predictive, the same initial trajectory would lead to the same extrapolation. Our results replicate a recent demonstration by Whitney and Murakami, in which the perceived displacement of a flash was influenced by a motion change that occurred after the flash. In our experiment, by directly comparing stimuli with an identical pre-flash trajectory to three different post-flash trajectories, we can demonstrate that the perceived displacement of the flashed and moving stimuli is a function of the movement after the flash. (Note that in the stopped case, there is no flash-lag effect at all*). Thus, forbearing any precognitive explanations, we are left to suggest that the perception attributed to an event at time to depends on what happens in to < t < to + h, where the magnitude of h will be determined by experiment 3.

Click for an MPEG demonstration .

Make sure you set your player to play every frame, or you might miss the flash...

Note on the movie: for the purposes of demonstration, this movie shows the 3 conditions sequentially (continuous, reversed, stopped), and the flash appears exactly in the middle of the ring each time. In the real experiments, conditions were randomly interleaved, and the flash was put 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.

*Note that the flash-terminated condition had been previously demonstrated by Romi Nijhawan, both in a 1992 ARVO abstract, and also in a talk at the Salk Institute in 1999, at which I was in attendance.