Vision, like all sensory inputs, except for olfaction, is relayed to the cerebral cortex by way of the thalamus. The thalamic relay for vision is the LGN. The LGN in humans and Old World monkeys has an obvious striped appearance, with six layers of neurons separated by interleaved fiber layers. Although, by the end of the nineteenth century, it was clear that the eye projects to the LGN, the pattern of termination of optic tract fibers was not well understood. The true picture was revealed by study of transneuronal atrophy and degeneration. Cells in the LGN that are deprived of their input from the eye shrink or die. Mieczyslaw Minkowski (1920), working in Zurich, studied the LGN of a monkey that had had one eye removed 8 months earlier and that of a 75-year-old woman who had had amblyopia due to a unilateral cataract for 38 years before she died. Minkowski saw that cells in the LGN layers opposite the blind eye, layers 1, 4, and 6, were atrophied. In the ipsilateral LGN, layers 2, 3, and 5 were affected. The technique of studying transneuronal atrophy has revealed the organization of the LGN in a large number of mammals. The six-layered pattern is virtually identical in the apes and the Old World primates.
In some cases, the existence of a hidden laminar pattern can be revealed by transneuronal atrophy. For example, in the squirrel monkey, Saimiri, the dorsal parvocellular region of the LGN is not obviously laminated. One year after unilateral enucleation, a clear six-layer pattern emerges which is similar to that of the Old World primates (Doty et al., 1966).
Was this article helpful?