Intermediate stage of sleep in the cat

Intermediate stage of sleep, previously described in the rat, was studied in the cat. The intermediate stage was a short-lasting episode, represented in the EEG activity by cortical spindles, which was accompanied by theta activity in the hippocampus. The cortical EEG spindles appeared simultaneously with hippocampal theta or on its background. Sometimes, geniculo-occipital waves were recorded during this stage. Such an episode of a few such episodes in sequence occurred frequently just before the paradoxical sleep and occasionally just after it. The amplitude of the positive fourth component of the somesthetic evoked potential induced in the S1 cortex by stimulation of the ventrobasal complex, was lower during the intermediate stage than during the other stages of the sleep-waking cycle. The EEG pattern of the intermediate stage was periodically observed under Nembutal anesthesia at the expense of paradoxical sleep and in acute cerveau isolé preparation. The results suggest that activatory ascending influences from the brainstem, which modulate waking processes, gradually decrease during slow sleep and are the lowest during the intermediate stage, when the influences for paradoxical sleep are still negligible.