Perception of apparent motion across the retinal midline following commissurotomy.

One subject (L.B.) with complete forebrain commissurotomy, another (D.K.) with posterior callosotomy, and 12 normal controls, were shown either single lights, simultaneous pairs, or successive pairs, presented either within the left or right visual fields or bilaterally. Regardless of location, all subjects scored at or near ceiling in discriminating: (1) simultaneous pairs from single lights, (2) successive pairs from single lights, (3) simultaneous pairs from successive pairs, and (4) leftward succession from rightward succession. However, with bilateral presentation, L.B. was often slow to respond to successive lights, and his accuracy in detecting bilateral succession deteriorated when successive presentations were intermixed with simultaneous pairs and single lights. These and other results suggest that three mechanisms may contribute to the discrimination of apparent motion: the detection of simultaneous events, a subcortically mediated switch in attention from first to second location, and cortical tracking between locations. Cortical tracking across the midline is incapacitated following complete forebrain commissurotomy.