Visual and Auditory Anomalies Associated with Albinism

One of the evolutionary characteristics of the mammalian visual system is the increase of binocular overlap of vision as eyes progress from being located on side of the head such as the guinea pig to the frontal position in Haplorrhine primates. Concomitantly as the proportion of temporal retina increases the number of uncrossed optic neurons at the optic chiasm expands from as little as 1% in guinea pigs to approximately 45% in primates including human beings (1, 2). Figure 1 illustrates the approximate distribution of crossed and uncrossed retinal ganglion cells (RGCs) in the visual pathways of normally pigmented human beings.C. L. Sheridan (3) compared the interocular visual pathways in split brain ocularly pigmented (hooded) rats and albino rats. Sheridan concluded “Perhaps the paucity of uncrossed fibers that characterized rodents in general is even further reduced in the albino. That year Lund (4) verified Sheridan's hypothesis anatomically. Lund stated albino rats display no organized uncrossed optic fibers. Lund agreed with previous estimates that pigmented rats possess up to 10% uncrossed optic fibers. Figure 2 is a diagram of crossed and uncrossed optic projections in wild-type rodents.Mammals with few uncrossed retinal ganglion cell fibers (RGCs) such as rodents and lagomorphs do not exhibit the laminated dorsal lateral geniculate nucleus (dLGN) seen in carnivores and primates including human beings. Carnivores and Haplorrhine primate dLGN contain up to six monocular layers with point-to-point representation of visual space organized in columns through these layers. The 90%+ crossed RGCs at the optic chiasm in rodents and lagomorphs fill the contralateral dLGN, with only a pocket of the ipsilateral dLGN devoted to the fewer uncrossed fibers. Figure 3 illustrates RGC fiber terminations in horizontal sections through the dLGN comparing black and albino rats. The fewer uncrossed fibers in albino rodents are also projected to the dLGN in a fragmented fashion as opposed to a close organization in pigmented rodents.