Cognitive functions communicated between the cerebral hemispheres by the corpus callosum was studied in two patients with commissurotomies at the Cognitive Neuroscience Laboratory, Department of Psychiatry, Dartmouth Medical School, Hanover, NH and the Department of Neurosciences, University of California at San Diego, CA. The patients were asked to judge whether pairs of words rhymed. One word in each pair was presented to the left visual field and the other to the right visual field. The two words in each pair either sounded and looked alike (R+ L+), sounded alike but looked different (R+ L−), sounded different but looked alike (R− L+), or both sounded and looked different (R− L−). The two commissurotomy patients differed in that one had sparing of some rostral and splenial fibers of the corpus callosum verified by MRI while the second patient had MRI-verified full callosal section. The patient with some sparing of fibers was able to perform the rhyming judgment significantly better than chance when the words both looked and sounded alike (R+ L+) whereas her accuracy did not differ from chance in the other three conditions. The second patient with full callosal section performed at chance in all conditions, and normal control subjects were significantly better than chance in all conditions except R+ L−. Both patients had callosal section performed at age 26 to control intractable epileptic seizures. These results indicated to the authors that the first patient was capable of comparing both phonologic and orthographic information across her hemispheres and this ability reflected the functioning of the callosal remnant fibers. The splenium of the corpus callosum interconnects visual association cortex and this patient's ability to transfer orthographic information was commensurate with the splenial locus of her spared callosal fibers. The surviving rostral fibers may have contributed to the transfer of phonologic information. 
COMMENT. The MRI has proved of value in defining the extent of brain lesions with greater precision than was previously possible and has provided more accurate information regarding the areas of the callosum that are cut in the split brain patient. This study has helped to specify the functional zones of the human callosum in regard to cognition. A second paper regarding magnetic resonance imaging morphology of the corpus callosum in monozygotic twins is published from the same Program in Cognitive Neuroscience of Dartmouth Medical School . There are wide variations in the size and shape of the human corpus callosum and measurements of size and shape revealed greater similarity in twin pairs than in randomly paired controls. The results were consistent with the view that the anatomy of the corpus callosum is under genetic control as well as being influenced by nongenetic factors.