In the 1940's a researcher by the name of Gray Walter discovered that when a person was subjected to a light flashing at a certain frequency their brain wave activity would fall in sync with the flashing of the lights. Since then a small body of scholarly research has been conducted on the subject and many devices have been been developed to entrain brain waves. Most of these devices include small lights mounted in a pair of eye glasses. These lights flash at given frequencies and are controlled by a small computer in the base unit. Today, most sound and light entrainment devices (SLEDs) also include a sound component. This sound pulses at a given frequency and is usually binaural. Binaural sound occurs when two distinct sine waves (pure tones) are played in each ear. For example, a person might hear a 440 hz tone in one ear and a 430 hz tone in the other ear; the ears will "hear" both tones but the brain (as shown by e.e.g.) will discern the difference between the tones, 10 hz. This 10 hz frequency is the alpha frequency for brain waves and is associated with relaxed states (light meditation, etc.) and imagery. SLEDs have also been used to entrain beta, theta and even delta states as well. The combined effects of having both the photic and the auditory portions of the brain stimulated in synchrony can be profound. SLEDs are currently being used by professional psychologists in their practices and by the general public for relaxation, stress management, Insomnia, mind expansion, accelerated learning and retention, breaking limiting beliefs, phobias, anxiety, sports training, promoting physical wellness, and most of all for fun. The possibilities are great and technology has been shown effective. However, the only way to truly appreciate the light sound experience is to try a mind machine for yourself.