Brainwave synchronization

Brainwave synchronization is the practice of entraining one's brainwaves to a desired frequency, by means of a periodic stimulus with corresponding frequency. The stimulus can be aural as in the case of binaural beats, or visual, as with a dreamachine, or a combination of the two with a mind machine.

Overview
Brainwave entrainment, also known as "brainwave synchronization", is concerned with frequency following response, a naturally occurring phenomenon where the human brain has a tendency to change towards the frequency of the dominant external stimuli applied to it. Brainwave synchronization with binaural beats is said to help 'synchronize the two hemispheres of the brain', hence the most famous incarnation of this example, Hemi-Sync (by Monroe Institute). There is, as with most quasi-scientific theories, some dispute over the actual truth in these claims. Brainwave entrainment has many historical, and often social, manifestations. Drum circles, chanting, and staring at a fire were all ways that our ancestors partook in the phenomenon, although participants were obviously oblivious to the method. Crowds at a rock concert, a deep conversation, and  being "on the same page" also serve to illustrate how brainwaves can "tune" to certain frequencies, although this is not particularly scientific. Much is not known about brainwaves and less about their entrainment, and most research is done by private institutions (like the one above) who make and sell self-help CDs. Brainwave Synchronization (or rather Frequency Following Response) is known to exist; but the effectiveness and amount of control is not yet "down to a science". It is also a private case of functional brain connectivity concept, whereas functional connectivity is defined as the temporal correlation between spatially-remote neurophysiological events, expressed as deviation from statistical independence (temporal correlation) across these events in distributed neuronal groups and areas, which produce the brainwaves.

Binaural beats
Brainwave synchronization (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 500 Hz tone and 510 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" (i.e. the 500 Hz in the example above), is also said by some to affect the quality of the transformative experience. Although some studies have reported 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 synchronization for emotional/mental disorders. The fixed, constant frequency of the 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.