Participants in the early Alzheimer's Disease (AD) spectrum, enrolled in ongoing or future clinical trials with JNJ-54861911 (Phase 1b or Phase 2 studies) will be provided the opportunity to participate in this study upon completion of their treatment period under the parent protocol. The study will consist of a Screening phase and 2 sequential treatment phases (a 12-month double-blind [DB] treatment phase [placebo controlled] and an open-label [OL] phase [active]) followed by an End-of-Treatment visit. Treatment in OL phase will continue until registration of JNJ-54861911; unless safety issues emerge as determined by the Data Review Committee (DRC) that would warrant termination of the study. Blood and cerebrospinal fluid (CSF) samples will be collected to evaluate the plasma and CSF pharmacokinetics of JNJ-54861911, as well as amyloid beta fragments. Participants' safety will be monitored throughout the study.