Confusion of space and time in the flash-lag effect.

The apparent lagging of a short flash in the relation to a moving object, the flash-lag effect (FLE), has so far been measured mainly in terms of illusory spatial offset. We propose a method of measuring the perceived temporal asynchrony of the FLE separately from its perceived spatial offset. We presented a moving stimulus that changed its colour at a certain moment. The observer indicated, in two different tasks, where and when the colour change occurred in relation to a stationary reference flash. Results show that the perceived time of the colour change was not congruent with the perceived location of the colour change: the colour change is perceived simultaneously with the flash, but is shifted in position. The presentation of the reference in the form of a flash is not critical for the occurrence of the FLE, because the same effect was obtained with a constantly visible reference signal, the position of which or time when it changed its colour were varied. The observer was not able to ignore the irrelevant dimension of the reference signal: the apparent time of the colour change was influenced by the position of the reference signal, and the apparent location of the colour change was influenced by the presentation time of the reference signal. The observer's inability to separate the spatial and temporal aspects of the moving stimulus clearly imposes certain limits on theories that are attempting to explain the FLE exclusively in terms of the perceived space and time.