Experiment Module: Effects of Visual Deprivation During the Critical Period for Development of Vision

In Cats

David Hubel and Torsten Wiesel’s experiments showed that if a kitten is deprived of normal visual experience during a critical period at the start of its life, the circuitry of the neurons in its visual cortex is irreversibly altered.

In each of a number of newborn kittens, one eyelid was sutured shut. The kitten was allowed to grow up that way, and when it reached adulthood (around 6 months), its eyelid was opened again. Recordings were than made of the electrophysiological activity in each of the kitten’s eyes. These recordings showed an abnormally low number of neurons reacting in the eye that had been sutured shut, and an abnormally high number in the other eye. Macroscopic observation of the visual cortex showed that the ocular dominance columns for the eye that had been left open had grown larger, while those for the eye that had been closed had shrunk.

Remarkably, Hubel and Wiesel also found that if the eye of an adult cat was sutured shut for a year, the responses of the cells in its visual cortex remain identical in all respects to those of a normal cat. Later experiments showed that suturing a cat’s eye shut had no effect on its visual cortex unless this visual deprivation took place during the first three months of the cat’s life.

In Primates

Other experiments have shown that this same phenomenon also occurs in primates, though the critical period is longer (up to age 6 months).

Austin Riesen reared monkeys in darkness for the first 3 to 6 months of their lives. When these animals were then introduced into a normal environment, they had great difficulties in distinguishing even the simplest shapes. It took them weeks or even months to learn how to tell a circle from a square—a task that a normal monkey learns in a few days.

Wiesel and Hubel also explored what happens in a monkey’s primary visual cortex when one of its eyelids is sutured shut for the first 6 months of its life. Normally, in monkeys, as in cats and humans, the two eyes work together to provide a single, three-dimensional image of the outside world, but this image is actually composed of two separate, slightly offset images on the two retinas. Wiesel and Hubel showed that it is not until the signals from the retinas reach the primary visual cortex that the brain begins to merge them into one, three-dimensional image.

In monkeys who had one eyelid sutured shut right after birth, when the eyelid was opened again at 6 months of age, the animals had lost practically all useful vision in the eye that had been sensorily deprived. Yet recordings of electrophysiological activity in the ganglion cells of the retina of that eye, and the lateral geniculate nucleus cells for that eye, showed that these cells’ visual fields were normal and functional. It was only the primary visual cortex cells for that eye that showed practically no activity. Other experiments in which both eyelids were temporarily sutured shut showed that normal development of connectivity in the visual cortex does not depend on the absolute activity of the neural pathways from the two eyes, but rather on competition between the relative activities of these two pathways.

As in other development processes for which there is a critical period, sensory deprivation does not have the same effect on adult animals—i.e., suturing shut one eyelid of an adult animal has no effect on the response of the visual cortex cells for that eye or for the other eye. In contrast, during the most sensitive part of the critical period, visual deprivation for as little as one week can have catastrophic effects on the animal’s vision for the rest of its lifetime.

In Humans

In humans, certain diseases can cause a cataract (a total or partial opacity of the lens) in one or both eyes. Cataracts can occur not only in adults but also in very young children. Cataracts can now usually be removed surgically. Studies of individuals who had such surgery at various times in their lives showed that in humans as in other animals, there is a critical period for the development of the sense of sight. These studies demonstrated for the first time that early environmental influences, and hence particular neural activity patterns during a critical period, can permanently alter the neural connections in certain areas of the human brain.