The Interpretation of Dreams by Sigmund Freud
2. Internal (subjective) sensory stimuli

All objections to the contrary notwithstanding, we must admit that the role of the objective sensory stimuli as producers of dreams has been indisputably established, and if, having regard to their nature and their frequency, these stimuli seem perhaps insufficient to explain all dream- pictures, this indicates that we should look for other dream-sources which act in a similar fashion. I do not know where the idea first arose that together with the external sensory stimuli the internal (subjective) stimuli should also be considered, but as a matter of fact this has been done more or less explicitly in all the more recent descriptions of the aetiology of dreams. "I believe," says Wundt (p. 363), "that an important part is played in dream-illusions by those subjective sensations of sight and hearing which are familiar to us in the waking state as a luminous chaos in the dark field of the vision, and a ringing, buzzing, etc., of the ears, and in especial, subjective irritations of the retina. This explains the remarkable tendency of dreams to delude the eyes with numbers of similar or identical objects. Thus we see outspread before our eyes innumerable birds, butterflies, fishes, coloured beads, flowers, etc. Here the luminous dust in the dark field of vision has assumed fantastic forms, and the many luminous points of which it consists are embodied in our dreams in as many single images, which, owing to the mobility of the luminous chaos, are seen as moving objects. This is perhaps the reason of the dream's decided preference for the most varied animal forms, for owing to the multiplicity of such forms they can readily adapt themselves to the subjective luminous images."

The subjective sensory stimuli as a source of dreams have the obvious advantage that, unlike objective stimuli, they are independent of external accidents. They are, so to speak, at the disposal of the interpretation whenever they are required. But they are inferior to the objective sensory stimuli by the fact that their claim to the role of dream-inciters- which observation and experiment have established in the case of objective stimuli- can in their case be verified with difficulty or not at all. The main proof of the dream-inciting power of subjective sensory stimuli is afforded by the so-called hypnogogic hallucinations, which have been described by Johann Muller as "phantastic visual manifestations." They are those very vivid and changeable pictures which with many people occur constantly during the period of falling asleep, and which may linger for a while even after the eyes have been opened. Maury, who was very subject to these pictures, made a thorough study of them, and maintained that they were related to or rather identical with dream-images. This had already been asserted by Johann Muller. Maury maintains that a certain psychic passivity is necessary for their origin; that it requires a relaxation of the intensity of attention (p. 59). But one may perceive a hypnogogic hallucination in any frame of mind if one falls into such a lethargy for a moment, after which one may perhaps wake up, until this oft-repeated process terminates in sleep. According to Maury, if one wakes up shortly after such an experience, it is often possible to trace in the dream the images which one has perceived before falling asleep as hypnogogic hallucinations (p. 134). Thus Maury on one occasion saw a series of images of grotesque figures with distorted features and curiously dressed hair, which obtruded themselves upon him with incredible importunity during the period of falling asleep, and which, upon waking, he recalled having seen in his dream. On another occasion, while suffering from hunger, because he was subjecting himself to a rather strict diet, he saw in one of his hypnogogic states a plate, and a hand armed with a fork taking some food from the plate. In his dream he found himself at a table abundantly supplied with food, and heard the clatter of the diner's forks. On yet another occasion, after falling asleep with strained and painful eyes, he had a hypnogogic hallucination of microscopically small characters, which he was able to decipher, one by one, only with a great effort; and on waking from sleep an hour later he recalled a dream in which there was an open book with very small letters, which he was obliged to read through with laborious effort.

Not only pictures, but auditory hallucinations of words, names, etc., may also occur hypnogogically, and then repeat themselves in the dream, like an overture announcing the principal motif of the opera which is to follow.

A more recent observer of hypnogogic hallucinations, G. Trumbull Ladd, follows the same lines as Johann Muller and Maury. By dint of practice he succeeded in acquiring the faculty of suddenly arousing himself, without opening his eyes, two to five minutes after gradually falling asleep. This enabled him to compare the disappearing retinal sensations with the dream- images remaining in his memory. He assures us that an intimate relation between the two can always be recognized, inasmuch as the luminous dots and lines of light spontaneously perceived by the retina produce, so to speak, the outline or scheme of the psychically perceived dream-images. For example, a dream in which he saw before him clearly printed lines, which he read and studied, corresponded with a number of luminous spots arranged in parallel lines; or, to express it in his own words: The clearly printed page resolved itself into an object which appeared to his waking perception like part of an actual printed page seen through a small hole in a sheet of paper, but at a distance too great to permit of its being read. Without in any way underestimating the central element of the phenomenon, Ladd believes that hardly any visual dream occurs in our minds that is not based on material furnished by this internal condition of retinal irritability. This is particularly true of dreams which occur shortly after falling asleep in a dark room, while dreams occurring in the morning, near the period of waking, receive their stimulus from the objective light penetrating the eye in a brightly-lit room. The shifting and infinitely variable character of the spontaneous luminous excitations of the retina exactly corresponds with the fitful succession of images presented to us in our dreams. If we attach any importance to Ladd's observations, we cannot underrate the productiveness of this subjective source of stimuli; for visual images, as we know, are the principal constituents of our dreams. The share contributed by the other senses, excepting, perhaps, the sense of hearing, is relatively insignificant and inconstant.