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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).
Click here for a more detailed discussion about the problems inherent in a latency difference model
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.
By clicking on the links to the left, you can view demonstrations of the stimuli we used to elucidate several aspects of the flash-lag phenomenon.
REFERENCES FOR FURTHER READING
Bachmann, T. Psychophysiology of Visual Masking (Nova Science, Commack, NY, 1994).
Baldo, M. V. & Klein, S. A. Extrapolation or attention shift? Nature 378, 565-6 (1995).
Eagleman, D.M. & Sejnowski, T.J. (2000) Motion integration and postdiction in visual awareness. Science. 287(5460). [Click here to download]
Eagleman, D.M. & Sejnowski, T.J. (2000) Response: The position of moving objects. Science. 289(5482):1107a. [Full text] [Science Online]
Eagleman, D.M., Sejnowski, T.J. (2000) Latency difference versus postdiction: Response to Patel et al. Science. 290(5494): 1051a. [Full text] [Science Online]
Eagleman, D.M. (2001) Visual Illusions and Neurobiology. Nature Reviews Neuroscience. 2(12): 920-6. [ Full text (pdf)]
Eagleman, D.M. & Sejnowski, T.J. (2001). The flash-lag illusion: distinguishing a spatial from a temporal effect, and why that matters for interpreting visual physiology. Journal of Vision, 1(3), 16a. [Abstract]
Eagleman, D.M. & Sejnowski, T.J. (2002). Untangling spatial from temporal illusions. Trends in Neurosciences. 25(6): 293. [Full text (pdf)]
Eagleman, D. M. & Sejnowski, T. J. A flashed stimulus perceptually lags a moving one due to rewriting of the past, not extrapolation into the future. Soc. Neuro. Abstr. 29:158.10 (1999).
Frohlich, F. W. Uber die Messung der Empfindungszeit. Zeitschrift fur Sinnesphysiologie 54, 58-78 (1923).
Khurana, B. & Nijhawan, R. Extrapolation or attention shift: Reply to Bardo and Klein. Nature 378, 566 (1995).
Kolers, P. & von Grunau, M. Shape and color in apparent motion. Vision Research 16, 329-335 (1976).
Krekelberg, B. & Lappe, M. Temporal recruitment along the trajectory of moving objects and the perception of position. Vision Res 39, 2669-79 2669-79 (1999).
MacKay, D. Perceptual stability of a stroboscopically lit visual field containing self-luminous objects. Nature 181, 507-508 (1958).
Nijhawan, R. Motion extrapolation in catching. Nature 370, 256-7 (1994).
Purushothaman, G., PaG., Patel, S. S., Bedell, H. E. & Ogmen, H. Moving ahead through differential visual latency. Nature 396, 424 (1998).
Rao, R.P.N., Eagleman, D.M., Sejnowski, T.J. (2001) Optimal smoothing in visual motion perception. Neural Computation. 13(6):1243-53. [Full text (pdf)] [Abstract]
Snowden, R. J. & Braddick, O. J. The combination of motion signals over time. Vision Res 29, 1621-30 (1989).11.
Snowden, R. J. & Braddick, O. J. The temporal integration and resolution of velocity signals. Vision Res 31, 907-14 (1991).
Weichselgartner, E. & Sperling, G. Dynamics of automatic and controlled visual attention. Science 238, 778-80 (1987).
Whitney, D. & Murakami, I. Latency difference, not spatial extrapolation. Nat Neurosci 1, 656-7 (1998).
Zeki, S. & Bartels, A. The asynchrony of consciousness. Proc R Soc Lond B Biol Sci 265, 1583-5 (1998).
For more detailed information, please see our manuscript, "Motion Integration and Postdiction in Visual Awareness", by D. M. Eagleman and T. J. Sejnowski (Science, 287(5460)).
Updated 12/2002, DME