MIT researchers have found that a subpopulation of brain cells in the part of the cortex that controls movements acquires novel firing patterns while an animal learns a new set of voluntary movements.
The results were published in the May issue of the journal Neuron by Emilio Bizzi, the E. McDermott Professor of Brain and Cognitive Sciences; MIT graduate student Camillo Padoa-Schioppa; and Chiang-Shan Ray Li of Chang Gung University in Taiwan.
The data indicate that the nervous system constantly reorganizes itself to deal with new motor acts needed in a new environment. Any time the nervous system encounters a new environment, a subpopulation of neurons change to accommodate the new conditions. This "suggests that neural plasticity is the rule rather than the exception," the authors wrote.
MONKEY SEE, MONKEY DO
The researchers concentrated on an area of the brain called "the primary motor cortex" responsible for voluntary movements. Motor commands issued elsewhere in the brain seem to be funneled through M1 before they are executed.
While other studies have shown that repeated finger movements by musicians, for instance, rapidly alter the area of M1 dedicated to that particular movement, the researchers say that no other studies have explored changes in neuronal activity during learning that involved a change in movement dynamics.
In this study, monkeys had to learn to move a joystick a new way to overcome a slight force when using a previously learned movement. This changes the dynamic of the movement, although the movement itself (the kinematics) looks identical once it is learned.
CHANGING THE BRAIN
Because learning new skills causes physical changes in the brain, the researchers' goal was to get a sense of what happens in the area of the brain devoted to movement before, during and after adaptation to a new movement dynamic.
Monkeys were taught to move a joystick that controlled a small square cursor at the center of a computer screen. They were supposed to move the cursor to targets that appeared sequentially at eight different locations around the screen.
Once they learned to do this quickly and accurately, they had to perform the same movements while an orthogonal force was being exerted on the joystick. At first, the monkeys' movements were shaky, but eventually they got just as good at reaching the targets with the force in effect (a process called adaptation).
The researchers tracked how these movements affected 162 individual neurons within the monkeys' brains. "We found that two different classes of memory cells coexist and balance each other after exposure to the force field," Padoa-Schioppa said.
The changes in memory cells suggested that the monkeys created an internal model for how to move the joystick to reach the target. This involves motor learning, not just motor performance.
These results may help explain the brain's ability to learn new things while not losing prior knowledge. "The network picks up new information and still acts properly when the information is not needed," Padoa-Schioppa said. "You want to learn something new without forgetting something old."
While it is known that the brain modifies itself to learn new things, this two-tiered cell function may be the mechanism that allows the network to simultaneously deal with old and new situations.
This work is supported by the National Institutes of Health.
A version of this article appeared in MIT Tech Talk on July 18, 2001.