An investigation of the line motion effect in a callosotomy patient

When a line is flashed instantaneously between two markers it can appear to propagate from one marker to the other. This illusion is known as the line motion effect. We investigated this effect in the two hemispheres of a callosotomy ("split-brain") patient. We found that both hemispheres perceived the line motion effect, and that flashing one of the markers biased the direction of motion away from that marker regardless of which hemisphere received the stimulus. In contrast, matching the width of the line to the width of one of the markers biased the direction of motion away from the marker only when it appeared in the left visual hemifield. This suggests that multiple mechanisms can contribute to the line motion effect, and that some of these mechanisms rely on different neural structures.