A previous study demonstrated the sensitivity of these connections

to alterations of the visual input (Levin et al., 2010). Visual pathways white matter analysis was performed in two steps: identifying the fiber bundles and evaluating their properties. Using a new probabilistic algorithm (Sherbondy et al., 2008a, 2008b), we could clearly identify the optic tract and the optic radiation composing the input fibers to the visual cortex as well as the output fibers from each hemisphere, which cross at the corpus callosum ( Figure 3A). Following fiber identification, we studied white matter integrity using directional diffusivity measures. By measuring diffusivity in multiple directions we obtained estimates of the principal diffusion direction (longitudinal) as well as the perpendicular direction (radial). The ratio of these two EGFR inhibitor values is similar to the fractional anisotropy (FA). We found that the properties of the achiasmic subject’s CHIR-99021 solubility dmso (AC2) visual pathways were within the range of 30 normally sighted control subjects

( Figure 3B). Finally, the cross-sectional area of the occipital fibers that connect right and left visual cortex was assessed ( Figure 3C). In normal sighted controls, there is a correlation between the cross-sectional area of this tract and the cross-sectional area of the entire callosum. The cross-sectional area of the achiasmic subject’s occipital callosal fiber group was smaller than that of controls, yet the overall size of his corpus callosum was small too ( Figure 3D). These results highlight that the white matter integrity at the resolution of our neuroimaging measurements is comparable to control subjects. Our results highlight both differences and similarities of the achiasmic compared to the typical human visual system. In achiasma, we found a highly atypical organization of the visual cortex consisting of overlapping visual hemifield maps with bilateral pRFs. In contrast, pRF sizes were in the

normal range as were the properties of all major visual pathways, in particular the geniculate-cortical and occipital-callosal fibers. Moreover, normal pRF sizes across early visual cortex in conjunction with the persistence of bilateral pRFs imply relatively unaltered cortico-cortical connections (Harvey isothipendyl and Dumoulin, 2011). Our results can be explained by conservative developmental mechanisms in human achiasma that largely preserve the normal visual pathways beyond the LGN. Both retinotopic and pRF mapping demonstrated an overlay of orderly retinotopic maps from opposing hemifields in the visual cortex, such that each cortical location represents two separate visual field locations, namely one in each hemifield. This intermixed representation could result from individual neurons with bilateral receptive fields, but also from the interdigitation of two different neural populations representing the contra- and ipsilateral visual field at the current fMRI resolution.