Theta rhythm

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 rhythms are observed during some sleep states, and in states of quiet focus, for example meditation (e.g. Aftanas & Golosheykin, 2005). They are also manifested during some short term memory tasks (reviewed in Vertes 2005). Some suggest that they reflect the "on-line" state of the hippocampus; one of readiness to process incoming signals (Buszaki, 2002).

This is parallelled by work in rodent models. Theta rhythms are very strong in rodent hippocampi during learning and memory retrieval, and are believed to be vital to the induction of long-term potentiation, a cellular model of learning and memory. A large body of evidence indicates that they are used in spatial learning and navigation (e.g. Buszaki 2005). Here, they seem to start in the dendrites of neurons in dentate gyrus and CA1 regions of the hippocampus (Buszaki 2002). Electrophysiological or pharmacological stimulation of the medial septum and the diagonal bands of Broca projecting to hippocampus also induces theta-like rhythms (Manseau et al. 2005).

The origins and functional significance of theta rhythms in both humans and animals remain unclear. However, it is likely that human sources of theta rhythm are similar, and that cholinergic projections from the basal forebrain to throughout the cerebral cortex drive the theta rhythm seen in human EEG patterns. Similarly, they show hippocampal theta rhythms that are probably mediated by inputs from the medial septum.