Abstract
In the absence of an optic chiasm, visual input to the right eye is represented in primary visual cortex (V1) in the right hemisphere, while visual input to the left eye activates only the left hemisphere. Retinotopic mapping reveals that left and right visual hemifield representations are overlaid in V1 in each hemisphere (Hoffman et al., 2012). To explain how overlapping hemifield representations in V1 do not impair vision, we tested the hypothesis (Victor et al., 2000) that visual projections from nasal and temporal retina create interdigitated left and right visual hemifield representations in V1, similar to the ocular dominance columns observed in control subjects. We used high-resolution fMRI at 7T to measure the spatial distribution of responses to left- and right-hemifield stimulation in one achiasmic subject. T2-weighted 2D Spin Echo images were acquired at 0.8 mm isotropic resolution, covering parafoveal regions of V1 (24 slices per 2 sec TR). The left eye was occluded while flickering checkerboards were presented to the right eye. Twelve sec presentations alternated between the left and right visual hemifield, separated by 12 sec mean field presentations. The subject performed a demanding orientation-discrimination task at fixation. A general linear model was used to estimate the responses of voxels in V1 to left- versus right-hemifield stimulation. The spatial distribution of voxels with preference for one hemifield or the other showed interdigitated clusters which densely packed V1 in the right hemisphere. The spatial distribution of hemifield-preference voxels in the achiasmic subject was comparable to the distribution of ocular-dominance voxels in a control subject, measured using standard techniques. These results are consistent with the hypothesis that visual hemifield representations interdigitate in achiasmic V1 following a similar developmental course to that of ocular dominance columns in controls.
Meeting abstract presented at VSS 2014