J. Neurosci. -- eLetters for Wahl et al., 27 (45) 12132-12138

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BehavioralSystemsCognitive:
Mathias Wahl, Birgit Lauterbach-Soon, Elke Hattingen, Patrick Jung, Oliver Singer, Steffen Volz, Johannes C. Klein, Helmuth Steinmetz, and Ulf Ziemann
Human Motor Corpus Callosum: Topography, Somatotopy, and Link between Microstructure and Function
J. Neurosci. 2007; 27: 12132-12138 [Abstract] [Full text] [PDF]

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Micro, Macro, and Function Relations of the Human Corpus Callosum Remain Challenging
Khader M Hasan (4 December 2007)

Micro, Macro, and Function Relations of the Human Corpus Callosum Remain Challenging 4 December 2007

Khader M Hasan,
Assistant Professor of Diagnostic and Interventional Imaging
University of Texas Health Science Center at Houston, Hosuton, Texas 77030
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Re: Micro, Macro, and Function Relations of the Human Corpus Callosum Remain Challenging

khader.m.hasan{at}uth.tmc.edu Khader M Hasan

Wahl et al. (2007) suggested in their Discussion that the diffusion-tensor- (DT-) based fractional anisotropy (FA) in the corpus callosum (CC) “reflects primarily fiber density rather than the degree of myelination or axon diameter” (Wahl et al., 2007; page 12137). To the best of our knowledge, this suggestion contradicts previous histology and DT-MRI reports that failed to demonstrate such association (Pierpaoli et al, 1996; Beaulieu, 2002). The study by Wahl et al. (2007) did not provide histology or callosal “functional activation” area measurements. They did not report transverse nor axial eigenvalues that would have provided a more specific surrogate of axonal packing and myelination (Drobyshevsky et al., 2005). The suggested hypothesis may explain qualitatively the observation: FA(genu CC) > FA(body CC), but it may not explain the trend that has been documented (see Hasan et al. 2005) by several DTI studies: FA(splenium CC) > FA(genu CC) > FA(genu CC). Although scant, published histological studies on both rhesus monkey (La Mantia and Rakic, 1990) and human CC (Aboitiz et al. 1992) did not report significant correlations between callosal axonal packing and callosal area that may be attributable to the fiber diameter, separation, and myelination distribution regional variability. The association of callosal areas with the corresponding FA needs some careful validation on controls. A recent DTI study on healthy adult controls by Westerhausen et al. (2006) reported that posterior callosal area correlated negatively with diffusion anisotropy, while Alexander et al. (2007) reported on healthy children a positive correlation between “corpus callosum volume” and the corresponding FA mean value. Thus, conclusive data from both MRI and histology in regards to the relations between the CC micro and macrostructure on and their interplay with function and behavior remain elusive even on the simplest of all brain structures.
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