Any conditions which could account for cognitive deficits in addition to AD, including but not limited to Vitamin B12 or folate deficiency, abnormal thyroid function, posttraumatic conditions, syphilis, multiple sclerosis or another neuroinflammatory disorder, Parkinson's disease, vascular or multi-infarct dementia, Huntington's disease, normal pressure hydrocephalus, central nervous system (CNS) tumor, progressive supranuclear palsy, subdural hematoma, etc.; Previous history of a seizure disorder, excepting cases where the first seizure or detection of epileptiform activity was within 5 years of screening and the patient is not prescribed an anticonvulsant; Significant systemic medical illnesses; Use of medications likely to affect CNS functions (e.g., benzodiazepines, narcotics); Severe renal dysfunction with creatine clearance < 30 ml/min, which would affect serum LEV levels; Participation in another AD clinical trial within 3 months of Screening, or any AD clinical trial, such as a vaccine, that has potential long-term effects; Treatment with another study drug or investigational drug within 30 days of Screening; Pregnant or lactating; Any other medical condition which is determined by the investigators to potentially create an undue risk for an adverse effect; Biomarker evidence unsupportive of a diagnosis of AD.