Motion

From Whitney & Cavanagh (2000). The display shows two pairs of linear gratings appearing to move toward or away from each other. When fixating at the center, the top circles look closer than the bottom circles; however, the top and bottom circles are aligned. This demonstrates that the direction of motion affects the perceived position of a moving stimulus.

From Whitney et al. (2008) . Here we see two dots moving toward each other that bounce in physical alignment. Most observers perceive the bounce locations to be misaligned (the dot on top appears shifted to the left or to the right of the one on the bottom). The misalignment is entirely a visual illusion caused by the motion of the dot. The perceived position of a moving object depends on the object’s motion and the motion of other objects in the scene, and this affects our everyday vision. A real-world example of how this causes a perceptual error applies to professional tennis referees, which players could exploit to their advantage.

From Whitney et al. (2000) . 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 models is 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.

From Whitney et al. (2000) . 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).

From Whitney and Cavanagh (2009) . 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.

From Whitney and Murakami (1998) . 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.