[Functional topography of the human corpus callosum]

The concept of a topographical map of the corpus callosum (CC) has emerged from lesion studies in humans and from anatomical tracing investigations in other mammals. We conducted the first in vivo study aimed at outlining the topographical organization of the normal human CC, using non-invasive functional magnetic resonance imaging (fMRI). We tested cortical and callosal activation by the BOLD effect during simple sensory stimulation (tactile, gustatory and visual) and simple motor tasks in 38 volunteers. The axonal organization of callosal white matter was also studied in 16/38 subjects, using diffusion tensor imaging (DTI) and diffusion tensor tractography (DTT). Activation foci evoked by taste stimuli were detected in most subjects in the anterior part of the CC, those elicited by motor tasks lay in the central portion of the body of the CC, and those elicited by tactile stimulation of different body regions lay in the posterior part of the body. Activation foci evoked by visual stimulation were seen in the splenium of the CC. Callosal fibers interconnecting the primary cortical areas activated by taste stimulation, motor tasks, and tactile and visual stimuli were shown by DTT. Anatomical correlates of the BOLD activation foci were demonstrated in the CC, with fibers crossing it at the level of the genu, anterior and posterior body, and splenium, respectively. This study demonstrates for the first time that the functional topographical organization of the human CC can be explored by fMRI in vivo. Our findings may have clinical implications, especially for neurosurgical planning.