Electroencephalography (EEG) is an electrophysiological non-invasive monitoring method to record electrical activity of the brain. Characteristic waveforms - e.g. the so-called Alpha-rhythm (sinusoidal-like waves with frequencies in 8–12 Hz range) - are believed to be produced by synchronous oscillations of very large groups of neurons. However, the underlying physiological mechanisms for the dynamic behaviour of the waveform sources themselves are for the most part unknown.
“The neuronal networks underlying some of these oscillations are understood (e.g., the thalamocortical resonance underlying sleep spindles), while many others are not (e.g., the system that generates the posterior basic rhythm)”. source: