FEST Writing Group

This is part of my attempt to absorb relevant facts from recent research about learning and central nervous system plasticity.

October 5, 2014

Brain Plasticity
Jim Edd Jones

The brain is dynamic, changeable, and continually fluctuating. Until the last 35-40 years, the central nervous system was thought to be fixed and unchangeable once it had completed its developmental growth. For the following 15 years, all but a few continued to believe that the brain was fixed.
Let’s look at some of the research that eventually turned this belief around.
The concept of cortical map is central. Wilder Penfield, in the 1930s and 40s, during many brain surgeries, electrically stimulated different parts of the brain and found which parts of the cortex were associated with different parts of the body. He might stimulate one group of cortical neurons and the person’s right pinky would twitch. He might stimulate another group and the person might report feeling an itch on their left cheek. Patiently repeating this procedure, Penfield compiled cortical maps of a person’s sensorium and motor actions. These maps showed which parts of the cortex were associated with reactions of different parts of the body. Penfield believed and others accepted that these maps were fixed and general from one person to the next. Always in the same place and unchangeable.
Michael Merzenich, in the late 1970s, early 80s, discovered that cortical maps can be changed. As you might expect, people for a long time did not accept his evidence; his research was often ignored or dismissed. His graduate student mentor at Johns Hopkins, Vernon Mountcastle, had developed a technique of recording the activity of a single neuron with a microelectrode. This allowed the drawing of much more precise cortical maps. When that is done, Merzenich found that experience changes the boundaries of the maps.
A couple of examples from Merzenich’s early work with monkeys.

First experiment. Cortically map a monkey’s hand. That roughly gives you areas on the cortex, side by side, which track the activity of the middle part of the hand and that of the adjacent outside parts.
Then Merzenich cut the peripheral nerve leading to the middle part of the hand, depriving the brain of any input coming from that part. After a few months, he repeated the cortical mappings. Then, there were two distinct areas, corresponding to the two outside parts of the hand. The cortical area which had corresponded to the middle part had disappeared, it had been appropriated by the two outside parts.
This experiment and findings by other researchers showed how loss of function can lead to loss of corresponding cortical map. Even the forced nonuse of a function, like using a sling to enforce nonuse of an arm can lead to loss of cortical map area.

Conversely, intense use and new learning will increase the area of the corresponding cortical map. Another experiment. Merzenich mapped the index finger functioning of a monkey. Then he had the monkey learn a difficult motor task. The monkey learned to use that index finger to lightly touch a spinning disk and slow the disk’s speed to a target speed. Not easy. After the monkey learned the task, requiring thousands of trials, they remapped that index finger. The cortical map representing that index finger had grown appreciably. This kind of result has been found repeatedly. For example, professional violinists have larger maps of the fingertips and fingers of their left hands, larger than average and larger than the maps for the fingers of their own right hand.
Conclusion. Cortical maps can grow and shrink with the right experience.

A study of human beings that provocatively shows some more of the time dimension of map change. In the 1990s, Alvaro Pascual-Leone developed a method of using transcranial magnetic stimulation to either stimulate or inhibit targeted areas of the cortex. In this way, he could make cortical maps of human beings.
One experiment done by Pascual-Leone’s team. Ten blind students were taught Braille over the course of one academic year. As part of their school day, they practiced Braille reading two hours a day, 5 days a week, with weekends off. They were cortically mapped Mondays and Fridays, focusing on maps of the finger tips used for the reading.
In the beginning, the Friday maps would show appreciable growth after 5 days of practice. By Monday, the maps would have shrunk to their original size.
Over time, this pattern changed. Slowly the Monday maps grew and the Friday maps had increasingly modest short term growth. The Monday maps appeared to correspond to long term skill growth, e.g. the Monday growth quantitatively correlated with increasing reading speed in words/minute.
These results speak for themselves. Cortical maps can change rapidly, but the rapid change is not necessarily solid change.

One last related result. Merzenich mapped a part of a single monkey’s cortex. Several weeks later, he mapped again, without the experimenter having put the monkey through any kind of procedure in the interim. Nevertheless, the cortical map showed changes. The conclusion is that the monkey’s and our cortical maps are dynamic and continually fluctuating, just from the impact of everyday ordinary experience.