Stopping the motion and sleuthing the flash-lag effect: spatial uncertainty is the key to perceptual mislocalization.

A moving object is perceived to lie beyond a static object presented at the same time at the same retinal location (flash-lag effect or FLE). Some studies report that if the moving stimulus stops moving (flash-terminated condition or FTC) the instant the flash occurs, a FLE does not occur. Other studies, using different stimuli, report that the FLE does, in fact, occur in the FTC. The FTC is thus a crucial turning point in theories of flash-lag. Unraveling the mystery of the FLE in the FTC will help unravel the mechanisms underpinning flash-lag and perhaps even perceptual localization in general. Our experiments show that eccentricity of the moving stimulus was a contributing factor, as were eccentricity of the flashed stimulus and spatial separation between the two stimuli. Other factors, such as contrast and offset of moving stimulus, also modulate the magnitude of the FLE in the FTC. We surmise that uncertainty in determining the position in space of a moving stimulus is a key requirement for the lag-effect. A lag-effect in the FTC challenges influential models, such as differential latency, motion extrapolation, and postdiction. Based partly on the notion of an asymmetric spread of activity that arises because of the sheer nature of motion and from a combination of established physiological mechanisms, we propose a schematic account of the present findings that subsumes previous psychological models and scaffolds past experimental findings. Copyright 2004 Elsevier Ltd.