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 world is more complete and the objectivity of the dream images is usually unquestioned, whereas in the waking state the hallucination is usually recognized as such; we may, however, be conscious that we are dreaming, and thus in a measure be aware of the hallucinatory character of our percepts. The physiological nature of (q.v.; see also ) and of dreaming is obscure. As a rule the control over the voluntary muscles in dreams is slight; the sleep-walker is the exception and not the rule, and the motor activity represented in the dream is seldom realized in practice, largely, no doubt, because we are ignorant, under these circumstances, of the spatial relations of our bodies. Among the psychological problems raised by dreams are the condition of attention, which is variously regarded as altogether absent or as fixed, the extent of mental control, and the relation of ideas and motor impulses. There is present in all dreams a certain amount of dissociation of consciousness, or of obstructed association, which may manifest itself in the preliminary stage of drowsiness by such phenomena as the apparent transformation or inversion of the words of a book. We may distinguish two types of dreams, (a) representative or centrally initiated, (b) presentative or due to the stimulation of the end organs of sense. In both cases, the dream having once been initiated, we are concerned with a process of reasoning, i.e. the combination of ideas suggested by resemblances or other associative elements. The false reasoning of dreams is due in the first place to the absence, to a large extent, of the memory elements on which our ordinary reasoning depends, and, secondly, to the absence of sensory elements.

Objectivity of Dreams.—In waking life we distinguish ideas or mental images from real objects by the fact that we are able under normal circumstances to dismiss the former at will. In sleep, on the other hand, we have, in the first place, no real objects with which to compare the images, which therefore take on a character of reality comparable to the hallucination of waking life; moreover, powers of visualization and other faculties are enhanced in sleep, so that the strength of dream images considerably exceeds those of the mental images of the ordinary man; changes in powers of attention, volition and memory help to increase the hallucinatory force of the dream. In the second place, the ideas of our dreams are presented in the form of images, which we are unable to dismiss; we therefore mistake them for realities, exactly as the sufferer from delirium tremens in waking life is apt to regard his phantoms as real.

Relations of Dreaming and Sleep.—It has been maintained by Hamilton and others (see below, Modern Views) that dreams invariably accompany sleep, and that we always find ourselves dreaming when we are awakened. But even if it were true that dreams were invariably experienced at the moment of waking, this would not by any means establish the invariable concomitance of dreams and sleep of all sorts; at most it would show that imperfect sleep is a condition of dreaming; in the same way, dreams before wakening, known to have taken place either from the recollection of the dreamer or from the observation of another person, may clearly be due to imperfect wakening, followed by a deepening of sleep. It is, however, by no means true that awakening from sleep is invariably accompanied by a dream; in considering the question it must be recollected that it is complicated by the common experience of very rapid forgetfulness of even a vivid and complicated dream, only the fact of having dreamt remaining in the memory; it is clear that amnesia may go so far that even the fact of dreaming may be forgotten. On the whole, however, there appear to be no good grounds for the assertion that we always dream when we are asleep. On the other hand, there is no proof that partial awakening is a necessary condition of dreaming.

Representative Dreams.—Centrally initiated dreams may be due to a kind of automatic excitation of the cerebral regions, especially in the case of those clearly arising from the occupations or sensations of the day or the hours immediately preceding the dream. To the same cause we may attribute the recalling of images apparently long since forgotten. Some of these revivals of memory may be due to the fact that links of association which are insufficient to restore an idea to consciousness in the waking state may suffice to do so in sleep. Just as a good visualizer in his waking moments may call up an object never clearly seen and yet distinguish the parts, so in sleep, as L. F. A. Maury (1817–1892) and others have shown, an image may be more distinct in a dream than it was when originally presented (see also below, Memory).

Presentative Dreams.—The dreams due to real sensations, more or less metamorphosed, may arise (a) from the states of the internal organs, (b) from muscular states, (c) from subjective sensations due to the circulation, &c., or (d) from the ordinary cause of the action of external stimuli on the organs of sense.

(a) The state of the stomach, heart, &c., has long been recognized as important in the causation of dreams (see below, Classical Views). The common sensation of flying seems to be due in many cases to the disturbance of these organs setting up sensations resembling those felt in rapidly ascending or descending, as in a swing or a lift. Indigestion is a frequent cause of nightmare—the term given to oppressive and horrible dreams—and bodily discomfort is sometimes translated into the moral region, giving rise to the dream that a murder has been committed. (b) Dreams of flying, &c., have also been attributed to the condition of the muscles during sleep; W. Wundt remarks that the movements of the body, such as breathing, extensions of the limbs and so on, must give rise to dream fancies; the awkward position of the limbs may also excite images. (c) Especially important, probably, for the dreams of the early part of the night are the retinal conditions to which are due the illusions hypnagogiques of the preliminary drowsy stage; but probably Ladd goes too far in maintaining that entoptic stimuli, either intra- or extra-organic in origin, condition all dreams. Illusions hypnagogiques, termed popularly “faces in the dark,” of which Maury has given a full account, are the not uncommon sensations experienced, usually visual and seen with both open and closed eyes, in the interval between retiring to rest and actually falling asleep; they are comparable to the crystal-gazing visions of waking moments; though mainly visual they may also affect other senses. Besides the eye the ear may supply material for dreams, when the circulation of the blood suggests rushing waters or similar ideas. (d) It is a matter of common observation that the temperature of the surface of the body determines in many cases the character of the dreams, the real circumstances, as might be expected from the general character of the dream state, being exaggerated. In the same way the pressure of bed-clothes, obstruction of the supply of air, &c., may serve as the starting-point of dreams. The common dream of being unclothed may perhaps be due to this cause, the sensations associated with clothing being absent or so far modified as to be unrecognizable. In the same way the absence of foot-gear may account for some dreams of flying. It is possible to test the influence of external stimuli by direct experiment; Maury made a number of trials with the aid of an assistant.

Rapidity of Dreams.—It has often been asserted that we dream with extreme rapidity; but this statement is by no means borne out by experiment. In a trial recorded by J. Clavière the beginning of the dream was accurately fixed by the sounding of an alarm clock, which rang, then was silent for 22 seconds, and then began to ring continuously; the dream scene was in a theatre, and he found by actual trial that the time required in ordinary life for the performance of the scenes during the interval of silence was about the same as in ordinary life. Spontaneous dreams seem to show a different state of things; it must be remembered that (1) dreams are commonly a succession of images, the number of which cannot be legitimately compared with the number of extra-organic stimuli which would correspond to them in ordinary life; the real comparison is with mental images; and (2) the rapidity of association varies enormously in ordinary waking life. No proof, therefore, that some dreams are slow can show that this mentation in others is not extremely rapid. The most commonly quoted case is one of Maury’s; a bed-pole fell on his neck, and (so it is stated)