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Jacques Loeb, Comparative Physiology of the Brain, and Comparative Psychology

Robert H. Wozniak
Bryn Mawr College

First published in English in 1900, Comparative Physiology of the Brain and Comparative Psychology[1] by Jacques Loeb (1859-1924) is an iconoclastic tour de force. In reducing psychical processes to associative memory, instinctive behavior to tropisms and chained reflexes, and central nervous system function to simple protoplasmic bridging between irritable sensory surfaces and contractile muscle tissue, Loeb challenged views long held sacred within biology and psychology. It is hardly surprising, therefore, that few of Loeb's most general conclusions were accepted by his contemporaries. [2] Yet his work -- objectivist in spirit, radical in its peripheralism, and dedicated to achieving the experimental control of behavior -- exerted a powerful influence on the direction taken by an emerging behaviorism. [3]

As the title implies, Comparative Physiology of the Brain and Comparative Psychology is a peculiar melange of neurophysiological and behavior studies. Begun during a trip to California in 1898, Physiology of the Brain was compiled largely from previous publications. It is a diffuse, rambling work, badly in need of a good editor. It is also a work that reflects the author's apparent need to achieve several related but only moderately compatible goals.

The first of Loeb's goals was to summarize his wide-ranging experimental work. Scattered over numerous publications in a variety of different journals, much of this work focused on the nature of animal and plant movement and on the relationship between animal movement and properties of the nervous system. By the turn of the century, Loeb had begun to drift away from such research toward experimental embryology, morphology, and physical chemistry. Physiology of the Brain gave him an opportunity to achieve closure on this chapter of his research career by articulating the thematic continuity implicit in his studies of nervous system/behavior relationships.

Second, Physiology of the Brain allowed Loeb, now academically well-established, to return to a theme -- the critique of theories of precise localization of function in the brain -- that he had first elaborated with disastrous results in his earliest work. Disaster arose when the young Loeb, who had previously criticized the work of localizationist Hermann Munk (1839-1912), [4] endeavored to present his anti-localization views before an 1886 meeting of German naturalists. [5] During discussion that followed his presentation, Loeb was publicly humiliated by Eduard Hitzig (1838-1907), a pioneer in the study of functional localization, and the effects of this humiliation remained with Loeb for many years. [6] The reader familiar with this story of Loeb's disaster cannot help but suspect that the extended critique of localization theories that appears in Physiology of the Brain was motivated in part by the author's need to wreak intellectual vengeance on the perpetrators of his earlier humiliation.

Third, Physiology of the Brain provided Loeb with a forum for extended discussion of his theory of psychical function. According to this theory, consciousness is simply a metaphysician's term for phenomena of associative memory, the ability to learn can function as a reasonably reliable criterion of associative memory, and the use of such a criterion can serve as a cornerstone for reconstructing comparative psychology along scientific lines. Although we cannot be certain, Loeb may have been pushed into this discussion by concern for priority arising after Albrecht Bethe[7] (1872-1954) published a paper in which, as Loeb tells us, "Bethe assumes associative memory as the criterion for the existence of consciousness, as I had done before." [8]

Given the manner of its compilation and its divergent goals, it is hardly any wonder that Physiology of the Brain strikes the modern reader as diffuse and repetitive. What made it an influential work in its day, however -- particularly among the younger generation of animal behaviorists -- was the dogged persistence with which it held extreme positions on issues of critical importance to comparative psychology. These included: a) the nature of reflex, tropistic, instinctive, and voluntary movement; b) the role, if any, of the central nervous system in the organization and control of reflex and instinctive movement; c) the nature of psychical processes; d) the question of cortical localization of function; and e) the issue of what ought to be the proper theoretical goal of a biological psychology.

For Loeb there was no principled distinction between reflex movement, tropism, and instinct. "Reflex" is simply the term used when reaction to an external stimulus involves only a part of the organism. Tropisms are simple reactions of the organism as a whole, and instincts are more complicated reactions composed of tropisms and reflex chains. Regardless of whether the animal's behavior is reflexive, tropistic, or instinctive, it consists of a reaction -- usually purposive and coordinated -- to an external stimulus. The stimulus causes a change in sensory nerve endings that is transmitted through sensory nerves to the central nervous system and from there through the motor nerves to muscle fibers, where it produces extension or flexion of the muscle.

Reviewing research on purposeful reflex actions, coordinated movements, tropisms, and instincts across a wide-range of invertebrate and vertebrate species, Loeb continually emphasized the point that the explanation of animal behavior from reflex to instinct requires nothing more than an awareness of external stimulus conditions, a knowledge of the overall morphology of the organism, and an understanding of the general protoplasmic characteristics of irritability, conductivity, and contractility. Rejecting the notion that instincts are "so purposeful and so complicated in character that nothing short of intelligence and experience could have produced them," [9] Loeb fashioned an account of animal movement that was objectivist, mechanistic, and focused on the determinants rather than the meaning of the animal's behavior.

Loeb was also radically peripheralist in his orientation. He bemoaned the loss of what he considered to be the original conception of the reflex in which the central nervous system is viewed as nothing more than a mirror-like device whose only function is to reflect stimulation from the sensory to the motor apparatus. Drawing on his own extensive research on tropistic movement, Loeb attacked the then prevalent alternative which held that reflexive activity -- purposive and coordinated as it is -- must be under regulatory control and that the mechanism for exerting that control lies within the central nervous system. Plant and animal tropisms, Loeb argued, are formally identical. Since plants have no nerves, there is no reason to believe that the tropisms of animals, or by inference the instincts that are composed of tropisms, require a central nervous system. In Loeb's view, the central nervous system is merely a protoplasmic bridge that provides more efficient conduction of stimulation from sensory surface to muscle; it contains no mechanisms for the coordination of complex movements in reflex action.

While Loeb's attention was heavily focused on lower forms of behavior, he recognized quite clearly that not all behavior is either reflexive, tropistic, or instinctive. Certain behaviors are spontaneous -- "determined by internal conditions of the living system." [10] More importantly, in many organisms spontaneous behaviors often involve "that complex of phenomena which are called psychic or conscious." [11] Eschewing purposefulness as a criterion of psychical process on the grounds that many non-conscious biological processes (e.g., plant tropisms or the development of the embryo) are obviously purposive, Loeb suggested that psychic phenomena "appear, invariably, as a function of...the activity of the associative memory." [12] By associative memory, Loeb had in mind:

"...the two following peculiarities of our central nervous system: First, that processes which occur there leave an impression or trace by which they can be reproduced even under different circumstances than those under which they originated...second ...that two processes which occur simultaneously or in quick succession will leave traces which fuse together, so that if later one of the processes is repeated, the other will necessarily be repeated also." [13]

No sooner had he proposed an invariable functional relationship between psychic phenomena and associative memory, however, than Loeb took his argument much further:

"I think it can be shown that what the metaphysician calls consciousness are phenomena determined by the mechanism of associative memory...I think that we are justified in substituting the term activity of associative memory for the phrase consciousness used by the metaphysicians...We will then consider the extent of associative memory in the animal kingdom instead of the extent of consciousness among animals...For the present, we can say that if any animal can learn, that is, if it can be trained to react in a desired way upon certain stimuli (signs), it must possess associative memory." [14]

This was a radical move. Without denying the existence or importance of the phenomena which "the metaphysician calls consciousness," Loeb effectively asked for a ban against the use of the concept of "consciousness" in comparative psychology. In its place he wished to substitute the activity of associative memory and for the activity of associative memory he wished to nominate an objective behavioral criterion -- learning.

Loeb's position on the localization of psychical functions was equally radical. Aware that learned behavior is heavily altered by general extirpation of the cerebral hemispheres, Loeb accepted the global assignment of associative memory to the cerebrum. As he put it, "the hemispheres are an essential organ for the phenomena of associative memory." [15] Specific localization of psychical functions in circumscribed regions of the brain, however, was anathema to Loeb. Citing facts of associative memory that run counter to a mosaic view of function (e.g., the fusion of stimulation from different sense organs into a unified image) and Goltz's finding that after extirpation of the so-called movement center for the left paw, dogs retain previously learned reactions with that paw even in the presence of serious movement disturbances, Loeb reworked the anti-localization argument that he had first made during his ill-fated presentation of 1886. Again, but with much greater skill, he attacked the localization views of Hitzig and Munk and unequivocally rejected the hypothesis of specific localization:

"Hitzig and Munk are wrong in interpreting the disturbances following the excision of a small piece of the cortex as psychic disturbances. In the majority of cases such slight lesions cause no disturbance, and where any is caused it is of such a character as could be produced by the lesion of a peripheral nerve. If we wish to produce psychic disturbances by a lesion of the brain, we must destroy extensive parts of both hemispheres." [16]

Echoing his argument against central nervous system regulation of reflex activity, Loeb concluded his attack on Hitzig, Munk, and localization theory in uncompromising style:

"The assumption of 'centres of association' is just as erroneous as the assumption of a centre of cošrdination in the heart. Association is, like cošrdination, a dynamical effect determined by the conductivity of the protoplasm. Associative processes occur everywhere in the hemispheres...just as cošrdination occurs wherever the connection between two protoplasmic pieces is sufficient. It is just as anthropomorphic to invent special centres of association as it is to invent special centres of cošrdination." [17]

Finally, Physiology of the Brain provided Loeb with an opportunity to take still another iconoclastic stand. Noting that any attempt to decide whether animals have intelligence necessarily plunges the biologist into a sterile discussion of the meaning of the term "intelligence," Loeb made a startling assertion. "The aim of modern biology," he wrote, "is no longer word-discussions, but the control of life-phenomena." [18] Identifying control as the preeminent scientific goal -- one which Loeb very much embodied in his own research -- was a significant move in the transformation of the science of animal behavior. Not only was Loeb's science to be oriented toward identifying the objective signs of psychical process and describing the objective conditions that determine the animal's reactions, it was to be a science whose theoretical goal was the control of learning and behavior.

This was a revolutionary creed. Thirteen years before John B. Watson (1878-1958) created behaviorism by defining psychology as "a purely objective experimental branch of natural science...(whose) theoretical goal is the prediction and control of behavior," rejecting "interpretation in terms of consciousness," and calling for "a unitary scheme of animal response...(that) recognizes no dividing line between man and brute," [19] Loeb had already drawn the blueprint for such a science. For this reason, it is of considerable interest to inquire into both the sources of Loeb's inspiration and the nature of the effect that his work had on the young John B. Watson.

Jacques Loeb was born in the small Rhenish town of Mayen in 1859. [20] The son of a wholesale merchant, he received a commercial education at the BŸrgerschule at Neuwied. In 1876, shortly after his father's death, Loeb moved to Berlin to take up banking. Within the year, however, he realized that his interests lay in science -- not in business. With the encouragement of an uncle who was a medieval historian at the University of Berlin, Loeb returned to school to obtain the classical background necessary for entrance to the University. Graduating from the Askanische Gymnasium in 1880, he spent a year at the Universities of Berlin and Munich studying medicine. Then in the summer of 1881, he traveled to Strasbourg to work under Friedrich Goltz.

In an era in which deterministic, physico-chemical reductionism was the dominant orientation within German medical physiology, [21] Goltz's contributions to the analysis of neurophysiological phenomena were more in line with those of organismic biology. As Pauly has described it:

"His [Goltz's] major concern was to capture the organism's complexity. In a long series of studies in the 1850s and 1860s on the central nervous system of the frog, Goltz sought to demonstrate the existence of a 'central adaptation capacity' in the organism. Decerebrate frogs were capable of 'self-regulation'; in the wake of trauma the various parts of the nervous system would interact to enable an organism to compensate and adapt as best it could in order to survive...Goltz believed that biological concepts such as dynamic interaction, self-regulation, and adaptation were the only way for natural scientists to deal successfully with complex neurophysiological phenomena." [22]

At the time of Loeb's arrival in Strasbourg, Goltz was in the midst of research on dogs' capacity to compensate for experimentally induced loss of cerebral tissue and deeply involved in controversy over the localization of cortical function. In keeping with his generally systemic orientation, Goltz was highly critical of "the modern phrenology"[23] practiced by the proponents of precise localization such as Eduard Hitzig and Hermann Munk. Citing his own experimental evidence that animals retain residual motor function even after the removal of much of the so-called motor cortex, Goltz argued against precise localization, which he viewed as "a classic example of the attempt to impose a superficial and false order on organic complexity." [24]

In 1884[25] and then again in 1886[26] Loeb sided with his teacher. Attacking both the method and the sensori-motor associationism of Munk, Loeb developed a functional, neurodynamic account of brain/behavior relationships. In line with the general functional principle that animal activities are determined by the need for self-preservation and reproduction, Loeb articulated a view of the body as "a system whose parts work...together for the good of the whole." [27] Within this framework, Loeb rejected the attempt to dissociate sensory and motor effects of cortical damage and emphasized the animal's ability to achieve progressive compensation for initial loss of function. Where the localizers attempted to isolate functions in specific regions of the cortex and to analyze overall brain function mechanistically in terms of associations between brain regions, Loeb, following Goltz, focused on the action of the whole brain conceived as an energy system in dynamic equilibrium.

In October 1886, Loeb accepted a position as Assistant to Adolf Fick (1829-1901) at WŸrzburg, where he stayed for two years. While Fick's interest in the application of physics to physiology had little impact on Loeb's thinking, this period was significant in two respects. First, it was at WŸrzburg that Loeb began to engage in psychological research. During the winter of 1886-1887, working with human subjects, Loeb combined psychological experiment with physiological analysis in an attempt to apply the dynamic psychophysiology developed in his first two papers to the analysis of the intact human brain and to account for space perception through purely physiological factors such as irritability, innervation, and motion.

It was also at WŸrzburg that Loeb met the botanist Julius Sachs (1832-1897). For more than ten years Sachs had been studying directed response to constant stimuli in plants (e.g., the orientation of seedlings toward light). These responses, known as "plant tropisms," had been shown to occur in relation to stimuli such as light, gravity, moisture, and properties of solid objects. As a botanist, Sachs's interest in tropistic plant behavior was closely related to his work on techniques designed to control plant growth. In this regard, his orientation to research on plant tropisms was largely descriptive: he was far more concerned with the direction and extent of tropistic effects than with their underlying mechanisms.

Loeb was immensely excited by Sachs's work. In the tropism he found a holistic response of the entire organism that might, he felt, help to explain orientation in space without recourse to sensorimotor associationism. In Sachs's concern with the prediction and control of plant behavior through the establishment of functional relationships between plant reactions and variables of stimulus energy, Loeb also found a model for the future pursuit of his science. He set immediately to work extending the concepts of geotropism and heliotropism to animals, [28] and by 1889 he had begun to compile his results for publication in monographic form. [29]

During this period Loeb also began to correspond with the physicist Ernst Mach (1838-1916). Mach was then in the process of elaborating an approach to science[30] that would make him famous among philosophers, and many of his ideas were of importance to Loeb. Throuhgout the more than ten years of their correspondence, [31] Loeb found in Mach a source of inspiration and support for a wide range of positions. These included the view that perception and behavior could be construed as biological functions, that it was possible to account for mental phenomena physiologically, that the cause of a phenomenon is simply the most notable condition of its change, and that identification of the functional relationships that allow for prediction and control of phenomena -- not the search for underlying causes -- is the goal of science.

In 1888, Loeb returned to Strasbourg as an assistant to Goltz. Almost immediately, however, he began to realize that his future at Strasbourg was extremely limited, and he resigned at the end of the year. From 1889 to 1891 he lived on his own, outside the university system, pursuing occasional research as an unpaid Fellow at institutes such as the Naples Zoological Station. In 1891, after marrying a young American whom he had met while visiting in Zurich, Loeb began to look for an academic position in the United States.

Within a few months Loeb had obtained a temporary position at Bryn Mawr College, where he remained for a year. In the Spring of 1892, on the recommendation of the biologist Charles Otis Whitman (1842-1910), Loeb was offered an assistant professorship in physiology at the newly established University of Chicago. He remained at Chicago until 1903 when he left first for the University of California, and then eventually for the Rockefeller Institute for Medical Research in New York, [32] where he remained until his death in 1924.

Although Loeb made many notable contributions to biological theory and research in the later part of his career, [33] his place in the history of behaviorism derives almost entirely from the influence he exerted during his sojourn at the University of Chicago. [34] It was there that he met a young and impressionable graduate student named John Broadus Watson. Watson had arrived at Chicago in 1900. At the suggestion of his psychology professor, James Rowland Angell (1869-1949), Watson had chosen to study neurology under Henry Herbert Donaldson (1857-1938) as one of the two minor fields required for the doctorate. As part of the neurology curriculum, he enrolled for coursework under Jacques Loeb.

Comparative Physiology of the Brain and Comparative Psychology had just been published. Loeb was full of enthusiasm for the application of physiology and biochemistry to the problems of associative memory. Watson was duly impressed, both with what he read and heard and by what he saw in Loeb's laboratory. Indeed, so struck was he by Loeb's approach that Watson seriously considered studying the learning abilities of brain-damaged dogs with Loeb for his dissertation. [35] Unfortunately, however, when he brought this idea to Angell and Donaldson, he was quickly given to understand that this was a dangerous path. As Watson described it in his autobiography, "Neither Angell nor Donaldson in those days felt that Loeb was a very 'safe' man for a green Ph.D. candidate." [36]

Given Loeb's iconoclasm, this was hardly a surprising reaction, and Watson, who was nothing if not career-oriented, must have realized quickly that Angell and Donaldson were right. Well outside the mainstream of American biological and psychological thought, Loeb was not a safe man to choose as a dissertation director, and Watson prudently chose the safer course, working under Angell and Donaldson instead. [37] On the other hand, like Loeb, Watson was an iconoclast by temperament. It would be surprising if a negative reaction from senior professors didn't heighten the attractiveness of Loeb's ideas for Watson. Be that as it may, it is clear from the tenor and the content of Watson's famous "behaviorist manifesto"[38] that no individual exerted a greater intellectual influence on Watson than did Loeb. [39] Comparative Physiology of the Brain and Comparative Psychology is a foundational text in the emergence of behaviorism.


1. Loeb, J. (1900). Comparative physiology of the brain and comparative psychology. New York: G.P. Putnam's Sons (translated and extensively revised from: Loeb, J. (1899). Einleitung in die vergleichende Gehirnphysiologie und vergleichende Psychologie. Leipzig: Verlag von Johann Ambrosius Barth; First London edition: Loeb, J. (1901). Comparative physiology of the brain and comparative psychology. London: John Murray. This reprint is based on the London edition. [Back to text]

2. Even fewer still would be accepted today. [Back to text]

3. See Pauly, P.J. (1987) Controlling life. Jacques Loeb and the engineering idal in biology. New York: Oxford University Press, p. 123 for a discussion of Loeb's impact on Robert M. Yerkes (1876-1956) and pp. 172-177 for a similar discussion of Loeb and John B. Watson. [Back to text]

4. Loeb, J. (1884). Die Sehstšrungen nach Verletzungen der Grosshirnrinde. PflŸger's Archiv fŸr die gesamte Physiologie des Menschen und der Tiere, 34; For the work of Munk, see, for example, Munk, H. (1881). Ueber die Functionen der Grosshirnrinde. Berlin: A. Hirschwald. [Back to text]

5. See Pauly (1987), pp. 30-31 for an interesting discussion of this debacle. [Back to text]

6. Ibid., p. 31. [Back to text]

7. Bethe, A. (1898). DŸrfen wir den Ameisen und den Bienen psychische QualitŠten zuschreiben? PflŸger's Archiv fŸr die gesamte Physiologie des Menschen und der Tiere, 70. [Back to text]

8. Loeb (1900), p. 220. Loeb continues: "He has evidently overlooked, or at least does not mention, my work on this subject." [Back to text]

9. Ibid., p. 6. [Back to text]

10. Loeb (1900), p. 9. [Back to text]

11. Ibid., p. 10. [Back to text]

12. Loeb (1900), p. 213. [Back to text]

13. Ibid. [Back to text]

14. Ibid., pp. 214, 216-218. [Back to text]

15. Ibid., p. 248. [Back to text]

16. Ibid., p. 274. [Back to text]

17. Ibid., p. 275. [Back to text]

18. Ibid., p. 287. [Back to text]

19. Watson, J.B. (1913). Psychology as the behaviorist views it. Psychological Review, 20, 158-177. p. 158. [Back to text]

20. Biographical details are drawn from Philip J. Pauly's excellent account of Loeb's life and work: Pauly (1987); and from: Flexner, S. (1927). Jacques Loeb and his period. Science, 66, 333-337. [Back to text]

21. See Pauly (1987), pp. 14-20. [Back to text]

22. Ibid., pp. 22-23. [Back to text]

23. Goltz, F. (1882). Ueber die moderne Phrenologie. Deutsche Rundschau, 45, 372, cited in Pauly (1987), p. 24. [Back to text]

24. Pauly (1987), p. 24. [Back to text]

25. Loeb, J. (1884). [Back to text]

26. Loeb, J. (1886). BeitrŠge zur Physiologie des Grosshirns. PflŸger's Archiv fŸr die gesamte Physiologie des Menschen und der Tiere, 39.[Back to text]

27. Pauly (1987), p. 26. [Back to text]

28. Loeb, J. (1888). Die Orientierung der Thiere gegen das Licht. (Thierischer Heliotropismus). Sitzungs-Berichte der Physikalisch-medicinischen Gesellschaft zu WŸrzburg, 1-5; and Die Orientierung der Thiere gegen die Schwerkraft der Erde. (Thierischer Geotropismus). Sitzungs-Berichte der Physikalisch-medicinischen Gesellschaft zu WŸrzburg, 5-10. [Back to text]

29. Loeb, J. (1890). Der Heliotropismus der Thiere und seine Uebereinstimmung mit den Heliotropismus der Pflanzen. WŸrzburg: Georg Hertz. [Back to text]

30. See Blackmore, J.T. (1972). Ernst Mach: His work, life, and influence. Berkeley: University of California Press. [Back to text]

31. See Pauly (1987), especially pp. 41-48[Back to text]

32. Loeb left Chicago for California in January 1903 and moved to the Rockefeller Institute in 1910. [Back to text]

33. See Pauly (1987), Chapters 5, 7 & 8. [Back to text]

34. One exception is Loeb's contribution to the exchange of views that became known as the Loeb-Jennings debate. See Pauly, P.J. (1981). The Loeb-Jennings debate and the science of animal behavior. Journal of the History of the Behavioral Sciences, 17, 504-5l5. [Back to text]

35. Watson, J.B. (1936). John Broadus Watson. In C. Murchison (Ed). A history of psychology in autobiography. Worcester, MA: Clark University Press, pp. 271-281. See especially p. 273. [Back to text]

36. Ibid. [Back to text]

37. Watson's dissertation was published as: Watson, J.B. (1903). Animal education. Chicago: University of Chicago Press. [Back to text]

38. Watson (1913). [Back to text]

39. See Pauly (1987), pp. 172-177 for an excellent analysis. [Back to text]