White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts.

[5] The other main component of the brain is grey matter (actually pinkish tan due to blood capillaries), which is composed of neurons.

The substantia nigra is a third colored component found in the brain that appears darker due to higher levels of melanin in dopaminergic neurons than its nearby areas.

[9] Multiple sclerosis (MS) is the most common of the inflammatory demyelinating diseases of the central nervous system which affect white matter.

[14] White matter lesions on magnetic resonance imaging are linked to several adverse outcomes, such as cognitive impairment and depression.

[16] Smaller volumes (in terms of group averages) of white matter might be associated with larger deficits in attention, declarative memory, executive functions, intelligence, and academic achievement.

[19] Nonetheless, regular aerobic exercise appears to either postpone the aging effect or in turn enhance the white matter integrity in the long run.

[19] Changes in white matter volume due to inflammation or injury may be a factor in the severity of obstructive sleep apnea.

[22] A 2009 paper by Jan Scholz and colleagues[23] used diffusion tensor imaging (DTI) to demonstrate changes in white matter volume as a result of learning a new motor task (e.g. juggling).

A more recent DTI study by Sampaio-Baptista and colleagues reported changes in white matter with motor learning along with increases in myelination.