The Hippocampal Theta Rhythm
The hippocampal theta rhythm is a well-known feature of the hippocampal electrogram in humans and other mammals although its functions remain partially unknown.15,49,29,39
Since the beginning, attention processes have been associated to the theta rhythm. Figure 1 shows a classical example on this matter. When a cat observed himself in a mirror exhibited a theta rhythm burst in the hippocampus.14
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Although more prominent in active wakefulness and paradoxical sleep, the hippocampal theta can also be observed during slow wave sleep.21,12 It has been related with phasic phenomena during paradoxical sleep,11,22 with movements5 and with autonomic control of the heart rate.32,34
Since the pioneering work of Scoville and Milner,40 almost five decades of research resulted in the recognition of the hippocampus as a brain region implicated in learning and memory processes42,4 in several species including humans.1,45 Besides, theta blocking by septal lesion provokes memory impairment.13
Moreover, the hippocampus is involved in the neural coding of spatial position28,51,2 necessarily associated with the sensory input and its processing. As an experimental animal traverses space, the hippocampal place neurones firing progressively changes to an earlier phase of the ongoing theta rhythm.23,44 This may be relevant to long-term potentiation which is sensitive to the theta phase, i.e., potentiation increases at the theta peak while depression is associated with the throughs.
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The theta wave may affect spatially distant neurons by inducing fluctuations in cellular excitability due to membrane potential oscillations.11,20 Moreover, intracranial recording from human cortices have revealed theta oscillations in several brain regions including the neocortex, suggesting that theta waves may not reflect volume conduction from the hippocampus but the existence of theta generators in the brain surface.17,18
Our approach was to study the hippocampal theta influences on the unitary activity of the sensory systems in the context of wakefulness and sleep. The interactions were present in both, sleep and waking behaviour.
By studying the unitary activity of several auditory nuclei, the lateral geniculate visual thalamic neurones and their relationship to the hippocampal theta, we have found that this rhythm may play a role as an internal clock. We postulate it constitutes a low frequency zeitgeber associating a temporal dimension to the processing of auditory signals in the lateral superior olive,47 inferior colliculus,31,47 primary auditory cortex,33,48 and to the visual processing in the thalamus.10
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