Comparative thresholds of cortex, vestibular system and reticular formation in wakefulness, sleep and rapid eye movement periods

Abstract

1. 1. Thresholds for evoking movement by stimulating cortex and vestibular nerves, and for behavioral and electrographic arousal by mesencephalic RF stimulation have been studied in 5 cats bearing implanted electrodes. These thresholds were recorded as the animals passed spontaneously from wakefulness into drowsiness, sleep, and low voltage fast ECoG and rapid eye movement periods (REMPs). 2. 2. Threshold for evoking cortico-spinal movement is lowest in the waking animal, increases in drowsiness, and becomes still higher in sleep. During REMPs threshold is lower than in sleep and is nearly as low as in relaxed wakefulness. 3. 3. In a single episode of low voltage ECoG, threshold for cortico-spinal movement is lower during bursts of REM than in the periods of ocular quiescence. 4. 4. Vestibular and RF stimulation thresholds increase from wakefulness to sleep similarly to cortical thresholds. By contrast, however, vestibular and RF thresholds are higher in REMPs than in sleep, and vestibular threshold is higher during REMs than in the lulls between bursts of ocular activity. 5. 5. We conclude that the neocortex is activated during REMPs, compared with sleep. 6. 6. Our findings, and other data suggest that sleep and REMPs represent qualitatively different processes rather than quantitatively different aspects (lightness or depth) of a single state of sleep.