Theta rhythms are one of several characteristic electroencephalogram waveforms associated with various sleep and wakefulness states. When seen in this fashion, they are between 4 and 8 Hz, and involve many neurons firing synchronously, probably in the hippocampus and through the cortex. Theta activity can be observed in adults during some sleep states, and in states of quiet focus, for example meditation (e.g. Aftanas & Golosheykin, 2005). These rhythms are associated with spatial navigation and some forms of memory and learning, especially in the temporal lobes. -Wikipedia