Same/different judgements about the direction and colour of apparent-motion stimuli after commissurotomy

One subject with complete forebrain commissurotomy (L.B.), another with posterior callosotomy (D.K.) and 6 normal controls were tested for their ability to make same/different judgements about the direction (Experiment 1) and colour (Experiment 2) of apparent-motion stimuli. In the first experiment, all of the subjects were able to judge whether the direction of apparent motion of two stimulus sequences was the same or different, regardless of whether the two sequences were presented unilaterally or bilaterally. In the second experiment, however, L.B. was unable to judge whether the colours of two lights comprising an apparent motion sequence were the same or different when the two lights were presented bilaterally although he was able to do so when both lights were in the same visual field. D.K. was able to make the discrimination on both bilateral and unilateral presentations but he performed more poorly in the bilateral condition than in either visual field. The control subjects performed consistently in all three locations. Given that same/different judgement tasks are a rigorous test of interhemispheric integration, the results indicate that the ability to discriminate apparent motion across the retinal midline is intact in split-brained subjects. Furthermore, the results support a dissociation in the interhemispheric integration of the direction and colour of apparent-motion stimuli after commissurotomy.