Brainwave entrainment

Brainwave entrainment or "brainwave synchronization," is any practice that aims to cause brainwave frequency to fall into step with a periodic stimulus having  a frequency corresponding to the intended brain-state (for example, to induce sleep), usually performed with the use of specialized medical software. It depends upon a "frequency following" response, a naturally occurring phenomenon where the human brain has a tendency to change its dominant EEG frequency towards the frequency of a dominant external stimulus. Such a stimulus may be aural, as in the case of binaural or monaural beats and isochronic tones, or else visual, as with a dreamachine, a combination of the two with a mind machine, or even electromagnetic radiation.

History
Brainwave entrainment has been noted or used in one form or another for centuries, from Shamanistic societies' use of drum beats to Ptolemy noting in 200 A.D. the effects of flickering sunlight generated by a spinning wheel. In the 1930s and '40s, with then-new EEG equipment and strobe lights, W. Gray Walter performed some of the first scientific research on the subject. Later, in the 1960s and '70s, interest in altered states led many artists to become interested in the subject, most notably Brion Gysin who, along with a Cambridge math student, invented the Dreammachine. From the 1970s to date there have been numerous studies and various machines built that combine light and sound. These efforts were aided by continued development of micro circuitry and other electronic breakthroughs allowed for ever more sophisticated equipment for measuring and inducing brainwave entrainment. One of the most important breakthroughs was the discovery of binaural beats, first published in Scientific American in 1973 by Gerald Oster. With the development of isochronic tones by Arturo Manns, combined with more sophisticated equipment, these discoveries led to many attempts to use brainwave entrainment in the treatment of numerous psychological and physiological conditions.

Binaural beats
Binaural beats deserve special mention because of the manner in which the desired frequencies are obtained. Brainwave entrainment may be achieved when audio signals are introduced to the brain causing a response directly related to the frequency of the signal introduced, called binaural beats. Two tones close in frequency generate a beat frequency at the difference of the frequencies, which is generally subsonic. For example, a 495 Hz tone and 505 Hz tone will produce a subsonic 10 Hz tone, roughly in the middle of the alpha range. The resulting subsonic tone may affect the state of mind of the subject. The "carrier frequency" (e.g., the 500 Hz in the example above), is also said by some to affect the quality of the transformative experience. Note that this effect is achieved without either ear hearing the pulse when headphones are used. Instead, the brain produces the pulse by combining the two tones. Each ear hears only a steady tone. Although some have claimed that these frequencies do provide help in treating certain medical conditions, there is not a wide acceptance by the medical community to adopt the practice of brainwave entrainment for emotional/mental disorders. A fixed, constant frequency of synchronization is less helpful than techniques such as classical neurofeedback or learning meditation, which naturally generate brain wave frequencies that differ from person to person and may vary from minute to minute.