The present disclosure provides systems and methods for at least one of preventing, reducing, and treating a level of or change in at least one of amyloid-β (Aβ) peptide, C-terminal fragment-β (β-CTF), β-secretase (BACE1), γ-secretase, neuroinflammation, and/or dementia (e.g., Alzheimer's disease or age-related decline) in a subject by inducing synchronized gamma oscillations in the brain of the subject using, for example, a stimulus-emitting device to emit a stimulus (e.g., light, sound, and/or haptic) at a frequency (e.g., about 40 Hz) that synchronously activates in vivo a specific cell type (e.g., fast-spiking-parvalbumin (FS-PV) immunoreactive interneurons) and/or brain region (e.g., a sensory cortex and/or hippocampus) of the subject.