The P&S Journal: Spring 1998, Vol.18, No.2
REM: It's About Eyes, Not Dreams
The function of rapid eye movement (REM) sleep may have more to do with vision than with dreams, says Dr. David Maurice, professor of ocular physiology. His research, reported in the Feb. 13, 1998, issue of Experimental Eye Research, suggests that the aqueous humor, the clear watery liquid in the anterior chamber just behind the cornea, needs to circulate to bring oxygen to the cornea from blood vessels in the iris. When the eyelids are closed during sleep the circulation slows dramatically, and the motion of rapid eye movement simply serves to "stir" the anterior chamber and prevent corneal suffocation.
In the 1950s, research found that sleepers could often recall a dream if they were awakened when their eyes appeared to be darting around beneath their eyelids. From these findings evolved the popular theory that during this REM sleep the brain is processing information gathered while awake, "rather like a store closing for business during inventory," says Dr. Maurice. "Because of its implications in the functions of the central nervous system, REM has been considered to be the domain of disciplines ranging from psychology to neuropharmacology, and it has not received attention from ocular physiologists."
Dr. Maurice proposed his theory for REM sleep when he learned of a young man whose eyes had been immobilized by an accident and whose corneas had become laced with blood vessels, presumably to supply the corneas with oxygen. Dr. Maurice knew that when the eyes are closed, oxygen can reach the cornea from the iris only by diffusion across the stagnant aqueous in the anterior chamber. Using mathematical calculations, he established that under these circumstances, the oxygen supplied will be insufficient and corneal suffocation, possibly leading to cell death, will result.
To demonstrate that aqueous circulation virtually stops when the eyelids are closed, Dr. Maurice stained the aqueous with a fluorescent dye and charted its currents. In a closed eye, freshly secreted unstained aqueous from behind the iris accumulated in front of the pupil as a dark pool. But when the eye opened, the pool quickly rose within the anterior chamber and began to circulate.
It has long been established that in an eye opened to cooler room air, convection currents based on heat differentials cause the aqueous to circulate. In other experiments, moving the eyes voluntarily immediately stirred up the fluorescence in the chamber so that it became uniform.
Dr. Maurice's "stagnant aqueous humor hypothesis" could explain why periods of REM sleep initially last about 20 minutes but become progressively longer as the night wears on. And the fact that the womb is at uniform temperature could explain why REM is so active in the fetus. If taking sensory inventory is the purpose of REM sleep, why does an unborn child spend many hours a day in REM? Likewise, why do animals born with sealed eyelids need REM? "It is quite possible," Dr. Maurice says, "that REM sleep evolved with the primary purpose of protecting the cornea."
And what of our dreams? Dr. Maurice does not deny that REM sleep is associated with such phenomena as dreaming, a rise in brain temperature, penile erections, and EEG changes. But he cannot see any physiological significance in these phenomena and suggests that they may result from the partial arousal necessary for REM to occur. "In any case," he writes, "my interests are in the plumbing, and I am happy to leave dreams to others."