Electroencephalography (EEG) is an electro-physiological 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 8–15 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 : Wikipedia
Theta waves are one of the mammalian brain's key neural electrical oscillation patterns, in the range of 4 - 7 Hz. (There are many others, see note below)
They were first described in 1938. and can easily be measured via sets of electrodes attached to the scalp (EEG).
“A major research problem has been to discover the “pacemaker” for the theta rhythm, that is, the mechanism that determines the oscillation frequency. The answer is not yet entirely clear, but there is some evidence that type 1 and type 2 theta depend on different pacemakers. For type 2 theta, the supramammillary nucleus of the hypothalamus appears to exert control (Kirk, 1998). For type 1 theta, the picture is still unclear, but the most widely accepted hypothesis proposes that the frequency is determined by a feedback loop involving the medial septal area and hippocampus”
Theta waves are just one of the group of electrical 'brainwaves' which can be routinely detected by EEG. They include :
No explanation is available for their generation and maintenance.
Also see :
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