A Study of JNJ-64042056 in Participants With Preclinical Alzheimer's Disease

- Elevated brain tau pathology defined as Braak 3 region of interest standardized uptake value ratio (ROI SUVR) greater than (>) 1.1 (or equivalent based on emerging data) on a screening tau PET scan, reviewed centrally by a qualified reader to enrich for probability of disease progression during the study

- Clinical Dementia Rating (CDR) global score of 0 at screening and baseline

- Mini Mental State Examination (MMSE) greater than or equal to (>=) 27 (with educational adjustment) at screening

- Able to read and write and with a minimum 5 years of formal education as reported by participant and study partner at screening

- A participant must be of non-childbearing potential

- History consistent with or known autosomal dominant AD (mutation identified in the family and/or participant)

- Fulfills diagnostic criteria for Alzheimer's Dementia or non-Alzheimer's Dementia, including, but not limited to Frontotemporal Dementia (FTD), Diffuse Lewy Body Dementia (DLBD), Vascular Dementia (VAD), alcoholic dementia, Parkinson's dementia, Korsakov, Creutzfeldt-Jakob or other prion diseases, Posterior Cortical Atrophy

- Diagnosis of Mild Cognitive Impairment (MCI)

- Vitamin B12 or folate levels below the central laboratory lower limit of normal, unless the investigator determines that supplementation is not required after randomization

- History of or current neurological disease other than preclinical AD that may make interpretation of possible new neurological signs or symptoms difficult