Theta rhythm
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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 normally absent in healthy awake adults but are physiological and natural in awake children under the age of 13 years. Nonetheless, 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). They can equally be seen in cases of focal or generalized subcortical brain damage and epilepsy. Interestingly, theta waves 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 (Buzsaki, 2002). Conversely, theta oscillations have been correlated to various voluntary behaviors (exploration, spatial navigation, etc.) and alert states (piloerection, etc.) in the rat (Vanderwolf, 1969), suggesting that it may reflect the integration of sensory information with motor output (for review, see Bland & Oddie, 2001).
Theta rhythms are very strong in rodent hippocampi and entorhinal cortex during learning and memory retrieval, and are believed to be vital to the induction of long-term potentiation, a potential cellular mechanism of learning and memory. A large body of evidence indicates that they are used in spatial learning and navigation (e.g. Buzsaki 2005). Here, they seem to start in the dendrites of neurons in dentate gyrus and CA1 regions of the hippocampus (Buzsaki 2002). Electrophysiological or pharmacological stimulation of the medial septum and the diagonal band 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 other 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 ascending brainstem synchronizing system via the medial septum.
A putative functional role of the theta rhythm has been put forth by Dr. Michael Hasselmo in a series of papers (Hasselmo et al. 2002, Hasselmo and Eichenbaum 2005). Based on evidence from electrophysiological studies showing that both synaptic plasticity and strength of inputs to hippocampal region CA1 vary systematically with ongoing theta oscillations (Hyman et al. 2003, Brankack et al. 1993, Pavlides et al. 1988), it has been suggested that the theta rhythm functions to separate periods of encoding of current sensory stimuli and retrieval of episodic memory cued by current stimuli so as to avoid interference that would occur if encoding and retrieval were simultaneous.
[edit] References
Aftanas 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.
Bland, B. H., & Oddie, S. D. (2001). Theta band oscillation and synchrony in the hippocampal formation and associated structures: the case for its role in sensorimotor integration. Behav Brain Res, 127(1-2), 119-136.
Brankack J, Stewart M, Fox SE (1993) Current source density analysis of the hippocampal theta rhythm: Associated sustained potentials and candidate synaptic generators. Brain Research 615(2):310-327.
Buzsaki G (2005) Theta rhythm of navigation: link between path integration and landmark navigation, episodic and semantic memory. Hippocampus 15(7):827-40.
Buzsaki G (2002) Theta oscillations in the hippocampus. Neuron 33(3):325-40.
Hasselmo ME, Bodelon C, Wyble BP (2002) A proposed function for hippocampal theta rhythm: Separate phases of encoding and retrieval enhance reversal of prior learning. Neural Computation, 14(4): 793-817.
Hasselmo ME, Eichenbaum H (2005) Hippocampal mechanisms for the context-dependent retrieval of episodes. Neural Networks, 18(9):1172-1190.
Hyman JM, Wyble BP, Goyal V, Rossi CA, Hasselmo ME (2003) Stimulation in hippocampal region CA1 in behaving rats yields LTP when delivered to the peak of theta and LTD when delivered to the trough. J Neurosci 23(37):11725-31.
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.
Pavlides C, Greenstein YJ, Grudman M, Winson J (1988) Long-term potentiation in the dentate gyrus is induced preferentially on the positive phase of theta-rhythm. Brain Res 439(1–2):383–387.
Vanderwolf, C. H. (1969). Hippocampal electrical activity and voluntary movement in the rat. Electroencephalogr Clin Neurophysiol, 26(4), 407-418.
Vertes RP (2005) Hippocampal theta rhythm: a tag for short-term memory. Hippocampus 15(7):923-35.
