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. These phenomena 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.
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
Mu, 8 to 12 Hz
Sigma (sleep spindle), 12 to 14 Hz
Sensorimotor rhythm (SMR), 12.5 to 15.5 Hz
It was Berger who first described the frequency bands Delta, Theta, Alpha, and Beta.