The goal of this line of research is to close in on a connection between physical mechanisms present in neural tissue and the perceptual functions that these mechanisms embody. Please follow the links to the left for an overview of the flash lag effect, the explanations previously forwarded to explain it, our hypothesis to explain our results, and several demonstrations.
"Life must be lived forward, but can only be understood backward" - Soren Kierkegaard, The Journals of Kierkegaard
(Please note: this page is not currently maintained, and has not been updated since 2007)
The flash-lag effect is a visual illusion wherein a flash and a moving object that appear in the same location are perceived to be displaced from one another (MacKay, 1958; Nijhawan, 1994). In recent years, two explanations have been forwarded, motion extrapolation and latency difference.
The first proposed explanation for the flash-lag effect is that the visual system is predictive, accounting for neural delays by extrapolating the trajectory of a moving stimulus into the future (Nijhawan, 1994; Khurana and Nijhawan, 1995). In other words, when light from a moving object hits the retina, a certain amount of time is required before the object is perceived. In that time, the object has moved to a new location in the world. The motion extrapolation hypothesis asserts that the visual system will take care of such delays by extrapolating the position of moving objects forward in time.
A second proposed explanation is that the visual system processes moving objects more quickly than flashed objects. This latency-difference hypothesis asserts that by the time the flashed object is processed, the moving object has already moved to a new position (Baldo and Klein, 1995; Whitney & Murakami, 1998; Purushothaman et al, 1998). The latency-difference proposal tacitly rests on the assumption that awareness (what the subject reports) is an on-line phenomenon, coming about as soon as a stimulus reaches its "perceptual end-point" (Zeki & Bartels, 1998).
Motion Interpolation and Postdiction
We have proposed a third alternative (Eagleman & Sejnowski, 1999ab, 2000): visual awareness is neither predictive nor on-line, but is instead postdictive, such that the percept attributed to the time of the flash is a function of events that happen in the ~80 msec following the flash. This postdictive framework is consistent with findings in other fields, such as backward masking in visual psychophysics (Bachmann, 1994), or the color-phi phenomenon (Kolers & von Grunau, 1976). In backward masking, a stimulus followed in rapid succession by a second stimulus can block or modify the perception of the first one. In the color phi phenomenon, 2 colored dots presented sequentially within a small time and distance will appear to have changed color in the middle of their apparent trajectory. Since the viewer cannot know what the color of the second dot will be until having seen the second dot, the only explanation is that the conscious percept attributed to the 'trajectory' of the dots is formed after the second dot has 'arrived' at its destination.
We find that the perception attributed to the time of the flash depends on events in the next ~80 msec after the flash, and also that the flash seems to reset motion integration mechanisms, perhaps by temporarily diverting attention. In this way, we can draw a correspondence between the flash-lag effect and the Frohlich effect (Frohlich, 1923), wherein the first position of a moving object entering a window is misperceived.
Scroll down to watch demonstrations of the stimuli we used to elucidate several aspects of the flash-lag phenomenon.
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.
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.
*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.
In the next experiment, to further verify that the initial trajectory has no bearing on the direction of the perceived displacement, we designed the flash and ring to appear on the screen at the same time, with the movement of the ring beginning only in the next frame (Fig 1b in manuscript). Thus, there is no trajectory (no previous motion) from which to extrapolate. The results are unchanged from Fig. 1a, strengthening the conclusion that only events after the flash determine the perception. This paradigm is analogous to the 'flash-initiated cycle' used by Khurana and Nijhawan (1995); however, the present result makes their interpretation of motion extrapolation untenable. The juxtaposition of our Fig. 1a and 1b suggests that the flash resets the motion integration in the visual system, making motion after the flash effectively like motion that starts de novo (as in Fig. 1b). One explanation may be that the flash temporarily redirects attention
Note on the movie: for the purposes of demonstration, this movie shows the 3 conditions sequentially (up, down, and 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 placed 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.
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.
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.
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.
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 .
TECHNICAL COMMENT: The Position of Moving Objects
Click here to download the Technical Comment, The Position of Moving Objects, with contributions by Krekelberg & Lappe; Whitney and Cavanagh; and Eagleman & Sejnowski.
Demonstration of the predictable stimulus:
Demonstration of the unpredictable stimulus:
TECHNICAL COMMENT: Differential latency, not postdiction, by Patel et al.
Click here to download the Technical Comment, Differential latency, not postdiction, by Patel et al, with Response by Eagleman & Sejnowski.