The primary objective of the study is to evaluate the efficacy of monthly doses of aducanumab in slowing cognitive and functional impairment as measured by changes in the Clinical Dementia Rating-Sum of Boxes (CDR-SB) score as compared with placebo in participants with early AD. Secondary objectives are to assess the effect of monthly doses of aducanumab as compared with placebo on clinical progression as measured by Mini-Mental State Examination (MMSE), AD Assessment Scale-Cognitive Subscale (13 items) [ADAS-Cog 13], and AD Cooperative Study-Activities of Daily Living Inventory (Mild Cognitive Impairment version) [ADCS-ADL-MCI].