<unk>  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.<unk>  rhythms are observed in awake children under<unk> ge of 13 years.<unk>  activity can be observed in adults during some sleep states, and in states of quiet focus, for example meditation (e.g. Aftanas & Golosheykin, 2005). These rhythms are associated with spacial navigation and some forms of memory and learning, especially in the temporal lobes.  They can equally be seen in cases of focal or generalized subcortical brain damage and epilepsy.<unk> -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,<unk>  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<unk>  rhythm is likely involved in spatial learning and navigation (e.g. Buzsáki 2005).<unk>  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<unk>  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<unk>  oscillations (Hyman et al. 2003, Brankack et al. 1993, Pavlides et al. 1988), it has been suggested that the<unk>  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<unk> -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<unk> -like rhythms (Manseau et al. 2005).
It is likely that human sources of<unk>  rhythm are similar to those found in other mammals, and thus it is likely that cholinergic projections from the basal forebrain drive the<unk>  rhythm seen in human EEG patterns. Similarly, humans show hippocampal<unk>  rhythms that are probably mediated by inputs from<unk> scending brainstem synchronizing system via the medial septum (see ).
Theta wave
A wave
The U Wave
T wave
P wave
V wave
QRS complex
Resting potential