Audio-visual entrainment

Audio-visual entrainment (AVE), a subset of brainwave entrainment, uses flashes of lights and pulses of tones to guide the brain into various states of brainwave activity. AVE devices are often termed light and sound machines or mind machines. Altering brainwave activity is believed to aid in the treatment of psychological and physiological disorders.

All of our senses (except smell) access the brain's cerebral cortex via the thalamus, and because the thalamus is highly innervated with the cortex, sensory stimulation can easily influence cortical activity. In order to affect brain (neuronal) activity, sensory stimulation must be within the frequency range of roughly 0.5 to 25 hertz (Hz). Touch, photic and auditory stimulation are capable of affecting brain wave activity. A large area of skin must be stimulated to affect brainwaves, which leaves both auditory and photic stimulation as the most effective and easiest means of affecting brain activity. Therefore, mind machines are typically in the form of light and sound devices.

Auditory or visual stimulation (AVS) can take a variety of forms, generating different subjective and clinical effects. The simplest form of stimulation is to present a series of random light flashes and/or sound pulses to a subject, such as from watching TV or cars drive by, and investigate the resulting subjective experiences or electroencephalography (EEG) effects. AVE, however, involves organized, repetitive stimulation at a particular frequency for a specific period of time, and the frequency of stimulation is reflected within the EEG. This is called "open loop" stimulation, or free-running entrainment, and is not contingent on monitoring brainwaves in any way. "Close loop" AVE would involve visual and auditory stimulation in response to one's EEG.

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