Single-Ascending-Dose Study of BIIB076 in Healthy Volunteers and Participants With Alzheimer's Disease

Key Inclusion Criteria - Healthy Participants * Must be in good health as determined by the Investigator, based on medical history and Screening evaluations. Key Inclusion Criteria - Participants with Alzheimer's Disease (AD) * Must meet all of the clinical criteria for mild cognitive impairment (MCI) due to AD or mild AD according to the National Institutes of Aging-Alzheimer's Association \[McKhann 2011\], and in addition must have the following: * Clinical Dementia Rating (CDR) global score of 0.5 for MCI due to AD or 0.5 or 1 for mild AD. * CDR Memory Box Score of ≥0.5. * Mini-Mental State Examination score between 18 and 30 (inclusive) at Screening. * Must have amyloid beta positivity confirmed at Screening Key Exclusion Criteria - Healthy Participants * Brain MRI findings that might pose a risk to the participant, or might prevent a satisfactory MRI assessment for safety monitoring. * History of any clinically significant cardiac, endocrine, gastrointestinal, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, or renal disease, or other major disease, as determined by the Investigator. * Current enrollment in any other drug, biologic, device, or clinical study or treatment with an investigational drug or approved therapy for investigational use within 30 days (6 months for biologics) or 5 half-lives, whichever is longer, prior to Day-1. * Contraindications to having a brain MRI (e.g., pacemaker; MRI-incompatible aneurysm clips, artificial heart valves, or other metal foreign body; claustrophobia that cannot be medically managed). * Contraindications to having an Lumbar Puncture (LP). Key Exclusion Criteria - Participants with Alzheimer's Disease (AD) * Any medical or neurologic/neurodegenerative condition (other than AD) that, in the opinion of the Investigator, might be a contributing cause to the participant's cognitive impairment (e.g.,current history of substance abuse, uncontrolled vitamin B12 deficiency or uncontrolled thyroid disease, stroke or other cerebrovascular condition, Parkinson's disease, Lewy body dementia, or frontotemporal dementia or head trauma), or could lead to discontinuation, noncompliance with study assessments, or safety concerns. * Diagnosis within 1 year prior to Screening and/or evidence of clinically significant (in the opinion of the Investigator) psychiatric illness including uncontrolled major depression, bipolar affective disorder, other psychiatric illness, and suicidal ideation. * Any documented prior history of chronic schizophrenia. * History of any clinically significant cardiac, endocrine, gastrointestinal, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary (including chronic obstructive pulmonary disease), neurologic, dermatologic, or renal disease, or other major disease, as determined by the Investigator. * Use of any medications for the treatment of comorbid conditions that have not been stable for at least 8 weeks prior to Day -1 and/or that are not expected to remain stable for the duration of the study. * Current enrollment or plan to enroll in any other drug, biologic, device, or clinical study or treatment with an investigational drug or approved therapy for investigational use within 30 days (6 months for biologics) or 5 half-lives, whichever is longer, prior to Day-1. * Contraindications to having a brain MRI (e.g., pacemaker; MRI-incompatible aneurysm clips, artificial heart valves, or other metal foreign body; claustrophobia that cannot be medically managed). * Brain MRI findings that might be a contributing cause of the participant's dementia, might pose a risk to the participant, or might prevent a satisfactory MRI assessment for safety monitoring. * Contraindications to having an LP. * History of, or ongoing chronic uncontrolled hypertension * History of unstable angina, myocardial infarction, chronic heart failure (New York Heart Association Class 3 or 4), or clinically significant conduction abnormalities (e.g., unstable atrial fibrillation) within 1 year prior to Day -1. * Medications with platelet anti-aggregant or anticoagulant properties, except the use of aspirin at a dose ≤325 mg per day. * For participants whose eligibility for study entry will be based on cerebral Aβ positivityas determined by amyloid PET (positron emission tomography), contraindication to having a PET scan (e.g., inability to lie flat or still for the duration of the scan) or intolerance to previous PET scans (i.e. previous hypersensitivity reactions to any PET radioligand or imaging agent, failure to participate in and comply with previous PET scans). NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.