Abnormal visual projection in a human albino studied with functional magnetic resonance imaging and visual evoked potentials. (Short Report).

The albino visual pathway is abnormal in that many fibres from the temporal retina project to the contralateral visual cortex. The visual projections in a human albino and a control have been investigated with fMRI and VEP during independent visual stimulation of both hemifields. Activity in the occipital cortex in the normal was contralateral to the stimulated visual field, whereas it was contralateral to the stimulated eye in the albino, independent of the stimulated visual field. Thus, the albino visual cortex is activated not only by stimulation in the contralateral visual field, but also by abnormal input representing the ipsilateral visual field. These novel findings help elucidate the nature of albino misrouting.

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Albinism has a profound effect on visual development and visual function. (1-3) In normal people, decussation of the optic fibres is loyal to the vertical meridian that passes through the fovea: fibres from the temporal retina project ipsilaterally to the lateral geniculate nucleus and cortex, whereas fibres from the nasal retina project contralaterally. In albinos, however, a higher than normal proportion of fibres from the temporal retina project contralaterally. The line of decussation is, as a consequence, shifted to the peripheral visual field. The impact of this abnormal visual projection on the cortical mapping of the visual field in human albinism has yet to be determined.

Clinically, the diagnosis of albinism is confirmed by measuring the visual evoked scalp potentials elicited by monocular full field pattern appearance (pattern onset-offset) stimulation. In albinos, the resulting visual evoked potential (VEP) is generally greatest at recording sites contralateral to the stimulated eye. It is this hemispherical VEP response asymmetry that is used as a diagnostic criterion in conjunction with the interocular reversal of this hemispherical asymmetry. (4) The reported sensitivity of using these criteria was remarkably high in one study (100% by Apkarian et al (6)), but other studies using similar criteria have reported lower sensitivities (45% by Bouzas et al (7); 83% by Soong et al (8); 18% by Jarry et al (9)). Furthermore, due to the limited spatial resolution, VEP investigations cannot contribute to specific questions of albino cortical organisation such as the representation of the visual field in the early visual areas and the degree of cortical magnification. Functional MR I (LMRI), however, has been used to elucidate visual cortical organisation in normal (10,11) and abnormal subjects (12-14) and has also been successfully employed to highlight lateralisation …