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 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.

Theta-frequency EEG activity is 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 (Buzsáki, 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). A large body of evidence indicates that theta rhythm is likely involved in spatial learning and navigation (e.g. Buzsáki 2005).

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 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.

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 (Alonso & Llinas, 1989; Chapman & Lacaille, 1999). 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 (Buzsáki, 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).

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 []).