A. Differences in electrical activity
The cortical activity in the cat during RPS is very similar to the arousal reaction. In man, however, the EEG during REM is different from the EEG recorded during wakefulness (arousal reaction with opening of the eyes). Nevertheless, the alpha or subalpha rhythm observed in the occipItal. region during sleep is similar to the rhythm recorded in awake humans with the eyes closed.
B. Similarities of the behavioral aspects of sleep
The REM recorded in man are very similar to those observed in the cat. They are accompanied by small movements of the extremities in both species and by movements of the angles of the lips in man and of the whiskers in the cat. The vegetative mani festations are also similar.
The similarity between the disappearance of muscle tone both in cat and man is very remarkable in the cases of pathological hypertony (decerebrate rigidity in the cat, decorticate rigidity in man). In normal humans, the total atony occurring during dreaming is not easy to determine since it is preceded by the hypotony of the first stages of sleep.
However, the total atony during nightmares was recognized by Kouretas and Scouras (1932) and recently R. Berger (1961) has observed a striking decrease of the muscle tone of extrinsic laryngeal muscles in normal humans at the onset of each phase of sleep with REM whereas EMG activity persisted during the other stages of sleep. This finding thus confirms that, in normal humans, sleep with REM is ac companied by a total decrease of muscle tone as in the cat.
That the REM periods of sleep are more profound than other stages in normal humans is supported by the rise of the arousal threshold to auditory stimulation. Whatever may be the neural mechanisms responsible for this increase of the depth of sleep, both RPS in the cat and sleep with REM in man stand apart as qualitatively different phenomena which appear periodically during sleep.
C. Structural similarities
Certainly, pathological brain lesions observed in man are not yet as numerous as the experimental lesions carried out in cats and only a very superficial analogy may be obtained.
Nevertheless, the patterns of sleep observed in both decorticate cats and humans are very similar, and the almost total disappearance of the phase of sleep with REM and decrease of muscle tone in humans with pontile lesions is analogous with the results obtained after coagulation of the pons in the cat.
D. Other functional aspects
In cats, the coagulation of the nucleus reticularis pontis caudalis suppresses RPS and produces the periodical appearance of some "hallucinatory behavior" after 3 or 4 days.
In man, sleep deprivation also produces periodic visual hallucinations and tactual misperceptions after the same period of time (Katz and Laudis 1935; Bliss et al. 1959; Brauchi and West 1959). The experiment of "dream deprivation" during two nights made by Dement (1960) may not have been long enough to induce such hallucinations.
It is certainly impossible to compare the subjective experiences of cats and humans. Nevertheless, it must be remembered that many psychoses are preceded by some insomnia. It is possible that dysfunction of the phase of sleep with REM could be responsible for the appearance of some psychotic states.
Thus, this short review of the differences and of the similarities between the RPS in the cat and the sleep period with REM in man permit the conclusion that it is very likely that dreaming occurs during a stage of sleep analogous to the RPS (it is, of course, impossible to state that the cat dreams during the RPS).
Dream activity would thus appear to be the subjective result of the cerebral mechanisms occurring during RPS. It is a periodic phenomenon elosely related with the decrease of muscular tone and the alteration of vegetative functions (which are not however related to it in a reciprocal fashion since they appear in decorticate subjects). Dreaming would thus depend upon the periodic coming into play, during sleep, of a neuronal pool situated in the reticular formation of the pons, related rostrally with the limbic system by ascending pathways different from those of the reticular activating system.
A more detailed analysis of the mechanisms and of the functions of dreaming is difficult and more experiments are needed in order to shed more light upon the mysterious activity occurring in the brain during dreams. Many problems are still not solved:
Why is the RPS, contemporaneous with an increase of the unit activity at the cortical and reticular levels (Evarts 1961; Huttenlocher 1961), always associated with a total decrease of the muscular activity? What is the function of this cerebral dream activity associated with muscular sleep which periodically interrupts the slow wave stages of sleep? On the other hand, this last stage does not seem to be associated with any subjective activity but is associated with some muscular activity.
The mechanisms of REM observed in the human during RPS, and which seem to be closely related to the visual scenery of dreams (Dement 1955) are also difficult to explain. Indeed, such eye movements are observed in a baby, in a blind child, in adult patients apparently unconscious for many years and in decorticate cats. The memory processes which are related with these scanning movements must be necessarily located in limbic or brain stem areas.
Dreaming, which periodically occurs during a quarter of our night's sleep, has for centuries aroused curiosity, religious or metaphysical speculations, and sometimes fear. For the neurophysiologist the key to understanding dreams has not yet been found and the functions of dreaming are still mysterious. However, as irrational as the contents of our dreams may be, their visual contents are integrated into the unity of our organism, since they are dependent, as are other vegetative and homeostatic mechanisms, on neurons located in the pons.