Truncation of the flash-lag effect by a fixed spatial landmark.

The flash-lag effect is a visual illusion where a moving image is perceived to be advanced in its spatial location relative to a flashed image. Multiple studies have shown that the flash-lag effect can be enhanced by increasing the uncertainty of the moving and/or flashed images. However, little is known about the effect of task-irrelevant visual objects on the flash-lag effect. We were interested to see whether a task-irrelevant spatial landmark might reduce uncertainty and hence reduce the flash-lag effect. We placed a fixed bar between moving and flashed bars while measuring the flash-lag effect in six participants. For most participants, the fixed bar substantially truncated the flash-lag effect. The effect was maximal when the fixed bar was aligned with the flashed bar and decreased when the fixed bar was positioned more peripherally. A second experiment with two participants used a smaller fixed bar; the smaller bar had less truncation effect in one participant, while the other participant showed similar truncation regardless of the fixed bar size. Our results support models that place the locus of the flash-lag effect in higher-order brain areas, e.g., the parietal lobe.