The development of callosotomy (or “split-brain”) surgery for the relief of intractable epilepsy offered the first opportunity to assess the roles of the cortical and subcortical connections in transferring information between the hemispheres. Some of the early research on interhemispheric transfer in these patients produced confusing and conflicting results, partly due to the lack of availability of noninvasive methods to verify that the entire corpus callosum had been resected. The advent of MRI scanning revealed that some patients had residual callosal fibers that had been inadvertently spared during surgery.1 In 1985 our research group reported such a case.2 We described MRI assessment of callosotomy in three patients. In one of these, MRI revealed areas of bright signal in the splenium and rostrum of the corpus callosum, which were assumed to reflect spared callosal fibers. Like most callosotomy patients, our patient fails most tests of interhemispheric information transfer.2,3⇓ Several studies have shown, however, that she is sometimes able to integrate information between the two hemispheres when the stimuli are visually presented words.4-6⇓⇓ We have speculated that …