A binaural beat is an auditory illusion perceived when two different pure-tone sine waves, both with frequencies lower than 1500 Hz, with less than a 40 Hz difference between them, are presented to a listener dichotically (one through each ear). For example, if a 530 Hz pure tone is presented to a subject's right ear, while a 520 Hz pure tone is presented to the subject's left ear, the listener will perceive the auditory illusion of a third tone, in addition to the two pure-tones presented to each ear. The third sound is called a binaural beat, and in this example would have a perceived pitch correlating to a frequency of 10 Hz, that being the difference between the 530 Hz and 520 Hz pure tones presented to each ear.
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History
The term 'binaural' literally signifies 'to hear with two ears', and was introduced in 1859 to signify the practice of listening to the same sound through both ears, or to two discrete sounds, one through each ear. It was not until 1916 that Carl Stumpf (1848-1936), a German philosopher and psychologist, distinguished between dichotic listening, which refers to the stimulation of each ear with a different stimulus, and diotic listening, the simultaneous stimulation of both ears with the same stimulus.
Later, it would become apparent that binaural hearing, whether dichotic or diotic, is the means by which the geolocation and direction of a sound is determined.
Scientific consideration of binaural hearing began before the phenomenon was so named, with the ideas articulated in 1792 by William Charles Wells (1757-1817), a Scottish-American printer, and physician at Saint Thomas' Hospital, London. Wells sought to theoretically examine and explain aspects of human hearing, including the way in which listening with two ears rather than one might affect the perception of sound, which proceeded from his research into binocular vision.
Subsequently, between 1776 and 1802, Giovanni Battista Venturi (1746 - 1822), an Italian physicist, savant, man of letters, diplomat, and historian of science, conducted and described a series of experiments intended to elucidate the nature of binaural hearing. It was in an appendix to a monograph on color that Venturi described experiments on auditory localization using one or two ears, concluding that "the inequality of the two impressions, which are perceived at the same time by both ears, determines the correct direction of the sound."
However, none of Venturi's contemporaries at the end of the eighteenth and beginning of the nineteenth centuries considered his original work worthy of citation or attention, with the exception of Ernst Florens Friedrich Chladni (1756-1827), a German physicist and musician, who is widely cited as the father of acoustics. After investigating the behavior of vibrating strings and plates, and examining the way in which sound appeared to be perceived, Chladni acknowledged Venturi's work, agreeing with him that the ability to determine the location, and direction of sound depended upon detected differences in a sound between both ears, including amplitude and frequency, subsequently denoted by the term 'interaural differences'.
Other significant historic investigations into binaural hearing include those of Charles Wheatstone (1802-1875), an English scientist, whose many inventions included the concertina and the stereoscope, Ernst Heinrich Weber (1795-1878), a German physician cited as one of the founders of experimental psychology; and August Seebeck (1805-1849), a scientist at the Technische Universität, Dresden, remembered for his work on sound and hearing. Like Wells, these researchers attempted to compare and contrast what would become known as binaural hearing with the principles of binocular integration generally, and binocular color mixing specifically. They found that binocular vision did not follow the laws of combination of colors from different bands of the spectrum. Rather, it was found that when presenting a different color to each eye, they did not combine, but often competed for perceptual attention.
Meanwhile, Wheatstone conducted experiments in which he presented a different tuning fork to each ear, stating:
It is well known, that when two consonant sounds are heard together, a third sound results from the coincidences of their vibrations; and that this third sound, which is called the grave harmonic, is always equal to unity, when the two primitive sounds are represented by the lowest integral numbers. This being premised, select two tuning-forks the sounds of which differ by any consonant interval excepting the octave; place the broad sides of their branches, while in vibration, close to one ear, in such a manner that they shall nearly touch at the acoustic axis; the resulting grave harmonic will then be strongly audible, combined with the two other sounds; place afterwards one fork to each ear, and the consonance will be heard much richer in volume, but no audible indications whatever of the third sound will be perceived.
Wheatstone's reference to the perceptual fusion of harmonically related tones were directly related to the principles examined by Wells. However, both their observations were ignored and remained uncited by contemporaraneous and subsequent German researchers of the following decades.
Venturi's experiments were repeated and confirmed by Lord Rayleigh (1842-1919), almost seventy-five years later. Other investigators of the late eighteenth and early nineteenth centuries, who were contemporaries of Lord Rayleigh, also investigated the significance of binaural hearing. These included Louis Trenchard More (1870-1944), a professor of physics, and Harry Shipley Fry (1878-1949), a lecturer in chemistry, both at the University of Cincinnati; H. A. Wilson and Charles Samuel Myers, both professors of science at King's College London; and Alfred M. Mayer (1836 - 1897), an American physicist, each of whom conducted experimental investigations with intent to discover the means by which human subjects ascertain the location, origin, and direction of sound, believing this to be in some way dependent on dichotic hearing, that is listening to sound through both ears.
Understanding of how the difference in sound signal between two ears contributes to auditory processing in such a way as to enable the location and direction of sound to be determined was considerably advanced after the invention of the differential stethophone by Somerville Scott Alison in 1859, who coined the term 'binaural'. Alison based his stethophone on the stethoscope, a previous invention of René Théophile Hyacinthe Laennec (1781-1826).
Unlike the stethoscope, which had only a single sound-source piece placed upon the chest, Alison's stethophone had two separate ones, allowing the user to hear and compare sounds derived from two discrete locations. This allowed a physician to identify the source of a sound through the process of binaural hearing. Subsequently, Alison referred to his invention as a 'binaural stethoscope', describing it as:
...an instrument consisting of two hearing-tubes, or trumpets, or stethoscopes, provided with collecting-cups and ear-knobs, one for each ear respectively. The two tubes are, for convenience, mechanically combined, but may be said to be acoustically separate, as care is taken that the sound, once admitted into one tube, is not communicated to the other.
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Neurophysiology
Cortical Oscillation and Electroencephalography (EEG)
The activity of neurons generate electric currents; and the synchronous action of neural ensembles in the cerebral cortex, comprising large numbers of neurons, produce macroscopic neural oscillations, which can be monitored and graphically documented by an electroencephalogram (EEG). The electroencephalographic representations of those oscillations are commonly called 'brainwaves'.
These neural oscillations are rhythmic or repetitive electrochemical activity in the brain and central nervous system. Such oscillations can be characterized by their frequency, amplitude and phase. Neural tissue can generate oscillatory activity driven by mechanisms within individual neurons, as well as by interactions between them. They may also adjust frequency to synchronize with the frequency of an external acoustic or visual stimulus.
The technique of recording neural electrical activity within the brain from electrochemical readings taken from the scalp originated with the experiments of Richard Caton in 1875, whose findings were developed into electroencephalography (EEG) by Hans Berger in the late 1920s.
Frequency bands of cortical neural ensembles
The fluctuating frequency of oscillations generated by the synchronous activity of cortical neurons, measurable with an electroencephalogram (EEG), via electrodes attached to the scalp, are conveniently categorized into general bands, in order of decreasing frequency, measured in Hertz (Hz) as follows:
- Gamma, 30 to 50 Hz
- Beta, 14 to 30 Hz
- Alpha, 8 to 14 Hz
- Theta, 4 to 8 Hz
- Delta, 0.1 to 4 Hz
In addition, three further wave forms are often delineated in electroencephalographic studies:
- Mu, 8 to 12 Hz
- Sigma (sleep spindle), 12 to 14 Hz
- SMR (Sensory motor rhythm), 12.5 to 15.5 Hz
It was Berger who first described the frequency bands Delta, Theta, Alpha, and Beta.
Neurophysiological origin of binaural beat perception
Binaural-beat perception originates in the inferior colliculus of the midbrain and the superior olivary complex of the brainstem, where auditory signals from each ear are integrated and precipitate electrical impulses along neural pathways through the reticular formation up the midbrain to the thalamus, auditory cortex, and other cortical regions.
Neural oscillations and mental state
Following the technique of measuring such brainwaves by Berger, there has remained a ubiquitous consensus that electroencephalogram (EEG) readings depict brainwave wave forms (patterns) that alter over time, and correlate with aspects of the subject's mental and emotional state, mental status, and degree of consciousness and vigilance. It is therefore now established and accepted that different electroencephalogram (EEG) measurements, including frequency and amplitude of neural oscillations, correlate with different perceptual, motor and cognitive states.
Furthermore, brainwaves alter in response to changes in environmental stimuli, including sound and music; and while the degree and nature of alteration is partially dependent on individual perception, such that the same stimulus may precipitate differing changes in neural oscillations and correlating electroencephalogram (EEG) readings in different subjects, the frequency of cortical neural oscillations (brainwaves), as measured by the EEG, has also been shown to synchronize with or entrain to that of an external acoustic or photic stimulus, with accompanying alterations in cognitive and emotional state. This process is called neuronal entrainment or brainwave entrainment.
Entrainment
'Entrainment' has been used to describe a shared tendency of many physical and biological systems to synchronize their periodicity and rhythm through interaction.
Brainwave entrainment
Brainwaves, or neural oscillations, share the fundamental constituents with acoustic and optical wave forms, including frequency, amplitude, and periodicity. Consequently, Huygens' discovery precipitated inquiry into whether or not the synchronous electrical activity of cortical neural ensembles might not only alter in response to external acoustic or optical stimuli but also entrain or synchronize their frequency to that of a specific stimulus.
Brainwave entrainment is a colloquialism for such 'neural entrainment', which is a term used to denote the way in which the aggregate frequency of oscillations produced by the synchronous electrical activity in ensembles of cortical neurons can adjust to synchronize with the periodicity of an external stimuli, such as a sustained acoustic frequency perceived as pitch, a regularly repeating pattern of intermittent sounds, perceived as rhythm, or a regularly rhythmically intermittent flashing light.
The frequency following response and auditory driving
The hypothesized entrainment of neural oscillations to the frequency of an acoustic stimulus occurs by way of the Frequency following response (FFR), also referred to as Frequency Following Potential (FFP). The use of sound with intent to influence brainwave cortical brainwave frequency is called auditory driving.
Auditory driving refers to the hypothesized ability for repetitive rhythmic auditory stimuli to 'drive' neural electric activity to entrain with it. By the principles of such hypotheses, it is proposed that, for example, a subject who hears drum rhythms at 8 beats per second, will be influenced such that an electroencephalogram (EEG) reading will show an increase brainwave activity at 8 Hz range, in the upper theta, lower alpha band.
Binaural beats and neural entrainment
One of the problems inherent in any scientific investigation conducted in order to ascertain whether brainwaves can entrain to the frequency of an acoustic stimulus is that human subjects rarely hear frequencies below 20 Hz, which is exactly the range of Delta, Theta, Alpha, and low to mid Beta brainwaves. Among the methods by which some investigations have sought to overcome this problem is to measure electroencephalogram (EEG) readings of a subject who is listening to binaural beats. Some researchers have published experimental results indicating that such listening precipitates auditory driving by which ensembles of cortical neurons entrain their frequencies to that of the binaural beat. Other researchers, such as Vernon et al. found that "broad-band and narrow-band amplitudes, and frequency showed no effect of binaural beat frequency eliciting a frequency following effect in the EEG." Similarly, Guruprasath and Gnanavel's 2015 study also found "no significant difference or frequency following effect elicited by either continuous or short burst stimulation was observed." Yet other studies found both EEG power increases and decreases as well as generally negative effects on synchronization, suggesting that, although entrainment doesn't occur, monaural and binaural beats could potentially be a noninvasive way to affect brain waves.
Binaural beats and music
Many of the aforementioned reports are based on the use of auditory stimuli that combine binaural beats with other sounds, including music and verbal guidance. This consequently precludes the attribution of any influence on or positive outcome for the listener specifically to the perception of the binaural beats. Very few studies have sought to isolate the effect of binaural beats on listeners. However, initial findings in one experiment suggest that listening to binaural beats may exert an influence on both low frequency and high frequency components of heart rate variability, and may increase subjective feelings of relaxation.
Non ordinary states of consciousness
The findings of some contemporary research suggests that listening to rhythmic sounds, especially percussion, can induce the subjective experience of a non ordinary states of consciousness (NOSC), with correlating electroencephalogram (EEG) profiles comparable to those associated with some forms of meditation, while also increasing the susceptibility to hypnosis. Specifically, some investigations show that the electroencephalogram readings attained while a subject is meditating are comparable to those taken while he or she is listening to binaural beats, characterized by increased activity in the Alpha and Theta bands.
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