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Binaural beats, or binaural tones, are auditory processing artifacts, or apparent sounds, caused by specific physical stimuli. This effect was discovered in 1839 by Heinrich Wilhelm Dove and earned greater public awareness in the late 20th century based on claims coming from the alternative medicine community that binaural beats could help induce relaxation, meditation, creativity and other desirable mental states. The effect on the brainwaves depends on the difference in frequencies of each tone: for example, if 300 Hz was played in one ear and 310 in the other, then the binaural beat would have a frequency of 10 Hz.
The brain produces a phenomenon resulting in low-frequency pulsations in the amplitude and sound localization of a perceived sound when two tones at slightly different frequencies are presented separately, one to each of a subject's ears, using stereo headphones. A beating tone will be perceived, as if the two tones mixed naturally, out of the brain. The frequencies of the tones must be below 1,000 hertz for the beating to be noticeable. The difference between the two frequencies must be small (less than or equal to 30 Hz) for the effect to occur; otherwise, the two tones will be heard separately, and no beat will be perceived.
For sound localization, the human auditory system analyses interaural time differences between both ears inside small frequency ranges, called critical bands. For frequencies below 1000 to 1500 Hz interaural time differences are evaluated from interaural phase differences between both ear signals. The perceived sound is also evaluated from the analysis of both ear signals.
If different pure tones (sinusoidal signals with different frequencies) are presented to each ear, there will be time-dependent phase and time differences between both ears (see figure). The perceived sound depends on the frequency difference between both ear signals:
Binaural beats can also be experienced without headphones; they appear when playing two different pure tones through loudspeakers. The sound perceived is quite similar: with auditory events that move through the room, at low-frequency differences, and diffuse sound at slightly bigger frequency differences. At bigger frequency differences, apparent localized sound sources appear. However, it is more effective to use headphones than loudspeakers.
Heinrich Wilhelm Dove discovered binaural beats in 1839 and published his findings in his book Repertorium der Physik. While research about them continued after that, the subject remained something of a scientific curiosity until 134 years later, with the publishing of Gerald Oster's article "Auditory Beats in the Brain" (Scientific American, 1973). Oster's article identified and assembled the scattered islands of relevant research since Dove, offering fresh insight (and new laboratory findings) to research on binaural beats.
In particular, Oster saw binaural beats as a powerful tool for cognitive and neurological research, addressing questions such as how animals locate sounds in their three-dimensional environment, and also the remarkable ability of animals to pick out and focus on specific sounds in a sea of noise (which is known as the cocktail party effect).
Oster also considered binaural beats to be a potentially useful medical diagnostic tool, not merely for finding and assessing auditory impairments, but also for more general neurological conditions. (Binaural beats involve different neurological pathways than ordinary auditory processing.) For example, Oster found that a number of his subjects that could not perceive binaural beats suffered from Parkinson's disease. In one particular case, Oster was able to follow the subject through a week-long treatment of Parkinson's disease; at the outset, the patient could not perceive binaural beats, but by the end of the week of treatment, the patient was able to hear them.
In corroborating an earlier study, Oster also reported gender differences in the perception of beats. Specifically, women seemed to experience two separate peaks in their ability to perceive binaural beats—peaks possibly correlating with specific points in the menstrual cycle, onset of menstruation and during the luteal phase. These data led Oster to wonder if binaural beats could be used as a tool for measuring relative levels of estrogen.
The effects of binaural beats on consciousness were first examined by physicist Thomas Warren Campbell and electrical engineer Dennis Mennerich, who under the direction of Robert Monroe sought to reproduce a subjective impression of 4 Hz oscillation that they associated with out-of-body experience. On the strength of their findings, Monroe created the binaural-beat technology self-development industry by forming The Monroe Institute, now a charitable binaural research and education organization.
There have been a number of claims regarding binaural beats, among them that they may simulate the effect of recreational drugs, help people memorize and learn, stop smoking, help dieting, tackle erectile dysfunction and improve athletic performance.
An uncontrolled pilot study of eight individuals indicated that binaural beats may have a relaxing effect. In absence of positive evidence for a specific effect, claimed effects may be attributed to the power of suggestion (the placebo effect).
In a blind study (eight participants) of binaural beats' effects on meditation, 7 Hz frequencies were found to enhance meditative focus while 15 Hz frequencies harmed it.
A further study conducted at Goldsmiths, University of London found that there was no main effect for the use of binaural beats in order to aliviate cold pain. Musicians, however, demonstrated themselves to be better at coping with the pain, suggesting that it may be the sound itself which is a distracting factor as opposed to any brainwave influence. 
The sensation of binaural beats is believed to originate in the superior olivary nucleus, a part of the brain stem. They appear to be related to the brain's ability to locate the sources of sounds in three dimensions and to track moving sounds, which also involves inferior colliculus (IC) neurons. Regarding entrainment, the study of rhythmicity provides insights into the understanding of temporal information processing in the human brain. Auditory rhythms rapidly entrain motor responses into stable steady synchronization states below and above conscious perception thresholds. Activated regions include primary sensorimotor and cingulate areas, bilateral opercular premotor areas, bilateral SII, ventral prefrontal cortex, and, subcortically, anterior insula, putamen, and thalamus. Within the cerebellum, vermal regions and anterior hemispheres ipsilateral to the movement became significantly activated. Tracking temporal modulations additionally activated predominantly right prefrontal, anterior cingulate, and intraparietal regions as well as posterior cerebellar hemispheres. A study of aphasic subjects who had a severe stroke versus normal subjects showed that the aphasic subject could not hear the binaural beats, whereas the normal subjects could.
Binaural beats may influence functions of the brain in ways besides those related to hearing. This phenomenon is called "frequency following response". The concept is that if one receives a stimulus with a frequency in the range of brain waves, the predominant brainwave frequency is said to be likely to move towards the frequency of the stimulus (a process called entrainment). In addition, binaural beats have been credibly documented to relate to both spatial perception and stereo auditory recognition, and, according to the frequency following response, activation of various sites in the brain.
Perceived human hearing is limited to the range of frequencies from 20 Hz to 20,000 Hz, but the frequencies of human brain waves are below about 40 Hz. To account for this lack of perception, binaural beat frequencies are used. Beat frequencies of 40 Hz have been produced in the brain with binaural sound and measured experimentally.
When the perceived beat frequency corresponds to the delta, theta, alpha, beta, or gamma range of brainwave frequencies, the brainwaves entrain to or move towards the beat frequency. For example, if a 315 Hz sine wave is played into the right ear and a 325 Hz one into the left ear, the brain is entrained towards the beat frequency 10 Hz, in the alpha range. Since alpha range is associated with relaxation, this has a relaxing effect, or if in the theta range, more alertness. An experiment with binaural sound stimulation using beat frequencies in the beta range on some participants and the delta/theta range on other participants found better vigilance performance and mood in those on the awake alert state of beta-range stimulation.
Binaural beat stimulation has been used fairly extensively in attempts to induce a variety of states of consciousness, and there has been some work done in regards to the effects of these stimuli on relaxation, focus, attention, and states of consciousness. Studies have shown that with repeated training to distinguish close frequency sounds that a plastic reorganization of the brain occurs for the trained frequencies and is capable of asymmetric hemispheric balancing.
|Frequency range||Name||Usually associated with:|
|> 40 Hz||Gamma waves||Higher mental activity, including perception, problem solving, fear, and consciousness|
|13–39 Hz||Beta waves||Active, busy or anxious thinking and active concentration, arousal, cognition, and or paranoia|
|7–13 Hz||Alpha waves||Relaxation (while awake), pre-sleep and pre-wake drowsiness, REM sleep, Dreams|
|8–12 Hz||Mu waves||Mu rhythm, Sensorimotor rhythm|
|4–7 Hz||Theta waves||Deep meditation/relaxation, NREM sleep|
|< 4 Hz||Delta waves||Deep dreamless sleep, loss of body awareness|
(The precise boundaries between ranges vary among definitions, and there is no universally accepted standard.)
The dominant frequency determines one's current state. For example, if in someone's brain, alpha waves are dominating, they are in the alpha state (this happens when one is relaxed but awake). However, other frequencies will also be present, albeit with smaller amplitudes.
The brain entraining is more effective if the entraining frequency is close to the user's starting dominant frequency. Therefore, it is suggested to start with a frequency near to one's current dominant frequency (likely to be about 20 Hz or less for a waking person) and then slowly decrease or increase it towards the desired frequency.
Some people find pure sine waves unpleasant, so a pink noise or another background (e.g., natural sounds such as river noises) can also be mixed with them. In addition to that, as long as the beat is audible, increasing the volume should not necessarily improve the effectiveness; therefore, using a low volume is usually suggested. One theory is to reduce the volume so low that the beating should not even be clearly audible, but this does not seem to be the case (see the next paragraph).
In addition to lowering the brain frequency to relax the listener, there are other controversial, alleged effects of binaural beats. For example, that by using specific frequencies, an individual can stimulate certain glands to produce desired hormones. Beta-endorphin has been modulated in studies using alpha-theta brain wave training, and dopamine with binaural beats. Among other alleged uses, there are reducing learning time and sleeping needs (theta waves are thought to improve learning, since children, who have stronger theta waves, and remain in this state for a longer period of time than adults, usually learn faster than adults; and some people find that half an hour in the theta state can reduce sleeping needs up to four hours; similar to another method of achieving a theta state, e.g. meditation;) some use them for lucid dreaming and even for attempting out-of-body experiences, astral projection, telepathy and psychokinesis. However, the role of alpha-wave activity in lucid dreaming is subject to ongoing research.
An uncontrolled pilot study of delta binaural beat technology over 60 days has shown positive effects on self-reported psychologic measures, especially anxiety. However only 15 people participated in this study, therefore, providing insignificant results. Of the few people that participated, there was reported a significant decrease in trait anxiety, an increase in quality of life, and a decrease in insulin-like growth factor-1 and dopamine, and it has been shown to decrease mild anxiety. Further research is warranted to explore the effects on anxiety using a larger, randomized and controlled trial. A randomised, controlled study concluded that binaural beat audio could lessen hospital acute pre-operative anxiety.
Another claimed effect for sound-induced brain synchronization is enhanced learning ability. It was proposed in the 1970s that induced alpha brain waves enabled students to assimilate more information with greater long-term retention. In more recent times has come more understanding of the role of theta brain waves in behavioural learning. The presence of theta patterns in the brain has been associated with increased receptivity for learning and decreased filtering by the left hemisphere. Based on the association between theta activity (4–7 Hz) and working memory performance, biofeedback training suggests that normal healthy individuals can learn to increase a specific component of their EEG activity, and that such enhanced activity may facilitate a working memory task and to a lesser extent focused attention.
A small media controversy was spawned in 2010 by an Oklahoma Bureau of Narcotics official comparing binaural beats to illegal narcotics and warning that interest in websites offering binaural beats could lead to drug use.
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