Address reprint requests to Dr. Tamminga, Maryland Psychiatric Research Center, University of Maryland, P.O. Box 21247, Baltimore, MD 21228. Images are excerpted with permission from Georgopoulos AP, Lurito JT, Petrides M, Schwartz AB, Massey JT: Mental rotation of the neuronal population vector. Science 1989; 243:234-236. Copyright 1989, American Association for the Advancement of Science.