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Theta rhythmImage:Eeg theta.svg Theta rhythms are one of several distinct and characteristic electroencephalogramwaveforms associated with various sleep and wakefulness states. When seen in this fashion, they are between 4 and 8 Hz, and involve a large number of neurons firing synchronously, probably in the hippocampusand through the cortex. It is observed during some sleep states, and in states of quiet focus, for example meditation(e.g. Aftanas & Golosheykin, 2005). It is also manifested during some short term memory tasks (reviewed in Vertes 2005, for example). Some suggest it to be 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 rodenthippocampusduring learning and memory retrieval tasks, 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 also exists indicating that they are used in spatial learning and navigation (e.g. Buszaki 2005). Here, they seem to be produced starting in the dendritesof neurons in dentate gyrus and CA1 regions of the hippocampus(Buszaki 2002). Electrophysiologicalor pharmacological stimulation of the medial septumand the diagonal bands of Brocaprojecting to hippocampus also induces theta-like rhythms (Manseau et al. 2005). The precise sources and functional significance of theta rhythm in both humans and animals remain unclear. However, it is reasonable to suspect that human sources of theta rhythm are similar, and that cholinergic projections from the basal forebrainto throughout the cerebral cortexdrive the theta rhythm seen in human EEG patterns. Similarly, they show hippocampal theta rhythms that are in turn probably mediated by inputs from the medial septum. ReferencesAftanas L, Golosheykin S (2005) Impact of regular meditation practice on EEG activity at rest and during evoked negative emotions. Int J Neurosci. 2005 Jun;115(6):893-909. Buszaki G (2005) Theta rhythm of navigation: link between path integration and landmark navigation, episodic and semantic memory. Hippocampus 15(7):827-40. Buszaki G (2002) Theta oscillations in the hippocampus. Neuron 33(3):325-40. Manseau F, Danik M, Williams S (2005) A functional glutamatergic neurone network in the medial septum and diagonal band area. J Physiol. 566(Pt 3):865-84. Epub 2005 May 26. Vertes RP (2005) Hippocampal theta rhythm: a tag for short-term memory. Hippocampus 15(7):923-35.
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