Sleep Hippocampal Theta Rhythm and Sensory Processing oleh Marisa Pedemonte and Ricardo A. Velluti

Introduction
Ancient human cultures have developed diverse forms of a device that, based on sensory
stimulation, is used to put babies to sleep: the rocking cradle. Vestibular and
somatosensory stimulation produced by the rocking movements, complemented by eye
closure and other stimulation such as constant temperature and the mother's voice/song
(lullaby) activating the auditory system, are able to induce sleep. On the other hand, it is
a common experience that reducing the sensory afferent volleys to the brain can
facilitate sleep.
A series of experimental data will be presented demonstrating the sensory input
relevance in the organization of the sleep and wakefulness cycle. Firing rate shifts in
auditory and visual neurones, changes in the pattern of discharge, and, most important,
the temporal correlation of the spike timing with the hippocampal theta rhythm, will be
set forth.
Sleep, a huge change in the brain physiology, depends on both, a series of active
processes and passive mechanisms, e.g., functional sensory deafferentation3,19 and
neural networks changing organization. Although many signs of active processes have
been shown, there are not enough experimental data to support a final decision about
the relative contribution of passive processes. However, both approaches may
be partially reconciled conceding that the deafferentation may be provoked by an
inhibitory influence acting, e.g., upon the ascending activating reticular system.
Our main purpose is to provide an experimental aspect of sensory data analysis, its
relation to sleep and the hippocampal theta rhythm as an internal zeitgeber (time giver)
for auditory and visual information processing.

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.
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.
Although more prominent in active wakefulness and paradoxical sleep, the hippocampal
theta can also be observed during slow wave sleep. It has been related with phasic
phenomena during paradoxical sleep, with movements and with autonomic control
of the heart rate.......further click http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=eurekah&part=A28911