In the 1970s, Edward Taub, then a behavioral psychiatrist, took a job as a research assistant in an experimental neurology lab, to better understand the nervous system. In the lab they were doing "deafferentation" experiments with monkeys.
Deafferentation is an old technique, used by the Nobel Prize winner Sir Charles Sherrington in 1895. An "afferent nerve," in this context, means a "sensory nerve," one that conveys sensory impulses to the spine and then the brain. Deafferentation is a surgical procedure in which the incoming sensory nerves are cut so none of their input can make this trip. A deafferented monkey cannot sense where its affected limbs are in space, or feel any sensation or pain in them when touched.
Taub, working with a neurosurgeon, A. J. Berman, deafferentiated the arms of several monkeys and, to make sure that they didn't just shift to using their unharmed arms, they put the good arms in a sling.
The prevailing theory at the time was that all movement was based on, and initiated by, the sensory reflex, and that if you destroyed the sensory part of information inflow, movement couldn't happen.
But much to Taub's amazement, the test monkeys started using their deafferentiated arms. Since their good arms were constrained from use in a sling, they had no choice but to rely on their sensation-less limbs. To Taub this meant there had to be independent motor programs in the brain that could initiate voluntary movement heedless of sensory input.
Taub also thought his finding might have implications for stroke recovery
because the monkeys, like stroke patients, had seemed utterly unable to move their arms. Perhaps some stroke patients, like the monkeys, might also move their limbs - if forced to do so.
Now open to the idea of plasticity, Taub forged ahead with deafferentation. He reasoned that if both arms were deafferented, a monkey should soon be able to move them both, because it would have to survive. So, he deafferented both limbs and, in fact, the monkeys did move both.
This finding was paradoxical: if one arm was deafferented, the monkey couldn't use it. If both arms were deafferented, the monkey could use both!
Then Taub deafferented the whole spinal cord, so that there wasn't a single spinal reflex left in the body and the monkey could not receive sensory input from any of its limbs. Still, it used its limbs.
Taub had another epiphany. He deduced that the reason a monkey didn't use its arm after a single limb was deafferented was because it had learned not to use it in the period right after the operation when the spinal cord was still in "spinal shock" from the surgery. Spinal shock can last from two to six months, a period when the neurons have difficulty firing. An animal in spinal shock will try to move its affected arm and fail many times during those months. Without positive reinforcement, the animal gives up and instead uses its good arm to feed itself, getting positive reinforcement each time, it succeeds.
Thus, the motor map for the deafferented arm — which includes programs for common arm movements — begins to weaken and atrophy, according to the plasticity principle of use it or lose it. Taub called this phenomenon "learned nonuse." He reasoned that monkeys that had both arms deafferented were able to use them because they'd never had the opportunity to learn that they didn't work well; they had to use them to survive.
But Taub thought he still had only indirect evidence for his theory of learned nonuse, so in a series of ingenious experiments he tried to prevent monkeys from "learning" nonuse. In one, he deafferented a monkey's arm; then, instead of putting a sling on the good arm to restrain it, he put it on the deafferented arm. That way the monkey would not be able to "learn" that it was of no use in the period of spinal shock. And indeed, when he removed the restraint at three months, long after the shock had worn off, the monkey was soon able to use the deafferented limb.
Taub next began investigating what success he could have teaching animals to overcome learned nonuse. He then tested whether he could correct learned nonuse several years after it had developed, by forcing a monkey to use the deafferented arm. It worked and led to improvements that lasted the rest of the monkey's life. Taub now had an animal model that both mimicked the effects of strokes when nerve signals are interrupted and limbs cannot be moved, and a possible way of overcoming the problem.
one that would transform the treatment of strokes.
Taub believed these discoveries meant that people who had had strokes or other kinds of brain damage, even years earlier, might be suffering from learned nonuse. He knew the brains of some stroke patients with minimal damage went into an equivalent of spinal shock, "cortical shock," which can last for several months. During this period each attempt to move the hand is met with failure, possibly leading to learned nonuse.
Stroke patients with extensive brain damage in the motor area fail to improve for a long period and, when they do, only recover partially. Taub reasoned that any treatment for stroke would have to address both massive brain damage and learned nonuse. Because learned nonuse might be masking a patient's ability to recover, only by overcoming learned nonuse first could one truly gauge a patient's prospects. Taub believed that even after a stroke, there was a good chance that motor programs for movement were present in the nervous system. Thus, the way to unmask motor capacity was to do to human beings what he did to monkeys: constrain the use of the good limb and force the affected one to begin moving.
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