The evolution of the human brain is marked not only by a major expansion of the neocortex but also by an enormous increase of the subcortical white matter. White matter tracts comprise the axonal output of some 1010 cortical projection neurons, which constitute a giant cellular network, recently referred to as the connectome. About half the human brain comprises myelinated axonal tracts that connect the cortex with subcortical structures and, most importantly, interconnect cortical areas with each other. Lesions affecting central nervous system white matter such as in stroke and demyelinating diseases typically cause acute motor and sensory deficits, but white matter abnormalities also cause cognitive dysfunction, including the kinds of abnormalities seen in psychiatric conditions.
Nave K, Ehrenreich H. Myelination and Oligodendrocyte Functions in Psychiatric Diseases. JAMA Psychiatry. 2014;71(5):582-584. doi:10.1001/jamapsychiatry.2014.189