Example of an EEG theta wave.
Theta rhythms are one of several characteristic electroencephalogram (EEG) waveforms associated with various sleep and wakefulness states of the brain. When measured in this fashion, they are between 4 and 8 Hz, and involve many neurons firing synchronously, in the hippocampus and through the cortex. Theta activity can be observed in adults during some sleep states, as well as in states of quiet focus (meditation, for example).[1]. These rhythms are associated with spatial navigation and some forms of memory and learning, especially in the temporal lobes.
Research findings in theta-wave activity
Theta-frequency EEG activity is also manifested during some short term memory tasks[2]. Studies suggest that they reflect the "on-line" state of the hippocampus; one of readiness to process incoming signals.[3] Conversely, theta oscillations have been correlated to various voluntary behaviors (exploration, spatial navigation, etc.) and alert states (piloerection, etc.) in rats[4], suggesting that it may reflect the integration of sensory information with motor output (for review, see Bland & Oddie, 2005)[5]. A large body of evidence indicates that theta rhythm is likely involved in spatial learning and navigation.[6]
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. 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[7][8][9], 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.
Underlying large-scale synchronization which results in rhythmic slow activity of field EEG are theta-frequency membrane potential oscillations, typically sodium-dependent voltage-sensitive oscillations in membrane potential at near-action potential voltages.[10][11] Specifically, it appears that in neurons of the CA1 and dentate gyrus, these oscillations result from an interplay of dendritic excitation via a persistent sodium current (INaP) with perisomatic inhibition.[3]
Electrophysiological or pharmacological stimulation of the medial septum and the diagonal band of Broca projecting to hippocampus also induces theta-like rhythms.[12]
It is likely that human sources of theta rhythm are similar to those found in other mammals, and thus it is likely that cholinergic projections from the basal forebrain drive the theta rhythm seen in human EEG patterns. Similarly, humans show hippocampal theta rhythms that are probably mediated by inputs from the ascending brainstem synchronizing system via the medial septum (see diagram[13]).
References
- ^ Aftanas, L. & Golosheykin, S. (June 2005), "Impact of regular meditation practice on EEG activity at rest and during evoked negative emotions.", International Journal of Neuroscience 115(6): 893–909, doi:10.1080/00207450590897969
- ^ Vertes, R.P. (September 7), "Hippocampal theta rhythm: a tag for short-term memory", Hippocampus 15(7): 923–35, doi:10.1002/hipo.20118
- ^ a b Buzsáki, G., "Theta oscillations in the hippocampus", Neuron 33(3): 325–40, doi:10.1016/S0896-6273(02)00586-X
- ^ Vanderwolf, C.H., "Hippocampal electrical activity and voluntary movement in the rat", Electroencephalography & Clinical Neurophysiology 26(4): 407–418, doi:10.1016/0013-4694(69)90092-3
- ^ Bland, B. H. & Oddie, S. D. (December 2001), "Theta band oscillation and synchrony in the hippocampal formation and associated structures: the case for its role in sensorimotor integration.", Behavioral Brain Research 127(1–2): 119–36, doi:10.1016/S0166-4328(01)00358-8
- ^ Buzsáki, G., "Theta rhythm of navigation: link between path integration and landmark navigation, episodic and semantic memory", Hippocampus 15(7): 827–40, doi:10.1002/hipo.20113
- ^ Hyman JM, Wyble BP, Goyal V, Rossi CA, Hasselmo ME, "Stimulation in hippocampal region CA1 in behaving rats yields LTP when delivered to the peak of theta and LTD when delivered to the trough", The Journal of Neuroscience 23(37): 11725–31, <http://www.jneurosci.org/cgi/content/abstract/23/37/11725>
- ^ Brankack, J.; Stewart, M. & Fox, S.E. (1993), "Current source density analysis of the hippocampal theta rhythm: Associated sustained potentials and candidate synaptic generators", Brain Research 615(2): 310-327, doi:10.1016/0006-8993(93)90043-M
- ^ Pavlides, C.; Greenstein, Y.J.; Grudman, M. & Winson, J., "Long-term potentiation in the dentate gyrus is induced preferentially on the positive phase of theta-rhythm", Brain Research 439(1–2): 383–387, doi:10.1016/0006-8993(88)91499-0
- ^ Alonso, A. & Llinás, R. (November 1989), "Subthreshold Na+-dependent theta-like rhythmicity in entorhinal cortex layer II stellate cells", Nature 342: 175–177, doi:10.1038/342175a0
- ^ Chapman & Lacaille, 1999
- ^ Manseau, F.; Danik, M. & Williams, S. (May 26), "A functional glutamatergic neurone network in the medial septum and diagonal band area", The Journal of Physiology 566(Pt 3): 865–84, doi:10.1113/jphysiol.2005.089664
- ^ Bland
- Canolty, R.; Edwards, E.; Soltani, M.; Dalal, S.S.; Kirsch, H.E.; Barbaro, N.M.; Berger, M.S. & Knight, R.T., "High gamma power is phase-locked to theta oscillations in human neocortex", Science 313(5793): 1626–1628, doi:10.1126/science.1128115
- Hasselmo M.E., Bodelon C., Wyble B.P. (April 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, doi:10.1162/089976602317318965, ISSN 0899-7667
- Hasselmo, M.E. & Eichenbaum, H. (November 2005), "Hippocampal mechanisms for the context-dependent retrieval of episodes", Neural Networks 18(9): 1172–90, doi:10.1016/j.neunet.2005.08.007, ISSN 0893-6080
See also
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