Dominant

DOMINANT (in psychology), means the predominance of one moment of behavior over another. In the latest German holistic psychology (Gestaltpsychologie), the concept of D. explains the process of the emergence of a whole image from its constituent parts. For example, a subject is shown an image of some object, transmitted only partially, with gaps, in sequential approximation to reality. The subject will guess what is depicted in this drawing, i.e., will try to form a whole from the parts. This tendency toward the whole will be greater the more objective expressiveness the proposed drawing possesses. This latter depends in turn on the structure, the combination of the parts making up the drawing, individual lines, strokes, etc. At a certain approximation to the actual image, the tendency to form a whole image is realized and always along the line of the main, dominant group of features in the structure of the drawing, giving it one unity or another. The dynamic tendency toward holistic formation associated with this group of features should be understood and designated as D. of the whole. From what has been stated, it is clear that the concept of D. in psychology should not be confused with the same concept in physiology.

DOMINANT (in physiology), a center of excitation in the central nervous system, capable of intensifying at the expense of external irritations, while exerting an inhibitory effect on the course of these latter (e.g., when a cat is prepared for defecation, irritation of motor points of the cortex of the hemispheres causes not the usual contraction of the limb muscles, but acceleration and intensification of defecation—Ukhtomsky, 1911). The principle of D., introduced into physiology by A. A. Ukhtomsky, indicates how the final result of irritation affecting the central nervous system is determined by the state of the centers at a given moment. Later works of Ukhtomsky and his school (from 1922) showed that even within one spinal cord, the presence of a dominant center of excitation—D.—changes the course of central reactions. Thus, if in a spinal (i.e., decapitated in a certain way) frog a D. is created in the sensory elements of the rubbing reflex (by poisoning the corresponding segments with strychnine or irritation of sensory nerves), then irritation that usually causes a flexion reflex will only intensify the rubbing reflex. With a sexual D. in a male frog, any irritation will only intensify the embracing reflex in the forelimbs, while the usual reflex to these irritations will be inhibited. From this it is seen that the final result of irritation affecting the central nervous system depends not only on its intensity (see below) and on which reflex arc this irritation falls upon, but also on intercentral relations, on the state of the centers at a given moment. D. can be considered one of the general principles of operation of nerve centers. Phenomena of D. have been found by Ukhtomsky's school in the ganglia of mollusks, Vetokhin—in the nerve ring of jellyfish, Popov—in the central nervous system of birds. Individual facts now explainable by the principle of D. can already be found in the old physiological literature of Freusberg and Exner. A particularly clear example was given by Vvedensky in 1880: in an excited state of the n. vagus center, irritation of afferent nerves acts as irritation of the n. vagus, in an excited state of the n. laryngei sup.—as irritation of this nerve. In later times, a number of similar examples can be found in Magnus, Baglioni, Graham Brown, Beritov, and others. But in all these cases, similar facts were considered as exceptions, deviations from the norm, 'perversions of reflexes' (Uexküll). A similar D. process was assumed by I. P. Pavlov in 1903 to explain the formation of a temporary connection. The question of the interaction of D. and conditioned reflexes is very complex; here one can only indicate that I. P. Pavlov spoke only about processes occurring in the brain. Sherrington considered that the result of irritation (or combination of irritations) of spinal afferent nerves depends only on the peculiarities of these reflex arcs as the substrate of specific reactions. Ukhtomsky, for the first time uniting and extremely expanding all previous observations, obviously proved that only morphological peculiarities and the character of action of nerve arcs participating in the reaction, without taking into account intercentral influences, in no case can explain all innervational processes. By emphasizing the state of the centers, highlighting all the fluidity and mobility of reactions in the nervous system, Ukhtomsky gave in principle a completely new path for studying the physiology of the central nervous system. It is necessary to especially emphasize that the principle of D. by no means is exhausted by the statement of the fact of the connection of excitation in the dominant center with external irritations. The principle of D. stands in a successive connection with the theory of parabiosis put forward by Vvedensky. For the school of the late N. E. Vvedensky, now headed by Ukhtomsky, inhibition (parabiosis) is only a quantitative modification of excitation; it represents excitation as well, however, having lost due to excessive intensification its oscillatory character. The same irritant can act either exciting or inhibiting depending on the degree of lability of the reacting apparatus, on its state. If the state of the tissue is given as definite, then excessively strong (or frequent) irritations will act inhibitingly, weaker (and rarer)—excitingly. From this point of view it is understandable that the dominant center in no case can be considered as a center of strong excitation. On the contrary, in this case, external irritations reaching the center, if excitation in D. is already close to culmination, can transfer it to a state of inhibition. Not the strength of excitation in the center, but the ability to summate and accumulate excitation makes it D. Works of Ukhtomsky's school have fully confirmed these views. D. is formed as a rule under the action of not excessively strong or frequent irritations (of course, here the duration of action of the irritant and the initial lability of the centers also have significance). Strong (or frequent) irritations cause not phenomena of D., but, on the contrary, phenomena of inhibition in the centers. The most favorable for the formation of D. are weak, tonic innervations. D. caused by them, as well as D. created by hormonal influences, are distinguished by special stability. Once created, a dominant can be maintained by a significantly weaker irritant and does not disappear immediately after the cessation of its action (inertia of D.). If the strength of the irritant forming D. was above the optimum, then after the cessation of the action of the irritant or with its weakening, D. may turn out to be intensified. The phenomena of inertia of D. are especially important in that here the significance of the factor of time, of the 'history of the system' for the work of the central nervous system is emphasized. On the basis of the above, D. can be characterized as follows: the dominant center is capable of summing up excitations. For this summation to take place, it is necessary that the dominant center possess increased excitability, increased sensitivity to diffusely spreading impulses in the central nervous system caused by external irritations. However, with excessively increased excitability, D. disappears, giving way to phenomena of hysteroidosis. With extremely increased excitability, even weak waves of excitation will act as strong ones and can therefore not reinforce excitation in D., but inhibit it. Further, for excitation of any center to influence other innervational processes, it must be stable, not fleeting. This stable excitation can be created, and all the more maintained, only by weak irritants. The last basic feature of D. is its inertia and ability to resume. Thus the principle of D. indicates not only one of the important factors of the direction of reactions of nerve centers, but also the conditions under which D. is formed, as well as the conditions under which phenomena of inhibition are observed in the centers. Putting in the forefront the accounting of the initial state of the central nervous system, emphasizing that this state in turn depends on the degree of self-excitation of the centers, on the current needs of the organism, and on the history of the system, the principle of D. allows considering as one whole all processes occurring in the central nervous system, and reveals great fruitfulness in the practice of scientific research.