Effects of 40 Hz Brain Stimulation Across Modalities: A Comparative Narrative Review

Gamma-band oscillations centered around 40 Hz play an important role in cortical communication, and their disruption has been documented as a neurophysiological feature of several neurodegenerative and neuropsychiatric disorders. This narrative review synthesizes preclinical and early-phase clinical evidence for 40 Hz non-invasive brain stimulation across five delivery modalities: (1) auditory stimulation, which leverages the 40 Hz auditory steady-state response (ASSR) to probe parvalbumin-positive (PV⁺) interneuron circuits and serves as a validated neurophysiological biomarker in schizophrenia; (2) visual stimulation, using luminance or invisible spectral flicker to induce steady-state visually evoked potentials (SSVEPs) and, in animal models, to activate microglial phagocytosis; (3) transcranial alternating current stimulation (tACS), which delivers sinusoidal sub-threshold membrane polarization at gamma frequency, with preliminary case-series evidence suggesting tau burden reduction and EEG-based biomarker changes in Alzheimer's disease; (4) repetitive transcranial magnetic stimulation (rTMS), offering focal cortical entrainment that, when combined with tACS in phase-synchronized protocols, produces sustained gamma enhancement in the dorsolateral prefrontal cortex; and (5) multisensory combined stimulation, which engages multiple convergent pathways and currently represents the approach with the most promising early translational signal, including cognitive stabilization and hippocampal volume preservation in small AD trials. While single-session entrainment does not reliably yield cognitive gains, multi-week applications have shown neurophysiological and preliminary biomarker-level changes in selected populations. It should be emphasized, however, that the human evidence base remains early-phase and largely derived from small, often uncontrolled studies; 40 Hz stimulation should accordingly be regarded as a biologically plausible and well-tolerated investigational approach rather than an established therapeutic intervention. Adequately powered, randomized, sham-controlled trials are required before clinical conclusions can be drawn.