The Flash-Lag Phenomenon: Object Motion and Eye Movements

Abstract

An object flashed briefly in a given location, the moment another moving object arrives in the same location, is perceived by observers as lagging behind the moving object (flash-lag effect). Does the flash-lag effect occur if the retinal image of the moving object is rendered stationary by smooth pursuit of the moving object? Does the flash-lag effect occur if the retinal image of a stationary object is caused to move by smooth-pursuit eye movements? A disk was briefly flashed in the center of a moving ring such that the ring center was completely ‘filled’ by the disk. In this display, observers perceived the flashed disk to lag such that it appeared only to partially ‘fill’ the ring center. The ‘unfilled’ portion (perceived void) of the moving ring was seen in the color of the background. With smooth pursuit of the ring, the flash-lag effect was eliminated, and observers saw the flashed disk centered on the moving ring. A strong flash-lag effect was observed when observers smoothly pursued a moving point target past a continuously visible stationary ring. Once again, the flashed disk appeared to only partially fill the center of the continuously visible stationary ring, yielding a vivid ‘perceived void’. These results are discussed in terms of neural delays and their compensation.

Saccades reveal that allocentric coding of the moving object causes mislocalization in the flash-lag effect

“…As a consequence, the extrapolation account would predict no dissociation between perceptual and motor responses in the flash-lag effect, contrary to what has been found for other visual illusions (Nijhawan, 1994(Nijhawan, , 2001; see also Nijhawan & Kirschfeld, 2003).…”

Through the eye, slowly; Delays and localization errors in the visual system

“…First, noting that the brain is getting only outdated information on the position of a moving object (as in smooth pursuit), Nijhawan 67,68 interpreted the flash-lag as an attempt at saccade by as much as 50 ms), as expected if the afferent visual delay is bypassed. These experiments confirm the results of perisaccadic mislocalization by using an excitatory drive applied directly within neural structures.…”

Physical, Neural, and Mental Timing

“…The flash is then perceived as lagging some distance behind the apparent position of the moving object. Nijhawan (1994Nijhawan ( , 1997Nijhawan ( , 2001 proposes that the nervous system predicts a motion path so that the perceived position of the moving object is close to veridical at all times. Due to its latency it takes a while before the flash is perceived (at its proper position), say at time t ϩ T, but in the meantime the moving object is already at a different position, it has moved a distance, VT, if V is its speed.…”