APA Dictionary of Psychology

a rare, progressive degenerative disorder of the central nervous system marked by involuntary, rapid eye movements (see nystagmus), muscular incoordination (see ataxia), and spasticity. It is caused by a mutation in the gene that controls the production of a specific myelin protein. Severity and onset vary widely, depending on the type of mutation, and extend from the mild, adult-onset, spastic paraplegia form to the severe form with onset at infancy and death in early childhood. [Friedrich Pelizaeus (1850–1917), German neurologist; Ludwig Merzbacher (1875–1942), German physician]