What Microelectrodes Tell Us About Critical Periods

Excerpted from the book: The Brain That Changes Itself

Stories of Personal Triumph from the Frontiers of Brain Science

By Norman Doidge, MD, Penguin Publishing, December, 2007

In the 1960s, just as Michael Merzenich was beginning to use microelectrodes on the brain, two other scientists, who had also worked at Johns Hopkins with Mountcastle, discovered that the brain in very young animals is plastic. David Hubel and Torsten Wiesel were micromapping the visual cortex to learn how vision is processed.

During 1964, David Hubel and Torsten Wiesel studied the short- and long-term effects of depriving kittens of vision in one eye.

They'd inserted microelectrodes into the visual cortex of kittens and discovered that different parts of the cortex processed the lines, orientations, and movements of visually perceived objects.

They also discovered that there was a "critical period," from the third to the eighth week of life, when the newborn kitten's brain had to receive visual stimulation in order to develop normally. In the crucial experiment Hubel and Wiesel sewed shut one eyelid of a kitten during its critical period, so the eye got no visual stimulation. When they opened this shut eye, they found that the visual areas in the brain map that normally processed input from the shut eye had failed to develop, leaving the kitten blind in that eye for life.

Clearly the brains of kittens during the critical period were plastic, their structure literally shaped by experience. When Hubel and Wiesel examined the brain map for that blind eye, they made one more unexpected discovery about plasticity. The part of the kitten's brain that had been deprived of input from the shut eye did not remain idle. It had begun to process visual input from the open eye, as though the brain didn't want to waste any "cortical real estate" and had found a way to rewire itself — another indication that the brain is plastic in the critical period. For this work Hubel and Wiesel received the Nobel Prize.

Critical Periods

The discovery of the critical period became one of the most famous in biology in the second half of the twentieth century. Scientists soon showed that other brain systems required environmental stimuli to develop.

It also seemed that each neural system had a different critical period, or window of time, during which it was especially plastic and sensitive to the environment, and during which it had rapid, formative growth. Language development, for instance, has a critical period that begins in infancy and ends between eight years and puberty. After this critical period closes, a person's ability to learn a second language without an accent is limited. In fact, second languages learned after the critical period are not processed in the same part of the brain as is the native tongue.

In fact, the psychological version of the critical period went back to Freud, who argued that we go through developmental stages that are brief windows of time, during which we must have certain experiences to be healthy; these periods are formative, he said, and shape us for the rest of our lives.

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If this post strikes a chord with you, we take brain plasticity possibilities a step further in Impossible Dream, the extraordinary story of triumph over disability told from the first-person perspective of a young woman living with autism.