Critical Period - Vision

Vision

In mammals, neurons in the brain which process vision actually develop after birth based on signals from the eyes. A landmark experiment by David H. Hubel and Torsten Wiesel (1963) showed that cats which had one eye sewn shut from birth to three months of age (monocular deprivation) only fully developed vision in the open eye. They showed that columns in the primary visual cortex receiving inputs from the other eye took over the areas that would normally receive input from the deprived eye. In general electrophysiological analyses of axons and neurons in the lateral geniculate nucleus showed that the visual receptive field properties was comparable to adult cats; however, the layers of cortex that were deprived had less activity and fewer responses were able to be isolated. The kittens had abnormally small ocular dominance columns (part of the brain that processes sight) connected to the closed eye, and abnormally large columns connected to the open eye. This did not happen to adult cats even when one eye was sewn shut for a year. Later experiments in monkeys found similar results.

In a follow-up experiment, Hubel and Wiesel (1963) explored the cortical responses present in kittens after binocular deprivation; they found it difficult to find any active cells in the cortex, and the responses they did get were either slow-moving or fast-fatiguing. Furthermore, the cells that did respond selected for edges and bars with distinct orientation preferences. Nevertheless, these kittens developed normal binocularity. Hubel and Wiesel first explained the mechanism, known as orientation selectivity, in the mammalian visual cortex. Orientation tuning, a model that originated with their model, is the concept that receptive fields of neurons in the LGN that excite a cortical simple cell are arranged in rows. This model was important because it was able to describe a critical period for the proper development of normal ocular dominance columns in the lateral geniculate nucleus, and thus able to explain the effects of monocular deprivation during this critical period. The critical period for cats is about three months and for monkeys, about six months.

In a similar experiment, Antonini and Stryker (1993) examined the anatomical changes that can be observed after monocular deprivation. The compared geniculocortical axonal arbors in monocularly deprived animals in the long term (4- weeks) to short term (6–7 days) during the critical period established by Hubel and Wiesel (1993). They found that in the long term, monocular deprivation causes reduced branching at the end of neurons, while the amount of afferents allocated to the nondeprived eye increased. Even in the short term, Antonini and Stryker (1993) found that geniculocortical neurons were similarly affected. This supports the aforementioned concept of a critical period for proper neural development for vision in the cortex.

In humans, some babies are born blind in one or both eyes, for example, due to cataracts. Even when their vision is restored later by treatment, their sight would not function in the normal way as for someone who had binocular vision from birth or had surgery to restore vision shortly after birth. Therefore, it is important to treat babies born blind soon if their condition is treatable.

Expression of the protein Lynx1 has been associated with the normal end of the critical period for synaptic plasticity in the visual system.