White Matter and Learning in the Brain
The brain is composed of two different types of tissue: gray matter and white matter. Traditionally, neuroscientists who have studied learning and memory have focused their attention on synaptic processes between neurons (found in gray matter) and consequently have largely ignored the role that glia and axons (found in white matter) play in facilitating learning and memory processes. Research funded by funded by Eunice Kennedy Shriver National Institute of Child and Health Development (NICHD) demonstrates that white matter plays a more important role in learning may have previously been thought.
Hamidi: Dr. Douglas Fields is a neuroscientist at the Eunice Kennedy Shriver National Institute of Child and Health Development (NICHD). Research from his lab has shown that white matter may play a more important role in facilitating learning in the brain than may have previously been thought.
Dr. Fields: Brain is composed of two tissues: gray matter and white matter. Gray matter, most people have heard of—that's the surface layer of the brain where the neurons and synapses and dendrites are located—but the connections between neurons are made possible through wire-like axons, so this is the white matter region of the brain which is composed of millions of bundles of axons that connect neurons and gray matter together.
Hamidi: Traditionally, learning has primarily been understood to be a function of new connections made at synapses and the connections made between neurons depends on the pattern of neuronal firing, or as any neuroscientist will tell you, neurons that fire together, wire together.
Dr. Fields: And this goes to the fundamental mechanism of learning, which is that neurons that fire together, wire together. Pavlov's dog is a good example where the food had to be presented at the same time as the bell to have the neuron that controls saliva become conditioned to respond to the bell. Neurons that fire together wire together.
Hamidi: So the question still stands: what does white matter or myelin have to do with anything? Well, in order to help neurons fire together, impulses must arrive at the neuron at the same time. In large brains, the distance between neurons can be quite large and to help the neurons fire together, the speed of impulse must be increased.
Dr. Fields: And what controls the speed of impulse flow in the brain is myelin. Myelin can increase the speed of conduction a hundred fold.
Hamidi: Increasing the speed of conduction is important, of course, but it's ialso mportant to keep in mind that faster is not always better. What's more relevant, Dr. Fields explains, is the synchrony of neuronal firing.
Dr. Fields: So we now realize that it's important, especially in higher-level cognitive function that the speed of impulse transmission between different parts of the brain involved in a complex cognitive function, that that speed be synchronized and optimized, just like a train system has to be highly synchronized.
Hamidi: Neuroscientists interested in studying learning have used various techniques for investigation. One such technique, used to study the brains of humans non-invasively is brain imaging. At first, Dr. Fields explains, scientists focused all their attention, yet again, on gray matter—since this was where the bulk of neurons are located.
Dr. Fields: But it also became apparent, eventually, that there were changes in white matter regions of the brain. So it’s the new human brain imaging that has shown differences in white matter and originally, in terms of pathology, and differences in ability, for example learning to play the piano, is associated with increases in white matter in specific tracts of the brain associated with these mental processes that are involved in, for example, musical ability, finger coordination.
Hamidi: Dr. Fields' own research has demonstrated on a cellular level that experience helps to drive changes in myelin which is further evidence that white matter is involved in learning.
Dr. Fields: Some of those glial cells that form myelin, were of particular interest and our work showed that when we made impulses, when we generated impulses in axons, myelin was increased.
Hamidi: All in all, it seems like neuroscientists are learning that white matter really does matter. For more information regarding the research from Dr. Fields’ lab visit nsdps.nichd.nih.gov/. This is Anahita Hamidi, National Institutes of Health, Bethesda, Maryland.