Anatomy of the corpus callosum reveals its function.

Editor's Note: These short critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and comentary. For more information on the format and purpose of the Journal Club, please see Review of Wahl et al. The corpus callosum (CC) comprises ax-ons connecting the cortices of the two ce-rebral hemispheres and is the principal white matter fiber bundle in the brain. As recently as the mid 20th century, the CC was thought to serve no other purpose than preventing the two hemispheres from collapsing on one another (Bogen, 1979). This cynical view was attributable to the failure of the Van Wagenen/Ake-laitis split-brain surgery to reveal strong disconnection symptoms. The work of Myers and Sperry (1958) in the 1950s changed this by definitively confirming the functional role of the CC in the inter-hemispheric transfer of visual information in animals. Since then, the structure and function of the CC have remained topics of continuous investigation (Zaidel and Iacoboni, 2003). In particular, researchers have asked how callosal structure relates to callosal function (i.e., hemi-spheric specialization and interaction). One pioneering study of callosal morphology relied on light microscopy of postmortem specimens to look at callosal structure and fiber composition (Aboitiz et al., 1992), but most anatomical studies of humans have relied on structural magnetic resonance imaging (MRI) mor-phometry of midsagittal cross-section views of the CC. In contrast, a few recent studies have used diffusion tensor imaging (DTI) methods to re-evaluate callosal topography (for discussion, see Wahl et al., 2007). These methods challenge the conventional partitioning schemes used to divide the CC into functionally significant regions (Witelson, 1989). In their recent article published in The Journal of Neuroscience, Wahl et al. (2007) revisit the issue of CC topography using functional MRI (fMRI) and DTI to assess callosal motor fiber (CMF) microstruc-ture in vivo. Their participants performed a simple, visually guided motor task that included alternating blocks of pursing lips, flexing left-hand fingers, right-hand fingers, left toes, and right toes, and a rest condition, all while being scanned. DTI analysis allows one to quantify the magnitude and direction of water diffusion (i.e., fractional anisotropy). Because diffusion of water molecules is hindered by the ax-onal membrane and myelin sheath, the molecules disperse primarily in the direction of the fiber tract, thus allowing one to infer the overall orientation …