The LEARN study a multicenter, observational study will that will evaluate the rate of cognitive change in approximately 500 clinically normal older individuals who "screen-fail" for the A4 trial on the basis of their screening PET imaging not demonstrating evidence of elevated amyloid accumulation (Aβ negative) but meet all other A4 study eligibility criteria. This study will leverage the A4 infrastructure and maximize the data acquired in screening a large number of well-characterized older adults for the A4 trial.

The LEARN observational cohort will provide a critical comparison group for the A4 placebo arm, and future trials in preclinical AD. Although accumulating longitudinal data suggest that older individuals with elevated Aβ burden are at increased risk of cognitive decline, it is important to demonstrate a differential rate of clinical decline between Aβe ("Aβ elevated") and Aβne ("Aβ not elevated") individuals on a standardized set of clinical outcomes. Over 2000 well-characterized, highly motivated older volunteers will "screen fail" for the A4 trial. The LEARN study will follow 500 of these individuals, matched as closely as possible to the two treatment arms, in this observation cohort. The LEARN study may selectively recruit from a specific range of SUVr that fall below the threshold for "elevated amyloid" in order to support analyses of the relationship of baseline SUVr to subsequent cognitive change and amyloid accumulation. The observational cohort will be followed for 384 weeks with identical clinical/cognitive testing performed every 24 weeks, running parallel to the A4 treatment study and open label extension.

Inclusion Criteria:

1. Consented to participate in the A4 study and previously met A4 demographic, cognitive and clinical criteria (e.g., Mini-Mental State Examination (MMSE); Clinical Dementia Ratin (CDR); Logical Memory test, part IIa (LMIIa); medications; medical history).
2. Has a florbetapir PET scan that falls below the Aβ threshold levels required for randomization into the treatment arms of the A4 trial.
3. In general, permitted medications should be stable for 8 weeks prior to LEARN Visit 1. Changes to medications that, in the opinion of the investigator, are not likely to impact LEARN Visit 1 assessments are permissible.
4. Has a study partner that is willing to participate as a source of information and has at least weekly contact with the subject (contact can be in-person, via telephone or electronic communication). The study partner must have sufficient contact such that the investigator feels the study partner can provide meaningful information about the subject's daily function.
5. In the investigator's opinion, is both willing and able to participate in all required procedures for the duration of the study (at least 240 weeks), including adequate literacy in English or Spanish and adequate vision and hearing to complete the required psychometric tests.

Exclusion Criteria:

1. Is receiving a prescription acetylcholinesterase inhibitor (AChEI) and/or memantine at LEARN Visit 1

2. Has current serious or unstable illness including cardiovascular, hepatic, renal, gastroenterologic, respiratory, endocrinologic, neurologic, psychiatric, immunologic, or hematologic disease or other conditions that, in the investigator's opinion, could interfere with the analyses of safety and efficacy in this study.

3. Has any contraindications for MRI studies, including claustrophobia, the presence of metal (ferromagnetic) implants, or a cardiac pacemaker that is not compatible with MRI.

4. Has a LEARN Visit 1 MRI scan with results showing >4 hemosiderin deposits (definite microhemorrhages or areas of superficial siderosis); or any amyloid-related imaging abnormalities - edema/effusions (ARIA-E).

5. Has received any exclusionary medication, including those with significant central nervous system (CNS) anticholinergic effects, within 3 months prior to LEARN Visit 1 or initiated at any point after screen. A full list of exclusionary medication will be provided in the relevant procedures manual.

6. Is currently enrolled in a clinical trial involving an investigational product or non-approved use of a drug or device, or concurrently enrolled in any other type of medical research judged not to be scientifically or medically compatible with this study. Participation in observational studies may be permitted upon review of the observational study protocol and approval by the Project Director or one of the ADCS Medical Monitors.

   For subjects participating in the optional Lumbar Puncture (LP, all of the above, plus:

7. Current use of anticoagulants, such as warfarin or dabigatran.

500 subjects without evidence of elevated ABeta on screening PET scan. Subjects will have consented to participate in the A4 trial and previously met demographic, cognitive and clinical criteria and have an A4 screening PET scan results that fallows below the ABeta threshold levels required for randomization into the treatment arms for the A4 trials.