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Abstract
Several tracing studies have established a topographical distribution of fiber connections
to the cortex in midsagittal cross-sections of the corpus callosum (CC). The most
prominent example is Witelson's scheme, which defines five vertical partitions mainly
based on primate data. Conventional MRI of the human CC does not reveal morphologically
discernable structures, although microscopy techniques identified myelinated axons
with a relatively small diameter in the anterior and posterior third of the CC as
opposed to thick fibers in the midbody and posterior splenium. Here, we applied diffusion
tensor imaging (DTI) in conjunction with a tract-tracing algorithm to gain cortical
connectivity information of the CC in individual subjects. With DTI-based tractography,
we distinguished five vertical segments of the CC, containing fibers projecting into
prefrontal, premotor (and supplementary motor), primary motor, and primary sensory
areas as well as into parietal, temporal, and occipital cortical areas. Striking differences
to Witelson's classification were recognized in the midbody and anterior third of
the CC. In particular, callosal motor fiber bundles were found to cross the CC in
a much more posterior location than previously indicated. Differences in water mobility
were found to be in qualitative agreement with differences in the microstructure of
transcallosal fibers yielding the highest anisotropy in posterior regions of the CC.
The lowest anisotropy was observed in compartments assigned to motor and sensory cortical
areas. In conclusion, DTI-based fiber tractography of healthy human subjects suggests
a modification of the widely accepted Witelson scheme and a new classification of
vertical CC partitions.