Simple differential latencies modulate, but do not cause the flash-lag effect

Abstract

When a brief static flash is presented in physical alignment with a moving target, the position of the flash can seem to lag behind that of the moving target. Various explanations exist. One of the most popular is the differential latency hypothesis. This assumes that neural latencies for moving stimuli are shorter than those for static stimuli. Accordingly, the flash lag would occur because perception is temporally fragmented, with moving stimuli seen before static. To test this, observers were asked to make different judgments concerning the same stimulus. Observers either judged if color changes in moving and static stimulus sections were synchronous, or whether the different stimulus sections were aligned at the time of a color change. If the flash-lag were driven by a simple differential latency, we would expect both judgments to be marked by a temporal advantage for moving stimuli. Our results suggest the contrary, as only the later judgment was marked by a flash-lag effect. The apparent timing of moving and static color changes was veridical. However, when we introduced a systematic differential latency, by modulating image contrast, both judgments were affected. Our data therefore suggest that a simple differential can modulate flash-lag type effects, but they do not cause the phenomenon.