Direct interhemispheric visual input to human speech areas.

Very little is known about the connectivity of the human cerebral cortex. Nonhuman primates often serve as a model, but they are very unsatisfactory when it comes to specifically human functions. Evidence from (human) lesion and activation studies indicates that Broca's and Wernicke's areas play a critical role in language functions, whereas the inferior temporal cortex of the right hemisphere tends to be associated with high-level visual recognition. We describe here monosynaptic interhemispheric input from the right inferior temporal cortex to Wernicke's and Broca's areas. The connections were traced in a brain with a right inferior temporal infarction by means of the Nauta method for anterogradely degenerating axons. Afferents were found both in Broca's and Wernicke's areas, with a higher density in the latter. Three organizational principles emerge from this study. First, the presence of direct connections from the right inferior temporal cortex to the speech areas indicates that human interhemispheric connections can be widely heterotopic. Second, the fact that connections from the inferior temporal cortex terminate in both Wernicke's and Broca's areas speaks in favor of parallel pathways in visuo-verbal processing. And third, the patchy distribution of visual interhemispheric afferents in Wernicke's area hints at a possible functional compartmentalization within this area.