The role of the corpus callosum in the representation of lateral orientation.

How do people recognize objects when they face in a novel lateral (left/right) orientation? The results of three experiments with a split-brain patient, who has a totally nonfunctional corpus callosum, demonstrate that the corpus callosum cannot play a critical role in allowing one to recognize mirror-reversed objects. First, both cerebral hemispheres could recognize mirror-reversed versions of pictures as accurately as the original renditions in an incidental memory task. Second, when asked to decide whether pictures faced the same way that they had originally, neither hemisphere performed better than chance in an incidental memory task--suggesting that the shape representations in the hemispheres do not specify lateral orientation. Third, neither hemisphere exhibited "priming" for lateral orientation, as assessed in an "object decision task", and only the left hemisphere exhibited priming for encoding the shape (independent of its lateral orientation).