Flash lag effect: Speeding up to get ahead?

Beena Khurana*, Katsumi Watanabe, Romi Nijhawan

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


In primates, visual processing takes place in a series of hierarchical steps. Cumulative neural delays should cause a moving object's perceived position to lag its instantaneous physical position. However, there is a paradox: When a moving object's instantaneous perceived position is measured by presenting a flash in spatial alignment with it, observers see the moving object as leading the flash. If delays in the processing of the moving and the flashed object are assumed to be the same, then the moving object must speed-up in order to be perceived ahead of the flash. Alternatively, the above stated "perceptual-lag-for-motion" premise entrenched in current thinking on neural delays requires re-examining. Six observers first participated in a standard flash-lag experiment. The flash-lag effect measured for each observer in this first experiment was then used to calculate the magnitude of speed increment that the moving object would have to undergo in order to be seen ahead of the flash. Next the same observers participated in a signal detection task where the signal was the speed increment calculated from the first experiment. Observers' performance was close to perfect in terms of distinguishing between trials with a speed increment from those without. The results indicate that the flash-lag effect occurs in the absence of any perceived speed increment. These results strongly oppose the " perceptual-lag-for-motion" premise, and provide further support for a visual mechanism that corrects for the delays for moving objects.

Original languageEnglish
Pages (from-to)394a
JournalJournal of Vision
Issue number9
Publication statusPublished - 2003
Externally publishedYes

ASJC Scopus subject areas

  • Ophthalmology
  • Sensory Systems


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