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Boris Sidis, Ph.D., M.D.

Boston: R. Badger, 1914




THE brain receives impressions from the external world. These impressions stimulate the activity of groups and systems of neurons and give rise to complex physiological processes having sensations as their concomitants. These processes stimulate activity of still more complex systems, groups of systems giving rise to physiological processes, having percepts as their accompaniment, and finally with the awakening of the activity of the highest constellations of neurons with their concomitant moments the highest form of consciousness, conceptual thought and judgment, is formed. There may be a disturbance in any of the several levels of psycho-physiological activity. Where the disturbance is chiefly of a perceptual character it gives rise to illusions and hallucinations.

        An illusion is a wrong perception of stimuli coming from an external source, a false perception of an external object. If an object, such for instance as a chair, is regarded as a man, the percept is illusory, and the perception is an illusion.

        Illusions may often appear in healthy individuals in waking states and more specially in dream states, in fact, art, such as painting, scenic effect produced in the dramatic world, etc., is based upon illusion. Illusions are common in psychopathic diseases, they may be induced in hypnosis and in other forms of subconscious states.

      Illusions occur most frequently in the insane. An hallucination differs from an illusion in the fact that the projection of the precept is more subjective in its nature. There may be nothing in the external world and yet the subject or patient perceives a person or hears a voice, for instance, when no sound is present. An illusion seems to be more peripheral in its nature while an hallucination seems to be more central in its character. There can be as many illusions or hallucinations as there are special senses. Hence they may be classified as visual, olfactory, gustatory, auditory, tactile.

        The physiological processes which operate in an illusion are as follows: In general the perception of an object consists in a synthesis of a present sensation with emerged subconscious, secondary sensations. Take for instance an apple, the perception of the apple may be by simple vision; we may see the outside general form, or perhaps only the general outline of the form and color, the one due to muscular eye movements and the other due to processes within the optic nerve.

        These sensations are peripheral incoming impressions, they would not in themselves constitute the perception of the apple. Along with them is aroused the spherical form of the apple, which could not be perceived by the visual sense alone, but depends also on muscular eye movements, and depends also on previous tactile and muscular sensations in the handling of the apple. The solidity and hardness of the apple, which depend on sensations given by the resistance and the weight of the apple, are due to muscular and tactile sensations. Then the peculiar taste and smell of the apple depend on previous gustatory and olfactory sensations. The average temperature of the apple is also an aid. All these psychic processes are awakened as soon as the eye perceives the apple.

        The eye itself cannot smell, or touch or weigh the apple; it cannot feel the solidity of the apple, and yet all these are present in the vision of the object. It is evident therefore that in seeing an apple the visual sensation awakens, by association, many subconscious psychic processes which form a synthesis in the percept, apple. This can be accomplished not only by vision, but also by the olfactory or tactile sensations, or even by auditory sensation, such as the peculiar sound which the paring of the apple produces.

        If A, B, C, D, E, F, etc., represent the associated physiological processes, and if A is peripherally awakened, then the others become synthetized and form the percept. The same occurs if B first appears, the rest of the processes may become synthetized, etc. In short, all these processes form a system, and if one is awakened all the rest may appear and become synthetized.

        Suppose now another synthesis, that is, one resulting in the percept of another object, is formed in which one, two or more of the elements of another system, elements which are common to both, are present. The two synthetized systems may become interchangeable by the principle of association by contiguity or resemblance. Thus it is possible that under certain circumstances, such as change of intensity of one or of some of the sensory components, or of lack of critical discrimination of the differences and details of the presented content instead of the one system exactly corresponding to the object being awakened, another one, which does not exactly correspond to the object is brought out in-stead. This will constitute an illusion. In art we find common examples when for instance artificial objects are made so as to represent as nearly as possible the real object.

        Living in a more or less constant environment, certain sensations naturally give rise to the most familiar systems, but when the environment is changed we often fall into illusions. Our environment, however, does not go on changing so rapidly as to give rise to illusions, and, therefore, our familiar percepts are correct, corresponding to external objects. These familiar percepts are adaptations between external objects and internal relations.

        Instead, however, of there being a disturbance in the external relations, in the environment, there may be derangement of the internal psychic processes. A stimulus therefore may give rise to sensations and instead of calling out the most familiar synthesis corresponding to this aggregation of external stimuli, calls out some other system, having something in common with the most familiar system. This gives rise to illusions. As a rule in ordinary illusions of common life, there is some similarity between the object and the illusory system awakened, but in insanity and under the influence of some toxic agents, the disturbance may be so great that but slight, inessential similarity is present.

        In illusions the peripheral sense organ may be in no way at fault, as a rule it is not, but rather the subconscious psychic systems which are excited and go to form the percept are at fault. In short, the fault is not in the sensation, but in the association.

        An aggregation of stimuli giving rise to peripheral sensory impressions and awakening sensations excite also subconscious psychic processes which in themselves are not of the nature of direct sensation, but rather of the nature of indirect sensation belonging to the most familiar systems of synthetized psychic processes forming familiar percepts.

        There are, however, cases, though rare and unusual in common life, in which an impression, giving rise to a special sensation, directly revives another impression also of a sensory character, giving rise to what we may call a secondary sensation. There is no external stimulus present to awaken this sensation, but it is awakened by the excitement of another special sense organ.

        It seems that in such persons there must be an association formed between two or more of the lower sensory centers. Thus for example there are persons who seeing a certain color, for instance, red, will at once hear a certain sound. This is not in the nature of a memory merely, the patient seems really to hear the sound.

        A sensation of smell may give rise to a sensation of light, etc. Sometimes the sensations may be related. For instance, a certain form, say that of a certain letter, may awaken the sensation of a certain color. In this way all kinds of combinations may be formed.

        There may be various combinations of primary with secondary sensations. We may have the following photisms and phonisms or synaesthesia:

Sound photism

Light phonism

Taste photism

Taste phonism

Pressure photism

Pressure phonism

Movement photism

Movement phonism

Smell photism

Smell phonism

Touch photism

Touch phonism

Temperature photism

Temperature phonism

        In fact any combination of primary with secondary sensation may be present.

        These secondary sensations are due to the formation probably of certain special, subcortical associations between the respective sensory centers.

        The following laws have been formulated:

            1. Photisms bright in color are produced by sounds of high quality, by intense pain, sharply defined sensations of touch. Dark photisms form opposite conditions.

            2. High phonisms are produced by bright light, well defined outlines, small objects, pointed forms. Low phonisms form opposite conditions.

            3. Photisms are associated with well defined forms, small, pointed; photisms are often produced by sounds of high pitch.

            4. Red, yellow, and brown are frequent photism colors; violet and green are rare, while blue stands between these extremes.

        Secondary sensations in their pure form, although found in normal persons, usually appear in those of neurotic tendency and in psychopathic states.


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