White matter tracts are signaling pathways in the brain made from bundles of white matter, axons coated in layers of myelin. Some of these tracts are laid down during fetal development, while others develop after birth, as people begin to interact with the environment and acquire skills. A brain imaging technique known as diffusion tensor imaging allows researchers to identify white matter tracts, and doctors can use this to compare a patient's brain with known tracts to see if there are abnormalities or unusual features in someone's brain.
These tracts could be thought of as signal cables. The myelin acts as a sheath to increase electrical conductivity, allowing for very rapid transmission of signals along a white matter tract. Some tracts, known as commissual tracts, span the hemispheres of the brain to communicate information between the left and right brain. In imaging, they can be seen snaking across the middle of the brain to create complete connections.
Association tracts run between lobes on the same half of the brain. Different lobes may need to be able to communicate information rapidly to make sense of sensory information and cognitive processes. Projection tracts can transmit information from the brain to the rest of the body. They can be extremely long to allow for conduction of signals across great distances.
Research on the brain demonstrates that in young people, if a white matter tract sustains trauma, the brain may be able to remap itself. It can take over neighboring tracts to find new ways to send the information. The younger people are, the more adaptable their white matter tracts will be, explaining why young children can recover well from head trauma and invasive brain surgery, including surgery to sever the corpus callosum, the structure connecting the two halves of the brain. This surgery may be used in people with severe seizures that do not respond to more conservative treatments.
In older people, it can be more difficult to recover from damage to the white matter tracts. People with brain damage caused by medications, trauma, surgery, tumors, degenerative diseases, and so forth can experience severe cognitive deficits. Deficits may cluster together, depending on the area of the brain involved and the degree of damage. Researchers interested in dementia and cognitive degeneration study white matter tracts to learn more about how key areas of the brain communicate, and what can be done to help people with cognitive deficits.