The interesting feature for us is the condition of the mind during sleep. That it is still capable of activity is shown by the fact that we dream. Since noises capable of exciting our attention when awake, as well as marked stimuli from somatic sources, will awaken us we know that the mind is still open to the reception of impressions. The chief question, however, is whether or not mental activity is constant in sleep or limited to certain periods, as in light sleep.

There are some who maintain that in deep sleep the brain is inactive, and that deep sleep is, therefore, dreamless. This is a contention difficult to prove or disprove conclusively. If we are unable to remember dreams, which are indicative of mental activity, this by no means proves that the mental faculties were totally at rest during sleep. As is well known, the memory of dreams is feeble, unless the dreams have been very vivid or have occurred near the period of awaking. Often a person will become conscious of having dreamt the night before by some thought during the day, which has some association with the dream, and which recalls it. Also, one frequently awakes, conscious of having dreamt, but is unable to recall the substance of the dream. In hypnotic sleep a person can talk and act and yet have no remembrance of such on awaking; the same is true of natural sleep-walkers, and those who talk or laugh or cry in their sleep. The writer has studied about thirty persons who talked habitually in their sleep: not only did these people not know that they talked in their sleep, except from hearsay, but the talking occurred at all periods of sleep, though most commonly after the first few hours of deep. The latter fact would help prove that dreaming occurs no matter what the depth of sleep. Further, by paying attention to the facial expressions of sleepers, it is possible to note various changes which we may consider indices of the moods of the dreams that are occurring. If those persons who claim that they never dream made it a point to try to recall dreams on awaking, they would often find that they would be, with practice, able to bring to mind either entire dreams or fragments of dreams.

Studying the question from another aspect, it has been shown that various changes may take place in the brain and the cerebral circulation at any period of sleep; these changes may be considered due, in part at least, to dreams, yet the individual often awakes unconscious of having dreamt.

For example, Mosso1 studied the appearance of the brains of several persons who had openings in the cranium. One of these was an idiot who had an opening as big as the palm of the hand. "In the period of exhaustion and stupor, the blood vessels of the brain seemed to relax, and at every contraction of the heart the pulsations became stronger. A faint noise which did not awake the patient was enough to produce a change in the brain and a more abundant gush of blood. It sufficed to touch him, or to approach him with the lamp; immediately, the volume of the brain increased, and a great elevation occurred in the curve of the pulse.

"Whenever we called him by name, it seemed as though an impetuous wave of blood rushed into the brain, causing the convolutions to swell. As this was invariably the case, there could be no doubt that the brain was still sensitive to the impressions of the external world, even during a heavy sleep."

In the case of Bertino, a mentally normal person, it was observed that the slightest noise during sleep disturbed the surface of the brain. "If the hospital clock struck the hour, or some one walked along the terrace, if I moved my chair, or if a patient coughed in the next room - everything, the slightest sound was accompanied by a marked alteration in the circulation in the brain."

1Fear, 1896, p. 72 et seq. English Translation; Longmans, Green ft Co.

Hammond and others who have noted changes in the cerebral circulation during deep attributed such changes as causes of dreams. It would be more accurate to consider them as effects of external or internal stimuli which may or may not be related to dreams. Asleep or awake we are constantly being affected by even the slightest alteration in the environment, often without our being consciously aware of such. If the alteration is marked it will obtain recognition in consciousness. However, by attaching instruments to a person's wrists and occupying his attention by conversation, we will find that a slight noise or other change in the environment which does not engage conscious attention is sufficient to cause an acceleration of the pulse; one may also note changes in the pupils. Yet the person is unaware of these alterations. The fact shows, however, that the circulation may be altered by external stimuli whether or not these occupy conscious attention.

Even ordinary thinking may cause changes in the physical activities, especially thinking associated with something of an emotional tone. Let a person think about an accident, for instance, and it will be found that it excites pulse, respiration, etc. Similarly, a dream may be concerned with scenes that arouse the emotions and thus cause alterations in the physical functions.

Changes in the cerebral circulation may thus be caused by dreams. They are, in sleep, more often the result of dreams than a cause of dreams. Quietness of the cerebral circulation is not to be taken as evidence that thought is abeyant. If we could examine the brain during waking life we would find situations identical with those that occur in sleep, namely, periods when the circulation was quiet and periods when it was disturbed. Yet in these quiet periods we would know that thought was going on. The quiet periods would coincide with thoughts with no emotional accompaniment; the disturbed periods with the reverse.

In an effort to prove that mental activity is going on at all periods of sleep various experiments have been made. The simplest of these was awakening the sleeper and asking him to mention what he had been dreaming about. In many cases the results were positive; in others negative. The most famous of these experiments were made by Maury, whose book, Le Somneil et let Reves,1 has been a standard on dream-life. His experiments were as follows: