Neurons that Fire Together Wire Together
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 1971, Michael Merzenich became a professor emeritus at the University of California at San Francisco, in the department of otolaryngology and physiology, which did research on diseases of the ear. Now his own boss, he began the series of experiments that would prove the existence of plasticity beyond a doubt. Thus, he spent much of the early 1970s mapping the auditory cortex of different species of animals, and he helped others invent and perfect the cochlear implant, which allows congenitally deaf people to hear.
The implant consists of a sound receiver, a converter that translates sound into electrical impulses, and an electrode inserted by surgeons into the nerves that run from the ear to the brain.
Merzenich went on to discover that animals of a particular species may have similar maps, but they are never identical. He also found that the maps of normal body parts change every few weeks. Every time he mapped a normal monkey's face, it was unequivocally different. Plasticity doesn't require the provocation of cut nerves or amputations. Plasticity is a normal phenomenon, and brain maps are constantly changing. When he wrote up this new experiment, Merzenich finally took the word "plasticity" out of quotes.
Merzenich discovered that Penfield's brain maps are neither immutable within a single brain nor universal but vary in their borders and size from person to person. In a series of brilliant experiments, he showed that the shape of our brain maps changes depending upon what we do over the course of our lives. But in order to prove this point he needed a tool far finer than Penfield's electrodes, one that would be able to detect changes in just a few neurons at a time.
It seemed reasonable to assume that if totally new maps were forming, then new connections must have been forming among neurons. To help understand this process, Merzenich invoked the ideas of Donald O. Hebb, a Canadian behavioral psychologist who had worked with Penfield.
In 1949 Donald O. Hebb, a Canadian behavioral psychologist who had worked with Penfield, proposed that learning linked neurons in new ways. He proposed that when two neurons fire at the same time repeatedly (or when one fires, causing another to fire), chemical changes occur in both, so that the two tend to connect more strongly.
Hebb's concept — actually proposed by Freud sixty years before — was neatly summarized by neuroscientist Carla Shatz: Neurons that fire together wire together. Hebb's theory thus argued that neuronal structure can be altered by experience.
Following Hebb, Merzenich's new theory was that neurons in brain maps develop strong connections to one another when they are activated at the same moment in time. And if maps could change, thought Merzenich, then there was reason to hope that people born with problems in brain map-processing areas — people with learning problems, psychological problems, strokes, or brain injuries — might be able to form new maps if he could help them form new neuronal connections, by getting their healthy neurons to fire together and wire together.
Starting in the late 1980s, Merzenich designed or participated in brilliant studies to test whether brain maps are time-based and whether their borders and functioning can be manipulated by "playing" with the timing of input to them.
In one ingenious experiment, Merzenich mapped a normal monkey's hand, then sewed together two of the monkey's fingers, so that both fingers moved as one. After several months of allowing the monkey to use its sewn fingers, the monkey was remapped. The two maps of the originally separate fingers had now merged into a single map. If the experimenters touched any point on either finger, this new single map would light up. Because all the movements and sensations in those fingers always occurred simultaneously, they'd formed the same map. The experiment showed that timing of the input to the neurons in the map was the key to forming it — neurons that fired together in time wired together to make one map.
In the final and most brilliant demonstration, Merzenich and his team proved that maps cannot be anatomically based. They took a small patch of skin from one finger, and — this is the key point — with the nerve to its brain map still attached, surgically grafted the skin onto an adjacent finger. Now that piece of skin and its nerve were stimulated whenever the finger it was attached to was moved or touched in the course of daily use.
According to the anatomical hardwiring model, the signals should still have been sent from the skin along its nerve to the brain map for the finger that the skin and nerve originally came from. Instead, when the team stimulated the patch of skin, the map of its new finger responded. The map for the patch of skin migrated from the brain map of the original finger to its new one, because both the patch and the new finger were stimulated simultaneously.
In a few short years Merzenich had discovered that adult brains are plastic, persuaded skeptics in the scientific community this was the case, and shown that experience changes the brain.
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.