Higher Nervous Activity

the activity of the higher centers of the central nervous system of animals and man “which ensures the normal and complex relations between the entire organism and the external environment” (I. P. Pavlov, Poln. sobr. trudov, vol. 3, 1949, p. 482), as opposed to the activity of the central nervous system in integrating different parts of the organism. The term “higher nervous activity” was introduced into science by I. P. Pavlov, who considered it synonymous with “psychic activity.” Thus, according to Pavlov, all forms of psychic activity, including human thought and consciousness, are components of higher nervous activity. A direct precursor of Pavlov in the study of higher nervous activity was I. M. Sechenov, who, in his work Reflexes of the Brain (1863), developed materialistic ideas on the reflex nature of psychic activity.

According to Pavlov, the basis of higher nervous activity in higher animals is conditioned reflexes formed in the higher centers of the central nervous system (in higher vertebrates and in man, predominantly in the cerebral cortex) in addition to complex unconditioned reflexes (instincts, emotions, and the like), that is, those forms of cerebral activity that arise mainly in the subcortical nuclei. Moreover, the cortex and the subcortical nuclei of the cerebrum, each with their specific functions, are in continual communication and interaction and work together as an integral mechanism. The basic physiological life-saving drives (hunger, sex, defense, and other motivations), which can be localized mainly in the subcortical centers, also serve as the basis for the formation of corresponding conditioned reflexes and thus are reflected in higher nervous activity as the product of the integral relationship between the cortex and the subcortical centers of the brain.

Perfect adaptation of an organism to its environment depends on the formation and extinction of various conditioned reflexes. Variability and dependence of the conditioned reflexes on the body and its external environment and their temporary nature have a paramount biological significance for the flexibility and accuracy of the organism’s adaptation to changes in the environment. Conditioned reflexes permit the organism, on the basis of certain often indirect signals functioning as conditioned stimuli, to prepare in time for favorable events and avoid unfavorable ones. They also help the organism to broaden its perception of objects and events in the surrounding world and its scope of activity. Unconditioned reflexes in higher nervous activity are not only the basis of all conditioned reflexes but are also, particularly the complex ones, the concentrated expression of the reinforced experience inherited from the preceding generations, as the manifestation of genetic memory.

The relative importance of conditioned and unconditioned reflexes in higher nervous activity changes in the process of the evolutionary development of the animal world. In the behavior of invertebrates and lower vertebrates innate forms of nervous activity predominate over acquired forms; in the process of phylogenetic evolution acquired forms of nervous activity gradually achieve a preponderance and emerge as the dominant ones. Moreover, these forms themselves undergo substantial changes: the conditioned reflexes become more complex and more perfected; their number is constantly increased; conditioned-reflex activity as a whole becomes an increasingly perfected and active means of adaptation to the environment, that is, it ensures the possibility of the organism’s existence in an ever wider range of living conditions.

According to Pavlov, the higher nervous activity of animals, even those at a high level of development (for example, dogs and monkeys), is, on the whole, a result of the complexity of diverse and heterogeneous conditioned reflexes of the first signaling system which is common both to man and to animals. In spite of the gradual development of speech, the conditioned reflexes of the first signaling system still continue to constitute the basic fund in the higher nervous activity of children in the first years of life and occupy a specific place in the higher nervous activity of humans at subsequent stages of growth. To this type of conditioned-reflex activity, Pavlov attributes sensations, ideas, and impressions derived by man from the external environment, including the social environment, which are formed without the use of verbal (i.e., word) signals. But in the case of man, along with the development of social forms of labor activity, “signals of a second order, symbols of those primary signals—in the form of words pronounced, heard, and seen—appeared, developed, and became highly perfected” (ibid., p. 576). This qualitatively new, higher, and more perfect second signaling system of reality, also based on conditioned-reflex mechanisms, is characteristic only of the higher nervous activity of man, exists in close interaction with the first signaling system, and plays a leading role in his conscious life, providing the basis for generalization and thought. While continually emphasizing the fundamental, qualitative difference between these two kinds of higher nervous activity, Pavlov at the same time also pointed to the organic connection between them, to the fact that the basic laws established in the working of the first signaling system must also govern the second system.

Pavlov distinguished the following six basic laws, or rules, of higher nervous activity. (1) The formation of conditioned reflexes, or neural arcs, depends on a combination of stimuli which provoke simultaneous spread of excitation from different cortical centers and also on the extreme reactivity and memory of the brain cells. (2) The magnitude of the conditioned reflex depends on the strength of the stimulus. (3) There may be a summation of conditioned stimuli. (4) Extinction occurs in the cortex as a result of a lack of reinforcement of a conditioned reflex, an extreme intensity of a conditioned stimulus, or the action of an unrelated stimulus. (5) Diffusion and concentration of neural processes in the cerebral cortex allows interaction between its separate parts and also generalization and specialization of conditioned reflexes. (6) The inverse functions of cerebrocortical centers of excitation and inhibition may coexist due to reciprocal inductibility of neural processes. The conditioned-reflex activity of the brain—a qualitatively specific form of reflex activity—obeys the principles of determinism and structuration (each function is determined by a specific structure) and is in accordance with the above rules. In the final analysis, higher nervous activity ensures the most delicate analysis and synthesis of diverse stimuli acting on the organism and the organism’s perfect adaptation to the environment.

Of vital importance for higher nervous activity is the dynamic nature of the specialization and localization of functions in the cortex of the cerebral hemispheres, which plays an important role in the reliability of its activity. Of vital importance are also the intensity and mobility of the neural processes, which, according to Pavlov, determine the physiological bases of the type of nervous system and of the nature of nervous activity. Pavlov’s work on higher nervous activity shed light on the phenomena of sleep and hypnosis.

Pavlov’s work on higher nervous activity has been continued by his students and followers in the USSR and abroad. The principal phenomenon of higher nervous activity— conditioned reflexes—is the object of current research in world neurophysiology and experimental psychology. Higher nervous activity is studied not only with the help of the diverse modifications of classical Pavlovian methods but also with the most recent macro- and microelectro-physiological and cytochemical methods. This opens up the possibility of studying both the cortical and the deep brain structures and of discovering the laws of the functions of individual nerve cells and the molecular basis of their activity.

Numerous studies, by both Russian and foreign scientists, have confirmed the correctness of the basic facts and laws of higher nervous activity established by Pavlov and his co-workers. At the same time, the mechanism of processes that take place in the brain and that are the basis of higher nervous activity is being discovered. Thus, by means of electrophysiological and neurochemical studies the important role of the reticular formation of the brain stem and the diencephalon in maintaining the “tension” and excitability of the cerebral cortex has been shown. This work confirms Pavlov’s hypothesis about the role of the subcortical centers that not only carry out unconditioned-reflex activity but also are the source of energy for the cerebral cortex, maintaining the cerebral cortex in an aroused and activated state. However, the views of a number of scientists concerning the leading role of this formation in conditioned-reflex activity and in cortical-subcortical interaction are questionable. Study of the formation of the conditioned reflex by means of electrophysiological methods has shown that upon repeated coupling of an unrelated signal with an unconditioned stimulus, the excitability of the corresponding cortical centers, especially that of the unconditioned stimulus, significantly increases, and this plays an important role in the formation of the conditioned reflex. Furthermore, it has been established that in the process of formation of a conditioned reflex the neural elements of those centers undergo yet other functional changes: they show more lability and continuous rhythmicity; they become more responsive to any changes in other parts of the brain. These studies confirm Pavlov’s view of the phenomenon of “facilitation” inherent in the entire nervous system, that is, of the “summation reflexes” which are based on the increased excitability or actual excitation of the corresponding neural structures.

The traditional ways of studying higher nervous activity have also led to progress: new types of conditioned reflexes (such as interoceptive, alternate, and binary reflexes) have been discovered; new phases in the formation of the conditioned reflex (the phase of initial localization) and of cortical inhibition (the phase of preventive inhibition) have been revealed; the mechanisms involved in the origin and localization of “inner” inhibition have been specified; information concerning the phylogenetic and ontogenetic evolution of higher nervous activity and the role of the ecological adequacy of signals in this activity has been extended. Of special importance are studies of complex behavioral reactions of experimental animals, carried out in natural surroundings or under conditions of traditional laboratory experiments. In these experiments new forms of complex, integrated conditioned reflexes (such as chain, situational, crossed, extrapolated, and orderly reflexes) have been discovered and studied. The basic stages in the formation of conditioned reflexes during the first months of a child’s life and the fact of the powerful stimulation of the development of the child’s speech by means of training his motor activity have been established.

The study of higher nervous activity has great theoretical and practical significance. It broadens the biological basis of dialectical materialism, confirms the correctness of Lenin’s theory of reflection, and serves as a weapon in the ideological struggle against manifestations of idealism.

The study of higher nervous activity is one of the greatest achievements of contemporary science: it has opened up a new epoch in the development of physiology; it has great significance for medicine since experimental results have provided a point of departure for physiological analysis and pathogenetic treatment (for example, by means of sleep) of various diseases of the human central nervous system; in addition, it has great significance in the realms of psychology, education, cybernetics, bionics (mathematical biology), the scientific organization of labor, and in many other branches of man’s practical activity.