Perception of apparent motion of colored stimuli after commissurotomy

One subject with complete forebrain commissurotomy (L.B.), another with posterior callosotomy (D.K.), and eight normal controls were presented with successive pairs of red and/or green lights, on either side of the retinal midline and within the left and right visual fields. All of the subjects could discriminate the direction of apparent motion in all three locations, although L.B. was poorer on bilateral than unilateral presentations. Moreover, on bilateral presentations, L.B. was significantly poorer at identifying the color of the first light than the color of the second light. In contrast, D.K., like the control subjects, was equally good at identifying the color of either light. These and other results provide evidence that a subcortical shift in attention enables L.B. to discriminate the direction of apparent motion across the midline. On the other hand, a more robust mechanism involving the middle temporal area of the cortex must be responsible for tracking motion in D.K. and the control subjects. On the basis of these findings, it is suggested that the superior colliculus may contribute to direction sensitivity in the middle temporal area by mediating shifts in spatial attention.