Visual function and brain organization in non-decussating retinal-fugal fibre syndrome

Functional neuroimaging, psychophysical and electrophysiological investigat ions were performed in a patient with non-decussating retinal-fugal fibre s yndrome, an inborn achiasmatic state in which the retinal projections of ea ch eye map entirely to the ipsilateral primary visual cortex. Functional ma gnetic resonance imaging (fMRI) studies showed that for monocularly present ed simple visual stimuli, only the ipsilateral striate cortex was activated . Within each hemisphere's striate cortex, the representation of the two he mifields overlapped extensively. Despite this gross miswiring, visual funct ions that require precise geometrical information (such as vernier acuity) were normal, and there was no evidence for the confounding of visual inform ation between the overlapping ipsilateral and contralateral representations . Contrast sensitivity and velocity judgments were abnormal, but their depe ndence on the orientation and velocity of the targets suggests that this de ficit was due to ocular instabilities, rather than the miswiring per se. Th ere were no asymmetries in performance observed in visual search, visual na ming or illusory contour perception, fMRI analysis of the latter two tasks under monocular viewing conditions indicated extensive bilateral activation of striate and prestriate areas. Thus, the remarkably normal visual behavi or achieved by this patient is a result of both the plasticity of visual pa thways, and efficient transfer of information between the hemispheres.