This research uses a striking visual illusion, the flash-lag effect, to study the normal functioning of the human visual system. One example of the illusion involves a line segment rotating about its center. A stationary segment is briefly flashed at each end of the rotating segment, such that the rotating and stationary segments are in alignment. Surprisingly, observers perceive the stationary segments not as aligned with the rotating segment, but rather as lagging slightly behind it. Even more striking is a version of the phenomenon involving color. If a red line is flashed within a stationary green bar, observers perceive the line as yellow (because mixing red and green light yields yellow, due to the way the visual system processes color). However, if a red line is flashed briefly within a rotating green bar, observers don't see a yellow line within a green bar; rather they see a red line trailing the green bar. Our hypothesis is that the flash-lag effects result from mechanisms within the visual system that compensate for delays in neural transmission time. Transmission of information from the eyes to the brain takes appreciable time; by the time information about a moving object reaches the brain, the object has moved some distance. Thus, we might expect the object to be seen not where it actually is, but rather where it was a short time ago. However, we suggest that the visual system extrapolates the registered location of smoothly moving objects to compensate for the effects of the delay. Thus, given collinear moving and stationary line segments, the moving segment is seen as leading the stationary segment because the visual system extrapolates the location of the moving but not the stationary segment. We will study several forms of the flash-lag effect under various conditions. The results will increase our understanding of how the visual system compensates for neural delays; also, the studies involving colored stimuli will allow us to test competing theories of color vision.
