THE ROLE OF THE CORPUS-CALLOSUM IN THE REPRESENTATION OF LATERAL ORIENTATION

How do people recognize objects when they face in a novel lateral (lef t/right) orientation? The results of three experiments with a split-br ain patient, who has a totally nonfunctional corpus callosum, demonstr ate that the corpus callosum cannot play a critical role in allowing o ne to recognize mirror-reversed objects. First, both cerebral hemisphe res 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 ori ginally, neither hemisphere performed better than chance in an inciden tal memory task-suggesting that the shape representations in the hemis pheres do not specify lateral orientation. Third, neither hemisphere e xhibited ''priming'' for lateral orientation, as assessed in an ''obje ct decision task'', and only the left hemisphere exhibited priming for encoding the shape (independent of its lateral orientation).