Retinal and Cortical Determinants of Cortical Magnification in Human Albinism
The human fovea lies at the center of the retina and supports high-acuity vision. In normal visual system development, foveal acuity is correlated with both a high density of cone photoreceptors at this location and a magnified retinotopic representation of the fovea in the visual cortex. Both cone density and the cortical area dedicated to each degree of visual space--the latter known as the cortical magnification function--steadily decline with increasing eccentricity from the fovea. In albinism, peak cone density at the fovea and visual acuity are reduced but appear to be normal in the periphery, thus providing a model to explore the correlation between retinal structure, cortical structure, and behavior. Here, we used adaptive optics scanning light ophthalmoscopy to assess retinal cone density and functional magnetic resonance imaging to measure cortical magnification in primary visual cortex of normal controls and individuals with albinism. We find that retinotopic organization is more varied in albinism than previously appreciated, yet cortical magnification outside the fovea is similar to that in controls. Moreover, cortical magnification in albinism and controls exceeds that which might be predicted based on cone density alone, suggesting that reduced foveal cone density in the albinotic retina may be partially counteracted by central connectivity. Together, these results emphasize that central as well as retinal factors must be included to provide a complete picture of aberrant structure and function in genetic conditions such as albinism.