The Tale of The Silver Spring Monkeys

Neuroscientist Michael Merzenich, in his experiments, showed that when sensory input from a finger was cut off, brain map changes typically occurred in 1 to 2 millimeters of the cortex. Scientists thought that the probable explanation for this amount of plastic change was the growth of individual neuronal branches.

Brain neurons, when damaged, might send out small sprouts, or branches, to connect to other neurons. If one neuron died or lost input, the branches of an adjacent neuron had the ability to grow 1 to 2 millimeters to compensate. But if this was the mechanism by which plastic change occurred, then change was limited to the few neurons close to the damage. There could be plastic change between nearby sectors of the brain but not between sectors that lay farther apart.

Merzenich's colleague at Vanderbilt, Jon Kaas, worked with a student named Tim Pons, who was troubled by the l-to-2-millimeter limit. Was that really the upper limit of plastic change? Or did Merzenich observe that amount of change because of his technique, which in some key experiments involved cutting only a single nerve?

Pons wondered what would happen in the brain if all the nerves in the hand were cut. Would more than 2 millimeters be affected? And would changes be seen between sectors?

The animals that could answer that question were the Silver Spring monkeys, because they alone had spent twelve years without sensory input to their brain maps. As the monkeys aged, their health deteriorated. One monkey had to be euthanized and by December 1989 another monkey, Billy, was also suffering and dying.

Mortimer Mishkin, head of the Society for Neuroscience and chief of the

Laboratory of Neuropsychology at the NIH's Institute of Mental Health, met with Pons and agreed that he could do one final experiment on this Silver Spring monkey before he too was euthanized.

In the experiment the monkey Billy was to be anesthetized and a microelectrode analysis of the brain map for his arm was to be done, just before he was euthanized. Because there was so much pressure on the scientists and surgeons, they did in four hours what would normally have taken more than a day. They removed part of the monkey's skull, inserted electrodes into 124 different spots in the sensory cortex area for the arm, and stroked the deafferented arm. As expected, the arm sent no electrical impulses to the electrodes. Then Pons stroked the monkey's face — knowing that the brain map for the face is adjacent to the map for the arm.

To his amazement, as he touched the face, the neurons in the monkey's deafferented arm map also began to fire — confirming that the facial map had As Merzenich had seen in his own experiments, when a brain map is not used, the brain can reorganize itself so that another mental function takes over that processing space. Most surprising was the scope of the reorganization. Fourteen millimeters, or over half an inch of the "arm" map, had rewired itself to process facial sensory input — the largest amount of rewiring that had ever been mapped.

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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