Illusory spatial offset of a flash relative to a moving stimulus is causedby differential latencies for moving and flashed stimuli

A flash that is presented adjacent to a continuously moving bar is perceive d to lag behind the bar. One explanation for this phenomenon is that there is a difference in the persistence of the flash and the bar. Another explan ation is that the visual system compensates for the neural delays of proces sing visual motion information, such as the moving bar, by spatially extrap olating the bar's perceived location forward in space along its expected tr ajectory. Two experiments demonstrate that neither of these models is tenab le. The first experiment masked the flash one video frame after its present ation. The flash was still perceived to lag behind the bar, suggesting that a difference in the persistence of the flash and bar, does not cause the a pparent offset. The second experiment employed unpredictable changes in the velocity of the bar including an abrupt reversal, disappearance, accelerat ion, and deceleration. If the extrapolation model held, the bar would conti nue to be extrapolated in accordance with its initial velocity until the mo ment of an abrupt velocity change. The results were inconsistent with this prediction, suggesting that there is little or no spatial compensation for the neural delays of processing moving objects. The results support a new m odel of temporal facilitation for moving objects whereby the apparent flash lag is due to a latency advantage for moving over flashed stimuli. (C) 200 0 Elsevier Science Ltd. All rights reserved.