Maturation of the Retinocortical Pathways

The optical characteristics of the eye change little between birth and adulthood. Axial length is approximately 60% of the corresponding distance in the adult, a difference that could lead to reduced retinal image size in the eye of the neonate. However, the reduced axial length is compensated by the smaller size of the pupil. Thus, the amount of light that falls on the retina per squared degree is essentially adult-like at birth ( Banks & Bennett, 1988 ). In the retina, ganglion cells mature earlier than any other type of cell in both humans and primates ( La Vail et al., 1992 ). Soon after the final stage of their development they begin to send axons to the dLGN (Hendrickson & Rakic, 1976 ). The initial overproduction of retinal ganglion cells is followed by a sharp reduction in the number of optic nerve axons that occurs around embryonic week 14 in primates and between the 20th and 30th embryonic week in the human fetus ( Provis et al., 1985 ). Some researchers believe that the elimination of ganglion cell axons is selective and largely restricted to those axons that synapse in the inappropriate eye-specific layer of the dLGN. This proposal is consistent with the observation that the emergence of eye-specific laminae in the dLGN coincides with the peak of ganglion cell death. In humans, the myelination of optic nerve fibers is nearly complete by two years of age ( Magoon & Robb, 1981 ).