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

Sight, taste, touch, and sound all access the brain's cerebral cortex to transmit sensation via the thalamus. 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 makes using it for brainwave entrainment impractical, 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.

AVS can take a variety of forms, generating different clinical effects. It involves organized, repetitive stimulation at a particular frequency for a specific period of time, with the frequency of stimulation reflected within the EEG. This is called "open loop" stimulation, or free-running entrainment.

AVS Info

(from Wikipedia)