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collaterals have been given off."1 Waller's quotations from both sides of the controversy show that "the differences of view have involved matters of fact far less than inferences from facts as embodied in words."'2 In fact, he calls attention to the inferences necessarily involved in the use of such terms for cerebral processes. "We are in presence of a central process associated with centripetal and centrifugal processes, and we have no right to say that the centre is motor, or sensory, or both, otherwise than inferentially."'3
Still another view of the relation of sensory and motor activities is presented by Külpe in his "Outlines of Psychology." His explanation of the rapid restoration of sensory functions in the cortex depends upon reference to physiological processes, while Mott's explanation depends upon the anatomical fact of the distribution of the fibres.
"All these phenomena (i. e., of recovery of sensation and loss of movement) are evidence that the specific function of a nervous excitation is as a rule dependent upon its normal place of origin: the peripheral organ in the case of sensory nerves, the central in the case of motor. It would seem, therefore, that we may compare the unknown processes of the sensory centres with the known phenomena of the motor periphery, and attempt in this way to gain a more definite idea of their nature. We must then correlate a particular sensation. with the excitation of a larger or smaller cortical area, according to the range of the peripheral stimulation, ... and lastly, we may designate the state of the nervous substance, in which the various cortical areas are capable of reproduction,-functional disposition. Just as the piano player uses hands and fingers for the most varied combinations of movements, so the same parts of the cortex may be concerned in very different forms of excitation."4
This brief review of the physiological basis for the distinction of sensory and motor centres indicates that the distinction is not by any means so definite as sensory and motor theories of attention have made it. Both on physiological and psychological grounds, it is more correct to speak of attention as sensori-motor than as sensory or motor. Even if, however, separate areas are assumed for sensations and movements, so that the two are anatomically distinguishable, it is universally admitted that they are as mutually dependent in physiological function as sensation and movement themselves are in psychological analysis. Attention must be sensori-motor: sensory in its contents, motor in its mani
1Op. cit., pp. 480, 469. A diagram accompanies the last passage cited, and makes clear the meaning of the somewhat obscure statement.
Op. cit., p. 346.
4""Outlines of Psychology," tr. p. 223.
festations, sensori-motor in its whole process.
In the acceptance of this opinion the sensori-motor controversy is ended so far as we are concerned. But we have still reached no explanation of attention, no true definition of it. We have a description of its phenomena-subjectively in sensations, objectively in movements,—and we refer both to one general, continuous, physiological process. But this process accompanies all consciousness, and sensation and movement are present in every phase of consciousness. We must look further for the peculiar physiological processes, and for the psychological conditions which are the basis of attention itself.
The Explanatory Theories of Attention.
The theories of the preceding chapter have been referred to as "descriptive"; those of the present chapter we call "explanatory." We have alluded to many facts and observations of attention which we may classify together as the outward expressions of the attention. We have also passed in review the reasons for characterizing attention as a sensory, a motor, or a sensori-motor process. But we have not yet been told what attention is. The theories have been either too narrow, defining it in terms of its secondary phenomena, or else they have been too broad, characterizing it in terms that apply to consciousness in general. The present chapter, however, is devoted to theories that do ascribe a specific function to attention. We have the essential, constitutive elements distinguished from the secondary, attendant phenomena, and the relations between the two are carefully analyzed. We have, thus, a definition of what attention is, and an explanation of its contents in the sense of a reference of all contents to fundamental principles of psychology and physiology.
There have been three types of explanatory theories. Attention is regarded (1) as essentially a facilitation of ideas; (2) as an inhibition of ideas; and (3) as both facilitation and inhibition.
I. Theories of attention as a facilitation of ideas. G. E. Müller is often referred to as the chief representative of the first type (e. g., by Külpe). From his statement of the thesis of his monograph "Zur Theorie der sinnlichen Aufmerksamkeit," we should expect to find him in the third class, with those holding a mixed theory of attention. His thesis maintains that "the capacity for acting on the mind which is possessed by certain physical processes in the central organ,
be modified by the activity of the sensible attention; they may be increased or diminished, or wholly destroyed." If we look only at Müller's discussion of the physiological changes involved in attention, we must admit that he investigates both sorts of modifications mentioned in his thesis, the inhibitory as well as the facilitatory. Külpe's criticism that Müller fails to discuss the phenomenon of inhibition ignores such passages as that extending from page 105 to page 110, as well as minor references scattered through the work.2 On the other hand Müller certainly lays far more stress on the reinforcing than on the inhibiting function of attention. In fact, when he is confining himself to the psychological analysis of attention, he does almost entirely ignore all changes except those which he describes as reinforcements of a sensation or idea. Hence, we shall concern ourselves here only with those parts of the theory which maintain that attention is a facilitatory process.
We may neglect Müller's presupposition of "a mind that absorbs a part of the energy with which a nerve acts upon it for the production of an equivalent of mental activity." His statement of his observations and most of his conclusions can easily be translated into the terms of a psychophysical theory.
In the summary of his discussion of voluntary attention, Müller mentions three ways in which attention reinforces its object. "We have found that the activity of the voluntary sensible attention consists, in many instances, in the effort to reproduce earlier sensations; in other cases, it may be evident merely in impulses imparted to motor nerves, and resulting in the adaptation of a sense organ;" this adaptation, in turn, may react on the central organs, reinforcing the idea through associations. We have, then, an adaptation of the sense organ to the stimulus, a facilitation of the excitation corresponding to the stimulus in the central nervous system, and correlated with these the recollection of similar ideas previously experienced.
The brief statement just given includes what is of greatest theoretical importance in Müller's discussion. The most valuable part of the work-the application of these principles to a very large number of concrete cases, we cannot dwell upon here. Müller's examples include references to Helmholtz, Wundt, Volkmann, G. H. Meyer, Lotze, and many others hardly less noteworthy. But his conclusions
Zur Theorie der sinnlichen Aufmerksamkeit," Leipzig (1873), p. 1. 2 E. g., pp. 14, 67, 71.
Op. cit., p. 70.
Op. cit., p. 103. See also p. 48.
are not systematically worked out, and his presupposition of a "mind" acting upon a "sensorium" has prevented his recognition of certain problems in attention which are the natural outcome of his theory, e. g., the question of our knowledge of the difference between increased intensity due to the stimulus and increased intensity due to the attention. Such problems will recur when we reach the discussion of the third type of the explanatory theories. Meanwhile we must examine the second type of theory, that which is directly opposed to Müller's, and which, both logically and chronologically, is the next in order for our investigation. The two chief representatives of the theory that attention is essentially inhibitory are Wundt and Külpe.
II. Theories of attention as an inhibition of ideas. Wundt's theory. Wundt analyzes the whole process of attention into the following subordinate processes:
"(1) Increase of the clearness of a definite idea or group of ideas, accompanied by the feeling which is characteristic of the whole process from the beginning; (2) Inhibition of other available impressions or memorial images; (3) Sensations of muscular strain, with the sense feelings which belong to them and which intensify the primary feeling; (4) Intensification of the sensory contents of the apperceived idea by these strain sensations through the medium of associative co-excitation. Only the first and second of these four part-processes are essential elements of every act of apperception. The third may be of very slight intensity, or even entirely wanting; the fourth is demonstrable only in cases where the third, of which it is a secondary consequence, attains a certain duration and intensity."
In our attempt to summarize Wundt's theory of attention, we shall take up the constitutive factors (part-processes) in the order just given. It is not the order in which they are discussed in the section on Aufmerksamkeit und Appercep tion, but it is evidently the order of their importance according to Wundt's own estimate.
The first process is "an increase of clearness of an idea with an accompanying feeling of activity." Clearness is distinguished from intensity.
"Since the intensity of the sensations which make up an idea exercises an undoubted influence on its clearness, these two concepts are often confused. But in strictness we can attribute intensity only to the sensation-elements, not to the idea itself.
The essential difference between the clearness of an idea and the intensity of its sensation-components is shown above all in the fact that an increase and decrease in clearness can occur without a simultaneous increase and decrease of sensation-intensity. If a continuous stimulus is allowed to act on a sense organ, even under conditions which preclude fatigue of the organ, it is more or less impossible to apperceive it continuously with the same clear1hys. Psychologie, IVte Aufl., II, 274.
ness and distinctness. A constant change in clearness is noticed, and this change appears to the subject as a process which is different from any objective oscillation in the intensity of stimulus. . If clearness and intensity are so entirely different, then the concept of the stimulus-limen acquires a double significance. As limen of intensity it means a limen of consciousness, since the entrance into consciousness (perception) of an idea depends upon the intensity of its sensory contents. The limen of clearness is something different; it is a limen of attention or apperception. Only impressions which lie above the limen of intensity can transcend the limen of apperception. But, if this is to happen, the subjective function of attention must also be discharged. We have proved, then, that an impression may oftentimes become clearer without growing stronger, and vice versa, and that the two processes are subjectively distinct. Still, this does not prevent their exerting a certain influence on each other. Ân intensive impression, provided that there are no special dispositions present to oppose it, is ordinarily apperceived more strongly than a weak impression. But undoubtedly certain influences may be exerted in the contrary direction as well; as when one makes an effort to recollect or to imagine, and tries to keep his ideas in consciousness at the highest possible degree of intensity. Many persons, it would seem, are successful in increasing the clearness of such images, but are absolutely unable to increase their intensity to any marked degree. Increase of clearness always precedes increase of intensity, and the latter always takes place more slowly, and appears as the accompaniment of strong strain sensations,the character of the muscular excitation naturally being in exact correspondence to the form of the apperceived idea. It is extremely probable that the intensification of sensations is a secondary effect, which may be-but is not necessarily-produced by certain concomitant phenomena of attention."1
Wundt's statements of the distinction between clearness and intensity have been quoted at length not only because the distinction is of such importance to his own theory, but also because the relation of the two phenomena is one of the most vital questions in any complete theory of attention. We shall return to this point later. We have now to consider the first part-process from its second point of view, viz., the feeling of activity which Wundt has associated with the increasing clearness of an idea.
It must be noticed that "feeling of activity" is frequently used as a general term, including the negative feeling of passivity as well as the positive feeling of activity; for Wundt has two classes of attention, the passive and the active, and if we use the Thätigkeitsgefühl as the characteristic feeling accompanying increased clearness of an idea, it must be remembered that the term also covers the feeling present in passive apperception. To simplify matters we shall consider the term in this section only in its restricted and more ordinary meaning.
'Op. cit., II, pp. 271-274.