Differential latencies and the dynamics of the position computation process for moving targets, assessed with the flash-lag effect

To investigate the dynamics of the position computation process for a moving object in human vision, we measured the response to a continuous change in position at a constant velocity (ramp-response) using the flash-lag illusion. In this illusion, flashed and moving objects appear spatially offset when their retinal images are physically aligned. The steady-state phase of the ramp-response was probed using the "continuous-motion" (CM) paradigm, in which the motion of the moving object starts long before the occurrence of the flash. To probe the transient phase of the ramp-response, we used the "flash-initiated cycle" (FIC) paradigm, in which the motion of the moving object starts within a short time window around the presentation of the flash. The sampling instant of the ramp-response was varied systematically by changing the luminance or the presentation time of the flashed stimulus. We found that the perceived flash misalignments in the FIC and CM paradigms were approximately equal when sampling of the ramp-response occurred after a relatively long delay from the onset of motion and, were significantly different when sampling of the ramp-response occurred at a relatively short delay. The systematic variations in the perceived misalignment between the moving and flashed stimuli as a function of stimulus parameters are compared to the predictions of our differential latency and to alternative models of position computation. (C) 2004 Elsevier Ltd. All rights reserved.