[Sensory processing could be temporally organized by ultradian brain rhythms]

Neuronal activity of sensory systems depends on input from the environment, the body and the brain itself. Various rhythms have been shown to affect sensory processing, such as the waking-sleep cycle and hippocampal theta waves, our aim in this revision. The hippocampus, known as a structure involved in learning and memory processing, has the theta rhythm (4-10 Hz), present in all behavioural states. This rhythm has been temporally related to automatic, reflex and voluntary movements, both during wakefulness and sleep, and in the autonomic control of the heart rate. On the other hand theta rhythm has been considered as a novelty detector expressing different level of attention, selecting the information and protecting from interference.

Our research is based on the hypothesis that sensory processing needs a timer to be processed and stored, and hippocampal theta rhythm could contribute to the temporal organization of these events. We have demonstrated that auditory and visual unitary discharges in guinea pigs show phase-locking to the hippocampal theta rhythm. This temporal correlation appears during both spontaneous and specific sensory stimulation evoked discharges. Neuronal discharges fluctuate between phase-locked and uncorrelated firing modes relative to the theta rhythm. This changing state depends on known and unknown situations. We have provoked, changing the visual stimuli, a power theta rhythm increment and the phase-locking between this rhythm and the lateral geniculate neurone discharge during wakefulness. In slow wave sleep results were different demonstrating that the ways of the inputs processing have changed.