EXPERIMENT 4
To further examine our interpretation, and to test the latency
difference model, we next separated the temporal coincidence of the
flashed and moving object.
Subjects were instructed to adjust the
angle of a "pointer" line (flashed for 1 frame) to point at the
beginning of the trajectory of the moving ring (Fig. 3 in manuscript).
The pointer was flashed, follcondition the flash and ring appeared on
the same frame (delta t = 0); in the remaining 4 conditions, the ring
did not appear until some delay after the single frame with the flash
(13 ms < delta t < 53 ms). The stimulus was repeated after a 1 sec
delay, and subjects were allowed to see a condition as many times as
they wished before committing to an answer. Regardless of the delay,
subjects adjusted the pointer to indicate a position an average of ~6
deg ahead of the actual starting position of the ring (same magnitude
as the displacements in Figs. 1 & 2). This demonstrates that subjects
do not perceive the starting position of the moving object (an
observation known as the Frohlich effect), but perceive instead, in
our interpretation, an interpolation of its past positions. The
latency difference model is not supported, for the outcome of a 'race'
between a flash and a moving object to a perceptual endpoint should be
changed by starting the flash first. Instead, the entirety of the
flash-lag effect in 1b can be explained by the fact that the starting
point of a moving object is interpolated (misperceived). Further, it
seems the traditional flash-lag effect (Fig 1a) is well explained by
our suggestion, above, that a flash resets motion integration.
Click here for an MPEG demonstration.
About the movie: for the purposes of demonstration, this movie shows 4
presentations for each different delay time. In the actual
experiment, subjects were allowed to watch as many presentations as
they wanted, and were instructed to adjust the angle of the flashed
'pointer' to point at the middle of the ring in its starting position.
When a subject was satisfied, he/she would hit the return key, and the
next trial would start, with a randomly chosen delay (delta t). In
this demonstration movie, the pointer is pointing at the veridical
starting position of the ring (0 degree displacement). The titles on
this movie are for demonstration purposes only, and were not shown to
subjects. Further, the size of the presentation is reduced for the
movie, and the frame rate will play differently on different browsers,
which means that the delay times will not correspond exactly to the
titles shown unless your browser plays this movie at 72 Hz .
For more information, please see our manuscript: D. M. Eagleman and
T. J. Sejnowski, "Motion Integration and Postdiction in Visual
Awareness", Science, 2000.
Updated 2/2000, DME