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The flash lag illusion or 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). Several explanations for this simple illusion have been explored in the neuroscience literature.
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 Integration and Postdiction
Eagleman & Sejnowski (2000abc) proposed a third alternative: 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. Eagleman & Sejnowski found that the perception attributed to the time of the flash depends on events in the next ~80 msec after the flash. In this way, they drew a correspondence between the flash-lag effect and the Fröhlich effect (Fröhlich, 1923), wherein the first position of a moving object entering a window is misperceived.
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