Increasing required neural response to expose abnormal brain function in mild versus moderate or severe Alzheimer's disease: PET study using parametric visual stimulation. - Mentis,M.J.; Alexander,G.E.; Krasuski,J.; Pietrini,P.; Furey,M.L.; Schapiro,M.B.; Rapoport,S.I. (Am J Psychiatry, 1998)