Retino-cortical relationships in albinism

Introduction: Albinism is a disorder of melanin synthesis that results in variable visual function. However, retinal disruptions in albinism do not fully explain the loss of visual acuity. While abnormal decussation of retinal afferents and disrupted retinotopic organization are known features of albinism, the role of cortical structure in establishing visual function in patients with albinism is poorly characterized. Understanding these aspects of the visual system will provide insight into the visual deficits associated with albinism. Methods: Two patients with oculocutatneous albinism 1B (OCA1B) underwent adaptive optics (AO) retinal imaging and functional magnetic resonance imaging (fMRI). Cortical maps were examined for abnormal retinotopy. In addition, cone density and cortical magnification were quantified over a 20 degree eccentricity. Results: Each patient exhibited a unique retinotopic map. Cortical magnification functions did not match cone density functions. Correction for cone-to-ganglion cell convergence at varying eccentricities showed a better, yet not complete, match between ganglion cell density functions and cortical magnification. Conclusion: Our results suggest that there is more variation in cortical organization in human albinism than is suggested by current conceptualizations. Additionally, disruptions to cortical magnification are not predicted by cone density alone. The cortical magnification, however, can be better predicted when accounting for convergence of visual neurons, but more subjects are needed to better delineate these relationships.