Magnetic Resonance Spectroscopy Study of Memantine in Alzheimer's Disease

24-week observational lead-in period, wherein patients already on a stable dose of donepezil, rivastigmine, or galantamine continue on that dose, followed by a 24-week open-label memantine period, wherein patients receive open-label memantine treatment titrated to a dose of 10 mg orally b.i.d., in addition to their ongoing stable cholinesterase inhibitor treatment