The Doctrine of Primary and Secondary Sensory Elements. BORIS SIDIS. Psychological Review, January and March, 1908. Pp. 44-68, 106-121.

        In continuation of earlier work on hallucinations, the author here develops a theory of perception, "based on a close analysis of the normal process of perception and substantiated by observations and experiments of abnormal mental life." When an object is perceived, some of its qualities are directly present to sense. Other qualities, which do not affect any sense organ at the moment, are, nevertheless, present in some form in the percept; according to the common doctrine, they are present in the form of images. Dr. Sidis, on the contrary, holds that the manner in which they are present in perception differs entirely from the way in which they are present in a memory image, such as we get when, without having the object present to any sense at the moment, we recall it and its qualities to mind. In the percept they have a sensuousness which they lack in the image. "We see, we perceive, the hard, heavy, smooth, resistant body of ice—all the elements have alike the intensity of sensation. The hardness, the smoothness, the bodily resistance are perceived by the visual sense and are visual, but as such they, of course, differ from the sensations xperienced by their appropriate sense organs, as when, for instance, the same sensations are given by touch or by muscular and kinesthetic sensations. Those muscular and tacto-motor sensations appearing as visual are not memory images, but they are actual sensations, they are secondary sensations; they are secondary sensory elements, which give the fullness of content to the percept having visual sensory elements as its nucleus."

        The author's meaning, which in the first part of his paper appears somewhat strained and obscure, becomes clearer when he passes to his pathological evidence, which refers first of all to the facts of synesthesia—sound-photisms, light-phonisms, and the rest. In these cases, a stimulus to one sense gives rise to what must be designated as, not images, but true sensations, belonging to other senses. In normal perception, the same sort of secondary sensations occur, the only difference being that they are closely fused with the primary sensation, whereas in synesthesia they are partially dissociated. In hallucinations, the dissociation is more complete. The author brings forward a number of cases in which hallucinations of one sense are traced to abnormal irritation of another sense; these give him the clue to his theory of hallucinations, which regards them, not as centrally originated, but as excited by the stimulation of some sense organ, the primary sensations due to such stimulation being then so far dissociated from the secondary sensations as to have no conscious connection with them, or even as to drop out of consciousness altogether. Were the primary and secondary sensory, elements partially fused, the result would be synesthesia; were they completely fused, the result would be normal perception. What distinguishes hallucinations from normal percepts is dissociation. "Percepts and hallucinations are of the same grain. A percept is a hallucination with the primary nuclear sensory elements present, a hallucination is a 'real' percept with the primary sensory elements absent.

        The main point of the doctrine is thus seen to be that the genuine sensory process can be aroused either by stimulation of the appropriate sense organ, or, secondarily, by the stimulation of other sense organs, but never in any other way. It can not be centrally aroused. What can be centrally aroused is a memory image, and this differs absolutely from the sensory process. "The image or idea is that bloodless, shadowy, fluttering affair which can no more attain the life of a sensation than a written letter can attain the power of sound." "The image of a light does not shine, the idea of a voice does not sound, and the representation of a perfume does not smell. . . The image, the representation, is essentially mediate, it is a mental substitute for the immediate experience of the sensation. The idea bears the same relation to the sensation as the photograph bears to the original, or rather as a symbol to the thing it represents. . . . There is not a particle of evidence to substantiate the view that ideas or images are copies of sensation in the sense of being weak sensations or 'centrally excited sensations.' There is nothing of the sensory in the idea." From these quotations it may reasonably be inferred that the author's imagery, in the Galtonian sense, is weak. But it is also clear that he is somewhat of an innovator in his definition of the term image. Following Galton, the image is usually defined in a descriptive way, as something like a sensation; and if an individual testifies that he is unable to call up images having this sensory quality, he is said, not to have images. But the author defines "image" in dynamic or causal terms. That is an image which can be recalled by memory, which is centrally excited. The question then comes whether the image, so defined, possesses the quality of a sensation; and the author, on the basis of his own introspection, decides that it does not. The sensory tang can be aroused, he finds, by peripheral excitation, either of  the sense organ appropriate to the particular sense quality aroused, or of other sense organs, but it can not be centrally aroused.

        Allowance made for his use of the term image, the author's theory of perception seems to differ in no respect from the traditional doctrine, for when the latter speaks of a percept as a blend of sensation and images, it means by images just what the author means by secondary sensory elements. The novelty of the author's view lies in his conception of "images" or memory content; and in the sharp line which he draws between this and sensation. On this point, the reviewer's introspection is on the whole in agreement with that of the author, though he does not follow him into such artificial antitheses as the following: "Sensations have intensity, but no vividness; images or representations have vividness, but no intensity." But it must be remembered that many observers insist on the sensuous character of memory content; and no acceptable theory of memory content can be made which not take account of these diverse introspective accounts.

        In one respect, it is practically certain that the author's distinction between percept and representation is too sharp. He would make them absolutely distinct, and would relate them to different cerebral processes and probably to the activity of separate organs in the brain. The process of representation is not in any respect a renewal of the process of perception. We do not recall past experiences of an object, but get in memory, a new and fundamentally different kind of experience of it. But this makes it hard to see how perception of an object enables us later to recall it; if the organs concerned in the two processes are different, no "trace" or modification of the organ by the first impression would operate to make possible a representation. Memory can not be conceived as something entirely new, wholly distinct from perception; the two processes must at least overlap, and their organs be in part the same. Such a view as that recently propounded by Mr. Marshall, according to which memory recalls part of the perceptual experience, namely, the less sensory part—not all, but some of the cerebral organs active in perception being also active in memory—seems adequate to the introspective facts which Dr. Sidis reports, and at the same time logically consistent. And if we add to this the further supposition that the sensory part of the perceptual process may be recalled, with an intensity varying from zero in those whose sensory imagery is nil to a high degree in those who report their images to be almost equivalent to sensations, then it would seem that all the observations were sufficiently accounted for.

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