Theta waves are one of the mammalian brain's key neural electrical oscillation patterns, in the range of 4 - 7 Hz.
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”