The effort to construct a satisfactory theory of inhibition has given rise, in recent years, to a good deal of discussion. Ever since it was discovered that the reflexes of the spinal cord are normally modified or restrained by the activity of the brain and Setschenow (1863) attempted to prove the existence of localized inhibition centers, the need of such a theory has been felt. The discussion, however, has been mainly physiological, and we cannot undertake to follow it here. The psychologist may not be indifferent, of course, to any comprehensive theory of nervous action. He works, indeed, under a general presumption which takes for granted a constant and definite relation between psychical and cerebral processes. But pending the settlement of the physiological question he may still continue with the study of facts to which general expression may be given under some theory of psychical inhibition not inconsistent with the findings of the physiologist.
A question of definition, however, confronts us here. Can we, it may be asked, speak of psychical inhibition at all? Does one conscious state exercise pressure on another, either to induce it, or to expel it from the field? 'Force' and 'pressure,' however pertinent to physical inquiries, are surely out of place in an investigation of the relations between the phenomena of mind. Plainly a distinction has to be made if we are to carry over the concept of inhibition from the domain of nervous activity to the conscious domain. Inhibition cannot, it should seem, have the same sense in both. We find, accordingly, that Baldwin, who defines nervous inhibition as 'interference with the normal result of a nervous excitement by an opposing force,' says of mental inhibition that it 'exists in so far as the occurrence of a mental process prevents the simultaneous occurrence of other mental processes which might otherwise take place.'[1]
Even here, it may be said, there is in the term 'prevents' an implication of the direct exercise of force. But if we abstract from any such implication, and conceive of such force as the term inhibition seems to connote, as restricted to the associated neural or physiological processes, no unwarranted assumptions need be imported by the term into the facts, and the definition may, perhaps, suffice.
Some careful work has been done in the general field of psychical inhibition. In fact, the question of inhibition could hardly be avoided in any inquiry concerning attention or volition. A. Binet[2] reports certain experiments in regard to the rivalry of conscious states. But the states considered were more properly those of attention and volition than of mere ideation. And the same author reports later[3] examples of antagonism between images and sensations, showing how the latter may be affected, and in some respects inhibited, by the former. But this is inhibition of sensations rather than of ideas. Again, Binet, in collaboration with Victor Henri,[4] reports certain inhibitory effects produced in the phenomena of speech. But here again the material studied was volitional. More recently, G. Heymans[5] has made elaborate investigation of a certain phase of 'psychische Hemmung,' and showed how the threshold of perception may be raised, for the various special senses, by the interaction of rival sensations, justly contending that this shifting of the threshold measures the degree in which the original sensation is inhibited by its rival. But the field of inquiry was in that case strictly sensational. We find also a discussion by Robert Saxinger,[6] 'Ueber den Einfluss der Gefühle auf die Vorstellungsbewegung.' But the treatment there, aside from the fact that it deals with the emotions, is theoretical rather than experimental.
In short, it appears that though much has been said and done upon the general subject of psychical inhibition, experimental inquiry into the inhibitory effect of one idea upon another—abstraction made, as far as possible, of all volitional influence—virtually introduces us to a new phase of the subject.
The term 'idea,' it should be noted, is here used in its broadest sense, and includes the memory image. In fact, the memory image and its behavior in relation to another memory image formed the material of the first part of the research, which alone is reported here. Apparatus and method were both very simple.
The ideas to be compared were suggested by geometrical figures cut out of pasteboard and hung, 25 cm. apart, upon a small black stand placed on a table in front of the observer, who sat at a distance of four feet from the stand. The diagrams and descriptions which follow will show the character of these figures.
Before the figures were placed in position, the subject was asked to close his eyes. The figures being placed, a few seconds' warning was given, and at the word 'look' the subject opened his eyes and looked at the objects, closing his eyes again at the word 'close.' The time of exposure was five seconds. This time was divided as equally as possible between the two figures, which were simultaneously exposed, the observer glancing freely from one to the other as in the common observation on which our ideas of objects are founded. At the end of the exposure the subject sat with closed eyes and reported the several appearances and disappearances of the ideas or mental images of the objects just presented. The conditions required of him were that he should await passively the entry of the rival claimants on his attention, favoring neither and inhibiting neither; that is to say, he was to remit all volitional activity, save so far as was necessary to restrict his attention to the general field upon which the ideated objects might appear, and to note what occurred on the field. The period of introspection, which followed immediately the disappearance of such retinal images as remained, after the closing of the eyes to the external objects, lasted sixty seconds. The reports, like the signals, were given in a just audible tone. They were in such terms as 'right—left,' 'small—large,' 'circle—star,' terms the simplest that could be found, or such as seemed, in any given case, most naturally or automatically associated with the object, and therefore least likely to disturb the course of the observation. And each report was noted down by the experimenter at the instant it was given, with the time of each phase, in seconds, as indicated by a stop-watch under the experimenter's eye.
It will be remarked that the attitude required of the observer was one which is not commonly taken. And it may be objected that the results of an attitude so unusual towards objects so ghostly and attenuated must be too delicate, or too complex, or influenced by too many alien suggestions, to be plumply set down in arabic numerals. The subjects, in fact, did at first find the attitude not easy to assume. A visual object may hold the attention by controlling the reflexes of the eye. But an ideational object has ordinarily no sure command of the conscious field save under the influence of a volitional idea or some strongly toned affectional state. But with a little practice the difficulty seemed to disappear. The subject became surer of his material, and the mental object gradually acquired the same sort of individuality as the visual object, though the impression it made might be less intense.
After a few preliminary experiments, figures were devised for the purpose of testing the effect of mere difference in the complexity of outline. That is to say, the members of every pair of objects were of the same uniform color-tone (Bradley's neutral gray No. 2), presented the same extent of surface (approximately 42 sq. cm.), were exposed simultaneously for the same length of time (5 seconds), and were in contour usually of like general character save that the bounding line in the one was more interrupted and complex than in the other.