Activity within rostral and intermediate ventral medullary surface areas, m
easured as 660 nm scattered light changes, was examined in six cats, (five
experimental, one control site) following 5% CO2 challenges during waking,q
uiet sleep, and rapid eye movement (REM) sleep states. Activity declined to
hypercapnia in all states, with a smaller decline in quiet sleep compared
to waking, and intermediate values in REM sleep, The decline occurred more
rapidly, with a shorter latency, during wakefulness, but with a much slower
return to baseline than during quiet sleep. During REM sleep, the latency
to nadir and recovery were greater than in other states. Regional patterns
of activation emerged which differed in extent of activation between states
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