Flash Lag
When a flash is presented next to a moving bar, the flash appears to lag behind the moving bar. These studies investigate why this visual illusion occurs.
2000
Illusory spatial offset of a flash relative to a moving stimulus is caused by differential latencies for moving and flashed stimuli.
A flash that is presented adjacent to a continuously moving bar is perceived 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 explanation is that the visual system compensates for the neural delays of processing visual motion information, such as the moving bar, by spatially extrapolating the bar’s perceived location forward in space along its expected trajectory. Two experiments demonstrate that neither of these modelsis tenable. The first experiment masked the flash one video frame after its presentation. 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 apparent offset. The second experiment employed unpredictable changes in the velocity of the bar including an abrupt reversal, disappearance, acceleration, and deceleration. If the extrapolation model held, the bar would continue to be extrapolated in accordance with its initial velocity until the moment 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 model of temporal facilitation for moving objects whereby the apparent flash lag is due to a latency advantage for moving over flashed stimuli.
Temporal facilitation for moving stimuli is independent of changes in direction.
A flash that is presented aligned with a moving stimulus appears to lag behind the position of the moving stimulus. This flash-lag phenomenon reflects a processing advantage for moving stimuli (Metzger, W. (1932) Psychologische Forschung 16, 176–200; MacKay, D. M. (1958) Nature 181, 507–508; Nijhawan, R. (1994) Nature 370, 256–257; Purushothaman, G., Patel, S.S., Bedell, H.E., & Ogmen, H. (1998) Nature 396, 424; Whitney, D. & Murakami, I. (1998) Nature Neuroscience 1, 656–657). The present study measures the sensitivity of the illusion to unpredictable changes in the direction of motion. A moving stimulus translated upwards and then made a 90° turn leftward or rightward. The flash-lag illusion was measured and it was found that, although the change in direction was unpredictable, the flash was still perceived to lag behind the moving stimulus at all points along the trajectory, a finding that is at odds with the extrapolation hypothesis (Nijhawan, R. (1994) Nature 370, 256–257). The results suggest that there is a shorter latency of the neural response to motion even during unpredictable changes in direction. The latency facilitation therefore appears to be omnidirectional rather than specific to a predictable path of motion (Grzywacz, N. M. & Amthor, F. R. (1993) Journal of Neurophysiology 69, 2188–2199).
The position of moving objects.
Eagleman and Sejnowski (1) recently proposed a “postdiction” model of the so-called flash-lag effect, in which a moving stimulus appears spatially to lead a flash, even though both stimuli are actually precisely aligned (2). According to postdiction, the moving object appears ahead of the flash because at each moment the object’s position is estimated by integrating forward in time; the flash resets all the integrals so that only those starting immediately after the flash will produce a position estimate, and the forward average is necessarily in advance of the position of the flash. A closer examination, however, shows that postdiction explains neither the flash-lag effect nor the Frohlich effect, and that our differential-latency model remains a viable account of the flash-lag phenomenon.
1998
Latency difference, not spatial extrapolation.
The inevitable neural delays involved in processing visual information should cause the perceived location of a moving stimulus to lag significantly behind its actual location. However, Nijhawan (1, 2, 3) has proposed that the visual system corrects the perceived location of the moving stimulus by extrapolating it along the trajectory of motion, so that the stimulus is perceived at its expected actual location. We provide new evidence to the contrary, demonstrating that the visual system does not compensate for neural delays but simply shows a reduced delay for moving stimuli.