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Lucretius, that remarkably inquisitive and shrewd observer of nature, surmised that the fidgetings of animals during sleep were linked to dreaming. Some 30 years ago a German investigator, R. Klaue, made a significant discovery with the electroencephalograph. He found that sleep progressed in a characteristic sequence: a period of light sleep, during which the brain cortex produced slow brain waves, followed by a period of deep sleep, in which the cortical activity speeded up. Klaue's report was completely overlooked at the time. In the 1950's, however, Nathaniel Kleitman and his students at the University of Chicago took up this line of investigation. Kleitman and Eugene Aserinsky found (in studies of infants) that periods of "active" sleep, alternating with quiescent periods, were marked by rapid eye movements under the closed lids. Later Kleitman and William C. Dement, in studies of adults, correlated the eye movements with certain brain-wave patterns and definitely linked these activities and patterns to periods of dreaming [see "Patterns of Dreaming," by Nathaniel Kleitman; SCIENTIFIC AMERICAN, November, 1960]. In 1958 Dement showed that cats may have periods of sleep similarly marked by rapid eyes movement and fast cortical activity. He called such periods "activated sleep."

Meanwhile at the University of Lyons Francois Michel and I had been conducting a series of experiments with cats. In the cat, which spends about two thirds of its time sleeping, the process of falling asleep follows a characteristic course, signaled by easily observable external signs. Typically the animal curls up in a ball with its neck bent. The flexing of the nape of its neck is a clear sign that the muscles there retain some tonus, that is, they are not completely relaxed. In this position the cat lapses into a light sleep from which it is easily awakened.

After about 10 to 20 minutes there I comes a constellation of changes that mark passage over the brink into deep sleep. The cat's neck and back relax their curvature, showing that the muscles have completely lost tonus : they are now altogether slack. At the same time there are bursts of rapid eye movements (eight to 30 movements in each burst) in either the side-to-side or the up-and-down direction, like the movements in visual use of the eyes. Occasionally these eyeball movements behind the closed eyelids are accompanied by a sudden dilation of the pupils, which in the main are tightly constricted during sleep. Along with the eye movements go events involving many other parts of the body : small tremors of muscles at the ends of the extremities, causing rapid flexing of the digits and now and then small scratching motions; very rapid movements of the ears, the whiskers, the tail and the tongue, and an episode of fast and irregular breathing.

It is somewhat startling to realize that all this activity goes on during a period in which the animal's muscular system is totally atonic (lacking in tension). The activities are also the accompaniment of deep sleep, as is indicated by the fact that it takes an unusually high level of sound or electrical stimulation to arouse the cat during this phase. The state of deep sleep lasts about six or seven minutes and alternates with periods of lighter sleep that last for an average of about 25 minutes.

To obtain more objective and specific information about events in the brain during sleep we implanted electrodes in the muscles of the neck and in the midbrain of cats. We used animals that were deprived of the brain cortex, since we wished to study the subcortical activities. In the course of extended recordings of the electrical events we were surprised to find that the electrical activity of the neck muscles disappeared completely for regular periods (six minutes long), and the condition persisted when sharp spikes of high voltage showed up now and then in the pontine reticular formation, situated just behind the "arousal center" of the midbrain. These electrical signs were correlated with eye movements of the sleeping animal. Further, we noted that in cats with intact brains both the abolition of muscle tonus and the sharp high-voltage spikes were strikingly correlated with the rapid eye movement and fast cortical activity Dement had described. These findings pre sented a paradox. It was surely strange to find fast cortical activity (generally a sign of wakefulness) coupled with complete muscular atony (invariably a sign of deep sleep) !

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