A disc moves in a circle at constant speed. When it reaches the top, a brief flash fires at exactly that position — disc and flash at identical screen coordinates, same frame. Yet observers consistently report the flash as lagging behind the disc: displaced toward where the disc was, not where it is.
The disc and flash were at the same place. The displacement is added by your visual system.
Romi Nijhawan first described the effect in 1994. He argued that the visual system compensates for its own processing delay by extrapolating moving objects forward in time. Because it takes ~80ms for a visual signal to travel from retina to cortex, the brain predicts where a moving object will be — not where it was — and positions the percept there. A brief flash has no future trajectory to extrapolate, so it lands where it was actually presented. The moving disc's percept is pushed forward; the flash's isn't. This creates a perceived spatial gap.
Eagleman and Sejnowski (2000) pressed on this with an omission variant: if the disc stops or disappears at the exact moment the flash fires, the lag vanishes — disc and flash appear coincident. This suggests the brain's extrapolation depends on the disc's future behavior, not just its current velocity. The lag is retrodictive: the brain reads information arriving after the flash (the disc's continued motion) and uses it to position where the flash appeared. The 80ms postdiction window from the flash-lag and the color phi phenomenon are the same window.
The reverse mode demonstrates this clearly. When the disc reverses direction at the flash moment, the flash appears ahead of the disc's new direction of travel — as if the brain correctly predicted the reversal. What actually happened: the reversal information arrived within the postdiction window, rewrote the flash's perceived position, and the result was delivered as a single coherent percept.
The simulation draws the disc and flash at identical screen coordinates. The blue crosshair marks where the flash was actually drawn. In continuous mode: the disc moves past the crosshair, and the flash appeared to be behind the crosshair's position — behind where it was drawn. In stop mode: disc and crosshair remain coincident. In reverse mode: the flash appears ahead of where the disc ends up. Same physical stimulus, three perceived locations, depending entirely on what the disc does after the flash.