FMRl mapping of visual projections in an achiasmatic subject

Purpose. We employ fMRI to investigate the conical organization of visual projections in a subject with isolated congenital absence of the optic chiasm due to a non-decussating retinal fugal fibre syndrome1. In a previous evaluation, the subject was found to have full monocular visual fields, even though scalp VHP topography suggested purely ipsilateral projections'. Methods. Cortical responses were obtained during monocular full field flashing stimulation and hemifield pattern reversing stimulation. A 1.5T GE MR scanner was employed to acquire echoplanar images during baseline and stimulation epochs. A 5 surface coil was centered at the occipital pole and 16 contiguous 3 mm thick slices were positioned over the calcarine sulcus with in-plane resolution of 1.5 x 1.5 mm. A single voxel analysis performed at multiple levels of significance yielded the location and frequency of voxels within primary visual cortex that were active during stimulation. Results. In response to full field monocular flash stimulation activation in primary visual cortex was seen only ipsilateral to the stimulated eye. In response to hemifield monocular stimulation, activation patterns elicited by ipsilateral and contralateral hemifields were generally found to co-occupy common regions of the ipsilateral primary visual cortex, to within the 1.5 mm resolution of the fMRI technique. Conclusions- These results reveal a remarkable plasticity of retinofugal projections and human cortical organization.