Micro-mapping the Mind with Microelectrodes
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
Michael Merzenich was a driving force behind scores of neuroplastic innovations and practical inventions. Merzenich's specialty is improving people's ability to think and perceive by redesigning the brain by training specific processing areas, called brain maps, so that they do more mental work. He has also, perhaps more than any other scientist, shown in rich scientific detail how our brain processing areas change.
Merzenich opted for Hopkins to do his Ph.D. in physiology under one of the great neuroscientists of the time, Vernon Mountcastle. In the 1950s was Mountcastle was demonstrating that the subtleties of brain architecture could be discovered by studying the electrical activity of neurons using a new technique: micro mapping with pin-shaped microelectrodes.
Microelectrodes are so small and sensitive that they can be inserted inside or beside a single neuron and can detect when an individual neuron fires off its electrical signal to other neurons. The neuron's signal passes from the microelectrode to an amplifier and then to an oscilloscope screen, where it appears as a sharp spike.
Merzenich would make most of his major discoveries using microelectrodes with monkeys. Micromapping allowed him to see differences that Penfield, with larger electrodes, could not.
In 1968, he and his colleague, Clinton Woolsey, micromapped the hand maps in the brains of several adolescent monkeys. They then cut a peripheral nerve to the hand, and immediately sewed the two severed ends close together but not quite touching. They waited seven months for the nerve to regenerate and then remapped the brains.
Merzenich assumed this second mapping would reveal a very chaotic, cross-wired sensorimotor area, but it didn't. Instead, the regenerated map was almost normal. This amazed the neurologists and completely changed their view of how the human brain forms maps to represent the body.
If a brain map could normalize its structure in response to abnormal input, the prevailing localist view that we are born with a hardwired system had to be wrong. The brain had to be plastic.
Merzenich would go on to collaborate in numerous studies researching brain plasticity. The most important ones were conducted with fellow neuroscientist Jon Kass at Vanderbilt University in Nashville. In the 1970s they did some extraordinary work with adult monkeys.
A monkey's hand, like a human's, has three main nerves: the radial, the median, and the ulnar. The median nerve conveys sensation mostly from the middle of the hand, the other two from either side of the hand. So, they cut the median nerve in one of the monkeys to see how the median nerve brain map would respond when all input was cut off.
They waited two months and then re-mapped the monkey's brain. As expected, they noticed that the portion of the brain map that serves the median nerve showed no activity when the middle part of his hand was touched. But what surprised the heck out of them was that when they stroked the outsides of the monkey's hand — the areas that send their signals through the radial and ulnar nerves — the median nerve map lit up! And the brain maps for the radial and ulnar nerves had almost doubled in size and invaded what used to be the median nerve map.
So, what does that tell you? It tells you that if the median nerve was cut, other nerves that still had an electrical connection would take over the unused map space to process their input. This was the first inkling that, when it comes to allocating brain-processing space, the real estate of brain maps is governed by competition and the principle of use it or lose it.
This further tells us that if we stop exercising our mental skills, we do not just forget them: the brain map space for those skills is turned over to the skills we practice instead.
I suppose competitive plasticity also explains why our bad habits, or any habits for that matter, are so difficult to break. Habits by nature are behaviors we repeat over and over and in so doing they lay claim to a great deal of space in a map. Unlearning is difficult when all the neural circuits are already tied up with doing the same thing over and over.
But back to Merzenich and his monkeys. He was not done with his plasticity experimenting. The next thing he did was amputate a monkey's middle finger. Again, he waited a couple of months, remapped the monkey's brain and what do you think he found?
"That the brain map for the amputated finger had disappeared and that the maps for the other fingers had grown into the space that had originally mapped for the middle finger."
"Exactly. Further evidence that brain maps are not only dynamic but that cortical real estate is allocated on the principle of use it or lose it.
Maps were dynamic. As Harvard's great historian of psychology of the time, Edwin G. Boring, put it, "One day's mapping would no longer be valid on tomorrow."
You have to understand, that he was bucking the system at this time. His theories of brain plasticity were heresy to the localists who still believed that the brain we are born with is essentially the brain we die with. That the brain's structure is fixed, and that our various sensory and motor processing systems are hardwired.
He laughs when he says it. "Let me tell you what happened when I began to declare that the brain was plastic. I received hostile treatment. I don't know how else to put it. I got people saying things in reviews such as, 'This would be really interesting if it could possibly be true, but it could not be.' It was as if I just made it up."
But you have to hand it to the guy, he didn't give up. He just kept plugging away with his plasticity experiments on monkeys.
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.