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.
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 oscillations, which can be monitored and graphically documented by an electroencephalogram (EEG). The electroencephalographic representations of those oscillations are typically denoted by the term ‘brainwaves’ in common parlance.
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 periodicity of an external acoustic or visual stimuli.
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.
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.
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