Diameter, Length, Speed, and Conduction Delay of Callosal Axons in Macaque Monkeys and Humans: Comparing Data from Histology and Magnetic Resonance Imaging Diffusion Tractography

A, FA values in different regions of the CC of the macaque monkey. The black curves refer to the FA values of the three animals used in this study, and the gray curves indicate the FA values of the four animals studied by Hofer et al. (2008). B, Normalized FA values across different callosal sectors (black bars) and comparison with data (gray bars) reported by Hofer et al. (2008). Abbreviations are as in Figure 1.

A, Overall view of callosal axons connecting different cortical areas of the macaque monkeys. B, Prefrontal axons cross the midline in the genu of the CC, motor (M1) axons cross in the midbody, somatosensory axons (S1) in the posterior part of the midbody, and visual (V1/V2) axons cross the midline in a selected zone of the splenium. White dashed lines separate fiber bundles crossing the CC in different sectors. The color-coded cube indicates the local mean direction of diffusion: red, left (L)–right (R); green, anterior (A)–posterior (P); blue, superior (S)–inferior (I).

A, Left, Microphotograph of a bundle of callosal axons emerging from a BDA injection site in dorsal prefrontal cortex (area 9) and coursing toward the corpus callosum. Four injections sites from different areas are also shown [reproduced with modifications from Caminiti et al. (2009) and Tomasi et al. (2012)]. Right, Reconstructed trajectory of callosal fibers labeled by BDA injections in area18 (dark blue) and area17 (light blue) at the 17/18 border. B, Outlines of selected sections (gray) and lateral ventricles (black) are also shown; calibration, 10,000 μm [reproduced with modifications from Caminiti et al. (2009)]. C, Comparison of the length of callosal axons from their origin to the midline as estimated by DTT and by histological reconstruction in the macaque monkey. D, Temporal delays to the CC midline computed by using data from the distribution of axon diameters, hence conduction velocities, with the conduction distance derived with DTT and with BDA fiber labeling, according to the three different methods illustrated in Materials and Methods.

Raw (A) and normalized (B) FA values in different sectors of the CC in humans and their comparison with the values obtained in corresponding callosal sectors of monkeys (C, D). In A, gray lines are data from individual subjects, and the black line indicates their average.

A, Overall view of the DTT-derived topography of the CC origination in different cortical areas of the human brain. B, Different panels show callosal axon bundles from prefrontal, primary motor, posterior parietal, and visual areas. Conventions and symbols are as in Figure 3.