Devices, systems, and methods for a treating dementia or Alzheimer's disease in a subject in need thereof. In one example, combined auditory and visual stimuli having a frequency of about 20 Hz to about 60 Hz, and more specifically about 40 Hz, are non-invasively delivered to the subject to induce synchronized gamma oscillations in at least one brain region of the subject. In particular, pursuant to various treatment and exposure protocols, combined auditory and visual stimulation (as opposed to auditory or visual stimulation alone) promotes a microglia response in the medial prefrontal cortex (mPFC). More generally, combined auditory and visual stimulation induces an extended microglia clustering response in the auditory cortex, the visual cortex, and the mPFC.