The pale appearance of freshly cut white matter comes from the myelin that ensheathes its axons. White matter consists primarily of long-range myelinated fibers that connect different cortical areas, link cortex to deep nuclei and spinal cord, and interconnect the two hemispheres.

Major tracts include the corpus callosum, the internal capsule, and the many association pathways that run within each hemisphere. Because myelin enables rapid conduction, white-matter integrity is essential for the timely coordination of distributed neural networks. Diffusion MRI now allows these tracts to be visualized and quantified in the living brain.

Diseases that preferentially attack white matter—multiple sclerosis, leukodystrophies, small-vessel vascular disease—produce deficits that reflect disconnection of the regions the tracts normally link. Age-related changes in white matter also contribute to cognitive slowing. Understanding white-matter organization has become as important as mapping cortical areas for explaining both normal function and neurological symptoms.

A lesion in a critical tract can disrupt a function even when the cortical areas themselves remain intact.