Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Hippocampal Theta Oscillations Are Travelling Waves

 

Nature 459, 534-539 (28 May 2009)

Hippocampal theta oscillations are travelling waves

Evgueniy V. Lubenov & Athanassios G. Siapas

Division of Biology, Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, USA

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Theta oscillations are a prominent 410-Hz rhythm in the hippocampal local field potential (LFP) of all mammals studied to date, including humans. During wakefulness they are associated with different species-specific behaviours, and they are invariably present during REM sleep. In the rat, theta oscillations always accompany voluntary movement and active exploration. Theta oscillations are essential for the normal functioning of the hippocampus, because manipulations that disrupt them produce behavioural impairments that mimic hippocampal lesions. The importance of theta oscillations is underscored by the fact that they reflect subthreshold membrane potentials and strongly modulate the spiking of hippocampal neurons. Furthermore, theta oscillations gate synaptic plasticity, because the timing of stimulation with respect to the phase of theta is important in determining the magnitude and direction of synaptic change. Theta oscillations therefore offer macroscopic access to the internal clock of the hippocampal circuit, responsible for temporally patterning its operation. Such clocking is essential for the temporal coding of spatial information by place cells, as evidenced by theta phase precession. In addition to coding position, theta phase precession ensures that the order of place-cell firing over behavioural timescales (seconds) is preserved and compressed within individual theta cycles and inside the window of plasticity. In the presence of spike-timing-dependent plasticity, the resulting compression of temporal sequences offers a mechanism for the formation of hippocampal memory traces. Furthermore, theta oscillations modulate activity not only in the hippocampus, but also in several subcortical, limbic and cortical structures.

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