Hubel, Wiesel, and Sensitive Periods

In the 50s and 60s, David Hubel and Torsten Wiesel wanted to find out how visual perception worked, so they jabbed a microelectrode into the visual cortex of an anesthetized cat and saw what kinds of visual stimuli made the cells in that part of the brain fire.

They then did a series of experiments with kittens, stitching one or the other eyelid together for a period during development.

It has long been known to psychologists and clinicians that depriving an animal or man of vision at an early age can lead to profound visual defects…

To assess the importance of visual experience we deprived kittens of vision in one eye at various times after birth, and for various lengths of time. In one kitten the lids of one eye were closed at an age of 9 weeks for a period of 1 month. While a number of [nerve] cells were driven from [connection with] both eyes, the balance was nevertheless still grossly abnormal, with an unusually large proportion of cells strongly preferring the normal eye…

Finally, it was found that a few months of deprivation in an adult cat were not enough to produce any cortical deficit or any morphological changes in the geniculate. These experiments, then, indicate that very young kittens are particularly susceptible to the effects of deprivation, and that the susceptibility decreases with each month of life, possibly even vanishing in the mature animal. It has long been known that in man, too, the effects of deprivation fall off with age, as shown by the profound visual deficit revealed when congenital cataracts are removed from children, as opposed to the good restoration of vision following removal of cataracts acquired late in life.

Stitching one eye shut at a critical period in the kitten’s development- for a month starting at 9 weeks after birth- would cause blindness in that eye, since the nerve connections would migrate from that eye to the other.Stitching an adult cat’s eye shut had no such effect- the cat could still start seeing again, even after two years of deprivation:

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In a further experiment, just putting a translucent covering over one eye (so that light but not patterns could come through) for a month would induce vision loss in that eye- the cells of the eye were fine, but the connections with the cortex were outcompeted by those from the other eye. Eventually, Hubel and Wiesel found that just a 3-4 day closure at a critical period would be enough for the kitten to permanently have one eye dominant over the other:

During the period of high susceptibility in the fourth and fifth weeks, eye closure for as little as 3-4 days leads to a sharp decline in the number of cells that can be driven from both eyes, as well as an over-all decline in the relative influence of the previously closed eye. A 6-day closure is enough to give a reduction in the number of oells that can be driven by the closed eye to a fraction of the normal. The physiological picture is similar to that following a 3-month monocular deprivation from birth, in which the proportion of cells the eye can influence drops from 85 to about 7 %.

Surprisingly, covering both eyes, or rearing the animal in complete darkness, could have less of an effect than covering one, since the connections from the working eye were no longer outcompeting the covered one in occupying space in the visual cortex.

In his 1981 Nobel lecture discussing these and other experiments, Wiesel makes the following conclusion:

Innate mechanisms endow the visual system with highly specific connections, but visual experience early in life is necessary for their maintenance and full development. Deprivation experiments demonstrate that neural connections can be modulated by environmental influences during a critical period of postnatal development. We have studied this process in detail in one set of functional properties of the nervous system, but it may well be that other aspects of brain function, such as language, complex perceptual tasks, learning, memory and personality, have different programs of development. Such sensitivity of the nervous system to the effects of experience may represent the fundamental mechanism by which the organism adapts to its environment during the period of growth and development.

So, why am I telling you this, given that the name of this blog is not “Mean Things Someone Once Did With A Cat?”

Because I think if you take the sensitive period idea seriously, it suggests something like the reverse of how we tend to think about education and the developing brain. Rather than being built up like a pyramid, brick by brick, the brain seeks out particular kinds of stimuli, at very particular times. Given the wide variety of people, those are likely to be very different stimuli, at very different times. I know several people who taught themselves to read when they were three or four or five; I also know perfectly bright people who might never have learned to read without enormous effort (on their part and their teachers) consciously being taught letters and sounds. Teaching school can often feel like herding cats, and like a litter of kittens, which ball of yarn strikes our fancy varies from cat to cat and time to time.

 

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