Binocular facilitation of cone-specific visual evoked potentials in colour deficiency

Neural compensatory mechanisms have been proposed, which preserve the binocular visual field in glaucoma, as well as cognition in Alzheimer's disease and motor function in Parkinson's disease. It is conceivable that comparable mechanisms operate to preserve function in congenital and/or dystrophic disease. In hereditary colour vision deficiency (CVD), we observed significant facilitation in the amplitude of the binocular cone-specific visual evoked potential (VEP) compared to the monocular amplitude for the cone type corresponding to the CVD. We propose that this finding may reflect preservation of function in hereditary colour vision deficiency.

Binocular and monocular L, M and S cone-specific VEPs were recorded from 12 colour vision deficient subjects and 17 with normal colour vision, confirmed to be CVD or normal on a battery of colour vision tests. Binocular VEP amplitudes were compared to monocular amplitudes within subjects and between subject groups.

Subjects with CVDs showed binocular facilitation of VEP amplitude (enhancement more than 2.0 times; mean: 2.8 times, p = 0.0003) for the cone type corresponding to their CVD. Mean facilitation of 2.8 times exceeded binocular enhancement for other cone types within CVDs (2.8 times versus 1.2 times) and compared to colour vision normals (2.8 times versus 1.2 times).

Hereditary CVDs show binocular facilitation of cone VEP signals for the cone type corresponding to their CVD. As CVD is typically assessed with foveal stimuli, our findings using wider-field binocular stimulation suggest that enhanced colour perception may occur in CVD across a more extensive area of visual field. These results may relate to binocular visual field enhancement in glaucoma and improved colour vision in CVD at supra-threshold levels of stimulation.