On the Works of P. P. Lazarev in the Field of Biological Physics*)

(On the Fifth Anniversary of P. P. Lazarev’s Death)

B. V. Deryagin

The main part of P. P. Lazarev’s investigations belongs to the field of biophysics. P. P.’s scientific activity began with investigations in physiological acoustics, and in the last period of his life it was concentrated entirely on questions of biophysics. At the same time, as P. P. himself repeatedly noted during his lifetime, most of the other directions in the broad field of his research, such as molecular physics and photochemistry, were closely connected with the development of biophysical problems; in the course of this work there often arose a number of problems of a molecular-physical and physico-chemical nature that required their solution. Thus, biophysical works are the most fundamental in P. P.’s creative output.

The main part of P. P.’s biophysical works consists of investigations on the ionic theory of excitation, the photochemical theory of vision, and investigations of the laws governing changes in the sensitivity of nerve centers—research that embraces almost a forty-year creative period (1904–1942).

In a brief survey it is quite impossible to encompass the variety of results obtained by P. P. and his school in this field, results that were set forth in large part by the author himself in a number of monographs and review articles.

I shall therefore dwell only on the principal results of these investigations. Their first stage relates to the development of the ionic theory of excitation. The starting point here was the classical work of Nernst and Loeb on the stimulation of nerves and muscles by electric currents and salt solutions. Proceeding from the idea that, when an electric current passes through semipermeable cell membranes, changes arise in ion concentration that can stimulate nerve endings or muscles, Nernst outlined a quantitative theory of the excitation of living tissues by an electric current.

*) The content of the article essentially coincides with a report read by the author at a session of the Division of Physical and Mathematical Sciences of the Academy of Sciences of the USSR in April 1947.

P. P. not only refined and developed Nernst’s theory, but also showed that, by generalizing the empirically discovered laws of the irritating action of salt solutions found by Loeb, and using the equations of diffusion of ions and their transport by electric current, it is possible theoretically to derive both the laws of the action of electric current found by Nernst and a number of other fundamental laws (for example, Pflüger’s laws of the change in the excitability of nerves and muscles under the action of electric current), linking into a single whole a large area of phenomena. P. P. returned more than once, and much later, to these fundamental questions of the theory of excitation. Thus, later the question of the laws of excitation of tissues by alternating and direct currents of different frequency and duration was examined in considerable detail. In particular, one should note the experimental investigations of S. N. Rzhevkin, carried out at the Institutes of Physics and Biophysics and of X-rays, which revealed the applicability of P. P.’s theory to currents with frequencies up to \(3 \cdot 10^5\) hertz, for which Nernst’s formula must be replaced by P. P.’s formula.

Another question considered by P. P. from the standpoint of ionic theory was the mechanism of conduction of nervous excitation and of its transition from neuron to neuron, as well as the mechanism of muscular contraction and its excitation. P. P. developed a diffusion-chemical theory of the propagation of nervous excitation, which explained a number of phenomena and provided an approach to a series of very difficult problems; for example, on the basis of diffusion theory, P. P. was able to calculate interneuronal distances and to give a number of other applications.

Remarkable are the investigations carried out at P. P.’s suggestion by Dr. G. G. Jaure, which showed the dependence of the periodic or aperiodic character of the contraction of both cardiac and motor muscles on the ionic composition of the surrounding liquid medium. G. G. Jaure’s experiments were a brilliant proof of the correctness of those predictions which P. P. made on the basis of the fundamental laws of the irritating action of ions, in particular the law of antagonism of monovalent and divalent cations.

A further important step taken by P. P. consisted in recognizing and further substantiating the proposition that all phenomena of excitation, including those of the sense organs, can be derived theoretically, and moreover quantitatively, from the generalized Loeb law of the irritating action of ions. This fundamental proposition constitutes the content of the ionic theory of excitation, to which the first large stage of the fruitful work of P. P., his pupils, and collaborators was devoted.

In the first place, the conceptions of ions as agents directly irritating nerve endings were applied by P. P. to vision, and later also to the other sense organs.

Unlike, however, irritation by salts and by electric current, in the case of vision and the other sense organs, between the initial irritants—light, sound—and the final irritants—ions—

included a complex mechanism specific to the various sense organs. In the case of vision, this mechanism, in accordance with Johannes Müller’s hypothesis, was to consist in the photochemical transformation of the photosensitive pigment present in the eye.

Thus P. P. was naturally faced with the task of developing the photochemical point of view. The second cycle of P. P.’s investigations was devoted to solving this problem. In essence, the theory of vision developed by him may to a considerable extent be considered independent of the conceptions of the ionic theory of excitation in the narrow sense*). The link is provided by the assumption that the products of the photochemical transformation of the photosensitive substances in the eye are precisely ions, which also irritate the endings of the optic nerves. However, the assumption that the products of the photochemical transformation are ionized had very little effect on the construction and the main results of P. P.’s theory of vision, which is essentially an application of the laws of photochemical kinetics to vision. At the same time, the close and deep connection that existed between P. P.’s studies in photochemical kinetics and the photochemical theory of vision is clear: the former served as the basis and starting point for the construction and development of the latter. Of fundamental importance here was the proposition that the rate of photochemical transformation is proportional to the absorbed energy within the limits of a simple absorption band, i.e. that, under these conditions, the energy yield is constant. The fact that, in the light of Einstein’s law of equivalence, one should speak of the constancy of the quantum yield rather than the energy yield changes very little (and rather formally) the conclusions of P. P.’s theory of twilight and color vision, and does not change the substance of the principal conclusions at all.

P. P.’s photochemical theory of vision wrote brilliant pages in the history of Russian science, received the widest recognition, and produced an entire pleiad of followers and continuators. The significance of what P. P. did is evident from the fact that, on just one of the sections of the photochemical theory of vision—the kinetics of changes in the sensitivity of peripheral vision in light and in darkness—there arose an entire literature containing, in the main, the development and refinement of P. P.’s theory. No less great is the significance of P. P.’s photochemical theory of color vision, which supplemented the Young–Helmholtz three-color theory with conceptions of the photochemical decomposition of three photosensitive pigments with different absorption curves.

The theory of color vision thus completed has, in the literature, quite justly been called the Young–Helmholtz–Lazarev theory.

P. P.’s investigations were far from being limited to the development of theory; at the same time they included its experimental verification and the obtaining of results important in applied respects.

*) This circumstance was emphasized by P. P. himself as well.

First of all, questions connected with threshold stimulations of vision and of other sense organs were considered. A number of studies were devoted to establishing and verifying the laws governing the action of brief and periodic illuminations of the retina. The practical importance of these phenomena of threshold stimulation can be illustrated by those important applications in photometry that were made by S. I. Vavilov and his collaborators, who developed a flicker method for measuring small luminances. The sensitivity of the eye to flashes of different duration underlies the theory of beacons and also plays an important role in a number of physical methods of measurement.

Subsequently the investigations of P. P. and his pupils were extended to suprathreshold stimulations and to the sensitivity of the eye to differences in illumination (the contrast sensitivity of the eye). Previously these questions had been treated only on the basis of the Weber–Fechner law, according to which a barely perceptible increment of stimulation (for example, of the brightness of light) is proportional to the magnitude of the stimulation itself.

Considering, on the basis of the ionic theory of excitation, those physicochemical processes that underlie sensations, P. P. was able to take an extremely important and bold step, reducing the Weber–Fechner law to the basic regularity of the irritating action of ions on nerve endings. This regularity, according to P. P., reduces to proportionality between the barely perceptible increment in the concentration of irritating ions and the existing concentration. Subsequently P. P. went still further, taking as a basis the proportionality between the increment $\Delta N$ in the number of stimulated nerve fibers that gives a barely perceptible increment of sensation, and the number $N$ of already stimulated fibers. It should be noted that the proportionality between $\Delta N$ and $N$ is not the only possible assumption. Later, the hypothesis was advanced of the constancy of $\Delta N$ for a barely perceptible increment of sensation. However, this is not of such fundamental importance as the basic idea put forward by P. P., namely that Fechner’s law should be regarded as a consequence of processes in the perceiving apparatus of one or another sense organ. Thus the self-sufficient, one might say metaphysical, character of the Weber–Fechner law is eliminated, and the way is opened to a materialist substantiation both of it and of other phenomena of sensory perception.

This line of research led to further important conclusions in the theory of vision. It is precisely the application of the “all-or-nothing” law of nerve stimulation—that is, the existence of either a zero or a maximal response of a nerve—that leads to substantial difficulties in explaining the eye’s ability to perceive gradations of illumination in accordance with the Weber–Fechner law: the eye would not be capable of sensations of different subjective brightness. This difficulty disappears, according to Lazarev, if one takes into account the discreteness both of the perceiving apparatus of the retina (rods and cones, connected with individual nerve ...

...endings), as well as light (photons). In this, by simple statistical arguments based on the existence of photons, P. P. related the intensity of illumination of the eye to the number of excited nerve endings, thereby laying the foundation for a quantum-statistical theory of vision.

This result is of outstanding fundamental importance. It is interesting to note that later quantum-statistical concepts were laid by a number of authors as the basis for the theory of the photographic process.

Further applications of quantum concepts in the theory of vision did not, during P. P.’s lifetime, attain the considerable development that they undoubtedly deserve. Let us note, finally, that applications of quantum concepts to threshold irritations of the eye were given not by P. P. himself, but by S. I. Vavilov, who subjected to experimental study those fluctuations of the visual threshold which are based on the small number of quanta necessary for threshold irritation. By this method S. I. Vavilov measured the threshold numbers of quanta, which is of great interest for the further development of the theory of vision.

Fluctuations of the visual threshold, having a physiological nature, were also investigated by P. P., who obtained very substantial data indicating that they are conditioned by processes in the centers of the brain.

All these results lead to the necessity of a statistical approach to the concepts of the absolute and contrast sensitivity of the eye, which, in turn, brings forward the importance of a statistical formulation in the theoretical interpretation of Fechner’s law.

A number of P. P.’s studies of this same cycle are devoted to the theory of photometric measurements and to other questions of great applied interest. Here too, as in other cases, it was not characteristic of P. P. to confine himself to a circle of purely theoretical questions alone. On the contrary, responsiveness to the demands of life and practice brought about great diversity in the subjects of the works of P. P. and his collaborators. Thus, a number of works were devoted to the scientific substantiation of camouflage and to the development of protective coloring for units of the Red Army—an area in which P. P. was a pioneer. The Weber–Fechner law was applied by P. P. to such a purely technical question as a method for detecting defects in glass. Many such examples could be cited.

The experimental study of the kinetics of the eye’s adaptation to darkness and light, carried out by P. P. on extensive material, led not only to the confirmation and deepening of the theory, but also to the posing of new problems.

The study of adaptation made it possible to separate changes in sensitivity dependent on physicochemical processes at the periphery from changes in the sensitivity of the centers of the brain; the detailed and extensive study of changes in the sensitivity of the centers of the brain...

brain under the influence of various physical and physicochemical factors, constituted the last cycle of P. P.’s biophysical investigations, broken off by his death.

We cannot, for lack of space, even enumerate all the results achieved here. We shall dwell only on a few that, from our point of view, are the most important and striking.

First of all, P. P. proved the indefatigability of nerve centers under the action of light or other stimuli, which is undoubtedly a fact of cardinal significance. A comparative study of the sensitivity of centers in people of different ages further showed that the average sensitivity is in a regular functional dependence on age; moreover, with increasing age the age curve rises from very small values, reaches a maximum at approximately 20 years, and then slowly declines, asymptotically tending toward zero.

Thus, the sensitivity of the centers of the human brain changes regularly over the course of life, and for different people the individual age curves of sensitivity are close to one another. There is no doubt that the discovery of this regularity, which in fairness should be named after Lazarev, is one of those major discoveries of natural science that are of interest not only to specialists, but also to any educated person.

The study of the sensitivity of nerve centers is undoubtedly of great applied interest. Indeed, P. P. and his collaborators gave numerous examples of a connection between various diseases and changes in the sensitivity of centers. Thus, measurements of the sensitivity of nerve centers, the methodology of which was worked out in detail by P. P., can serve for purposes of disease diagnosis. It further turned out that the sensitivity of centers undergoes certain variations following the administration of medicinal substances; this provides a means for objective monitoring of the action of drugs.

Thus, here too scientific and applied questions are closely intertwined.

P. P.’s interest in the problem of the activity of nerve centers and, in general, of higher nervous activity was not limited only to the study of their sensitivity.

The phenomena of the mutual influence of the sense organs—for example, vision and hearing—the causes of the indefatigability of nerve centers, and the electrical phenomena in them were also subjected to theoretical consideration; all these phenomena were interpreted from the standpoint of the ionic theory of excitation and of various physicochemical processes accompanying excitation. Thus, P. P.’s work is of enormous interest from the point of view of a materialist interpretation of higher nervous activity, perhaps the chief citadel of all possible vi-

statistical theories. One of the remarkable conclusions is the following. The activity of the centers of the brain is accompanied by variable electromotive forces connected, according to P. P., with the periodic chemical reactions underlying this activity. The variable electromotive forces must send electromagnetic waves into space, which may perhaps be picked up by the centers of another subject. Thus, according to Lazarev, there exists a fundamental possibility of transmitting thoughts at a distance. To avoid misunderstandings it should be emphasized, as P. P. himself always emphasized, that this assumption has only the character of a physical hypothesis and is in no way experimentally proven; this hypothesis aroused considerable interest among specialists and at the same time serves as a vivid example of P. P.’s capacity for bold conclusions, combined with work of a meticulous and specialized character.

P. P.’s biophysical investigations were not limited only to those that belonged to the cycles of work listed above; thus, P. P. devoted a number of works to questions of the migration, reproduction, and survival of organisms, to questions of the growth of living tissues, and so on. These works, however, were relatively few in number.

In conclusion I would like to dwell on those general features of P. P.’s creativity which are closely connected with his image as a scientist and are well known to all his pupils and collaborators.

P. P. possessed that fundamental gift necessary to every major research scientist, which consists in the ability to pose major problems and to choose a broad, timely, and fruitfully developing field of investigation. At the same time P. P., always by his own personal example and in conversations, emphasized the special importance for our national science of developing its own directions and fields of investigation, without which our science cannot preserve due independence and occupy a fitting place in world science. P. P. unswervingly followed this precept, which is so consonant with us now, throughout all his activity, creating a genuinely new and independent direction in biophysics, one that far outstripped science abroad. This life’s work of P. P. best characterizes him as a true patriot of his Motherland, who so ardently took to heart its interests and its greatness.

P. P. possessed exceptional capacity for work and energy, without which the vigorous scientific, public, and popularizing activity that he conducted would have been impossible. The merits of P. P. in the matter of cultivating numerous cadres of researchers and scientists should especially be noted. It should also be noted that P. P.’s collaborators and pupils always obtained independence early, which always gave the best results.

P. P. devoted special attention not so much to the “training” of beginning workers as to imparting to them general advice and inspira-

in them the proper spirit and methodology of scientific research. No less important a role in the growth of young researchers in P. P.’s institute and laboratories was played by his personal example and by that scientific temperament which captivated and inspired others.

The freedom that all capable workers soon received under P. P., the breadth of his scientific views, and the diversity of his own fields of research led to the fact that P. P.’s pupils now work in the most varied areas, so that from a narrow or formal point of view it might be difficult to speak of a single scientific school. The fields of research of his pupils include biophysics, optics, photochemistry, molecular physics, geophysics, i.e., the very areas in which P. P. himself worked. At the same time, many of P. P.’s pupils are developing branches of these disciplines which, strictly speaking, are not a continuation of their teacher’s work. This is essentially inevitable, given the nature of the relationship between teacher and pupils which P. P. considered correct and in which he sacrificed his personal interests as a researcher to the task of forming independent scientists.

This last, greatest task, too, was brilliantly solved by him. There can be no doubt that P. P.’s pupils, despite all the diversity of their fields of work, are united by that spirit of inquiry and by those common views on science and principles of scientific work which P. P. tirelessly instilled.

In particular, P. P. always emphasized the necessity of being guided by ideas in scientific work, as the first condition of its fruitfulness, warning against works which, even if carried out at a high technical level, being devoid of ideas are in essence also barren. P. P., like no one else, knew how to inspire faith that, when the idea has been correctly found, there are no insoluble problems, just as there are no boundaries for scientific research.

These and other thoughts and precepts of P. P. are well remembered by his numerous pupils, who will always gratefully recognize how much they owe to their teacher.