BULLOCK, THEODORE HOLMES

(b. Nanking, China, 16 May 1915; d. La Jolla, California, 19 December 2005)

c omparative biology, neuroscience, neuroethology.

Bullock had a career that spanned the development of twentieth-century neuroscience. Trained as a zoologist and comparative biologist, he was a key pioneer in the birth of the discipline of neuroethology—the study of the neural basis of naturally occurring animal behavior—as well as neuroscience more generally. Rather than being associated with a single scientific success, Bullock’s contributions are as diverse as the biological world he studied. With a career that involved neurophysiological investigations into every taxonomic group of animals, he made important discoveries in nonhuman sensory physiology, created new tools and experimental techniques for neuro-physiology, and expanded our understanding of how nerve cells communicate. But perhaps more than his role in generating this or that new datum point, Bullock was known throughout his career for his ability to synthesize what was currently understood and to set the agenda for future work. By presenting his fellow scientists with a clear and fair assessment of what is actually known and what was not, in fact, known—regardless of the prevailing zeitgeist—Bullock played a crucial role in the development of neuroscience and neuroethology.

Life and Times. One of four children of Presbyterian missionaries, Theodore (“Ted”) Holmes Bullock was born in Nanking, China; and spent the first thirteen years of his life there before his family returned to Southern California in 1928, just prior to the Great Depression. He received his AB and his PhD (in zoology) from the University of California, Berkeley (in 1936 and 1940, respectively). His PhD work—on the nervous system of enteropneusts, also known as acorn worms, a group of organisms closely related to the chordates—was conducted under the supervision of S. F. Light. One benefit of his graduate stipend was finally feeling established enough to marry, and he did so, to Martha Runquist, who became his lifelong partner. After spending a number of years on the East Coast and in the Midwest, associated with both the Yale and University of Missouri Schools of Medicine and the Marine Biological Laboratory, he returned to the West Coast in 1946, joining the Zoology faculty at the University of California, Los Angeles (UCLA). Finally, in 1966, he and Martha moved down the coast to the University of California, San Diego, to help Robert Livingston create the world’s first Department of Neurosciences. It is with that department in the School of Medicine and with the Scripps Institution of Oceanography (where he directed the Neurobiology Unit) that he was associated for the final forty years of his research career. Although he had retired in 1982, Bullock was still actively publishing as an emeritus professor when he passed away of natural causes on 19 December 2005. He was survived by Martha, a son, a daughter, and five grandchildren.

Bullock had a long and rich research career that spanned seven decades. In the main, he was a comparative biologist who took neurophysiological phenomena as his traits of taxonomic comparison. Other comparative biologists take bone structure or, more recently, genetic markers, as their point of comparison across the different taxa. Bullock, however, was interested in what could be learned about evolution from the study of the diversity of nervous systems. This also led him into the field of neuroethology, with its comparative approach to the study of animal behavior and nervous systems.

His career was marked not by one or two significant discoveries or theoretical innovations, but by a large range of important work in the lab, in the field, in the classroom, and in the development of the discipline of neuroscience to which he was one of many midwives. However, there are highlights to his career that give one a flavor of his contributions to science.

Bullock, the Iconoclast. Bullock often embraced the role of heretic. For example, whereas the majority of those working in sensory physiology concentrated their attention on those senses enjoyed by humans, Bullock tackled senses of a nonhuman variety. During the 1950s at UCLA, Bullock, together with Friedrich Diecke and Raymond B. Cowles, investigated the pit organ of rattlesnakes, an organ located just below the eyes. Previous behavioral work had demonstrated that snakes with a pit organ were able to strike accurately at prey targets even when their eyes were covered. This behavior, together with previous anatomical discoveries, suggested the hypothesis that the pit organ mediated a unique modality of radiant heat perception independent of changes in ambient temperature; that is, that the pit organ was an infrared receptive sensory organ. Bullock and his colleagues confirmed this hypothesis by a careful study of the neurophysiology of pit organs of live rattlesnakes presented with a variety of controlled stimuli (Bullock and Cowles, 1952; Bullock and Diecke, 1956).

Bullock’s love of the underappreciated or overlooked scientific hypothesis is also evident in his contributions to cellular neurophysiology. He challenged the ubiquity of the chemical synapse as the locus of communication between neurons. While he never denied the importance of the standard model of neural communication— whereby the arrival of an action potential at the axon terminal causes the release of neurotransmitter chemicals into the synaptic cleft leading to changes in the electrical state of the postsynaptic neuron—he incessantly urged his colleagues not to be blind to other potential means of communication between neurons. For example, while working with Bullock, Akira Watanabe discovered the existence of direct electrical connections between neurons in cardiac ganglion of lobsters (Bullock and Watanabe, 1960). (Bullock’s lab was where Susumu Hagiwara had carried out the first intracellular recordings in these neurons a few years earlier.) Later work revealed that they had inadvertently discovered a significant means of neural communication: gap junctions, in which electrical activity travels directly between conjoined cells, without chemical mediation. Interestingly, while some see this work as pointing to the later discovery of gap junctions, Bullock himself believed that there is more going on in the cardiac ganglion preparation. In addition to the presence of electrical synapses there, he also held that work from his lab indicated the presence of lower frequency electrical connections—so called slow potentials (Bullock, 1996).

In addition, Bullock is generally credited, along with his colleague Hagiwara, with the first discovery of electroreceptive sensory cells in electric fish (Bullock and Chichibu, 1965; Bullock et al., 1961). These cells had been suspected at least since Hans Lissmann had proposed in the 1950s that these fish possess a nonhuman sense of electric fields, but Bullock and colleagues were the first to establish what cells were carrying out this function (and thereby establishing that the sense existed [Keeley, 1999; Bullock, 1974]). This work was later taken up by his former postdoctoral student, Walter Heiligenberg, who is credited with making the electrosensory system of the weakly electric fish, Eigenmannia, one of the best understood vertebrate sensorimotor system in neurobiology (Bullock and Heiligenberg, 1986; Bullock et al., 2005).

Bullock, the Synthesizer. One example of Bullock’s contribution to the then nascent field of neuroscience is his influential review titled “Neuron Doctrine and Electro-physiology” (1959). This paper documents “a quiet but sweeping revolution” (p. 998) in neurobiology, as a result of developments specifically from neurophysiology. According to Bullock’s review, Santiago Ramón y Cajal’s Neuron Theory or Reticular Theory? (Ramón y Cajal, 1954) asserted that, “the nerve cell and its processes, together called the neuron, form the cellular units of the nervous system which are directly involved in nervous function” (Bullock, 1959, p. 997). As Bullock’s review spells out,

whereas Ramón y Cajal’s doctrine is largely structural or anatomical in nature, contemporary work on the physiology of the nervous system has spurred change in key aspects of the doctrine as a functional or neurophysiological one. (Bullock himself was a key innovator of techniques, experimental preparations, and methods in neurophysiology.) Bullock’s review crystallized then nascent concepts such as that: the nerve impulse (now known as an “action potential”) is a special property of neuronal axons, not the cell as a whole; impinging excitation does not become nerve impulse directly but spreads to special firing zones; distal dendrites may not affect firing of a given neuron, but may influence other neurons through local graded potentials (Watanabe’s discovery in Bullock’s lab, discussed above, motivated this proposal); and that multiple integrative zones within neurons provide evaluation actions. In sum, Bullock synthesized an understanding of the behavior of neurons in terms of the physiological contributions of different components that constitute them. He showed how neurophysiologists have developed an emerging theory of the dynamics of dendrites, cell body, and axon. This review, and the questions it posed, influenced the direction of neurophysiology for several decades afterward.

Another example of the “synthetic” role of Bullock in the development of twentieth-century neurobiology is his two-volume 1965 work, Structure and Function in the Nervous Systems of Invertebrates (cowritten with G. Adrian Horridge), which is still considered a landmark work of invertebrate neurobiology. Weighing in at over 1,700 pages, this work brings together much that had been learned by that date about the neuroanatomy and neurophysiology of spineless animals from the relatively primitive Protozoa and Porifora through the Deuterostomes (whose neural organization begins to resemble that of the vertebrates). They note that this “work is primarily for reference but it is not an encyclopedia or compendium.

Rather, it attempts a synthetic, personal evaluation of the state of our information” (p. viii). This goal of synthesis is a hallmark of Bullock’s career, in that he sees as his goal the creation of a coherent account of nature by extracting principles and regularities out of a bewildering array of evolved organisms: “If recognizing some of the gaps and pointing to the nearest relevant reports is a step forward, this work can pretend to be a contribution” (p. xi). This title exists in only a single edition, as the explosion of research that began in the 1960s made it impossible to publish ahead of the literature in this way again. The smaller, less comprehensive Introduction to Nervous Systems (Bullock, Orkand, and Grinnell, 1977) allowed him to bring some of the key developments up to date.

Bullock as Midwife and Honoree. In addition to his research contributions to neuroscience, Bullock also contributed to the birth and early development of two closely related contemporary disciplines: neuroscience and neuroethology. He also represented those fields in the larger world of science, as well as the larger world in general. He traveled widely and hosted innumerable foreign scientists in his laboratory. He served as president of the American Society of Zoologists (1965; now the Society for Integrative and Comparative Biology), the third president of the Society for Neuroscience (1973–1974) and was the first president of the Society for Neuroethology (1984–1987).

Bullock was awarded a number of honors during his career, most notably the Karl Spencer Lashley Prize from the American Philosophical Society (1968) and the Ralph W. Gerard Prize from the Society for Neuroscience (1984). He was inducted as a member of both the American Academy of Arts and Sciences (1961) and National Academy of Sciences, U.S.A. (1963). He was also named a Queen’s Fellow in marine biology in Australia, and received honorary doctorates from the University of Frankfurt and the University of Loyola Chicago. His name also graces a building in Manaus, Brazil (one of the many place to which he made research field expeditions during his career): the Bullock-Heiligenberg Laboratory of Behavioral Physiology.

The Importance of Diversity. Finally, the key concept to understanding Bullock’s approach to scientific inquiry is diversity. He brought to bear a diversity of techniques to study a diversity of biological phenomena in a diversity of organisms. He pioneered many new techniques in electrophysiology. Over his career, in addition to what has been described above, he published on topics as diverse the evolution of myelin, phylogenetic taxonomy, acclimation, and the measurement of metabolism, as well as ecological physiology. Nowhere is Bullock’s love of scientific diversity more evident than in the wealth of organisms that he studied (in addition to those already mentioned): Aplysia, bats, catfish, corals, crabs, crayfish, cuttlefish, elephants, earthworms, frogs, hagfish, manatees, octopus, porpoises, rats, ratfish, rays, salamanders, sea lions, sea urchins, sharks, sloths, squid, starfish, tuna, turtles, not to mention humans. The point of this breadth is not a short attention span or perversity. The point is Bullock’s conviction that taxonomic breadth and diversity of data form the single most important feature of evidence related to evolutionary hypotheses. Just as Charles Darwin did not just look only to domesticated pigeons to support his far-reaching theory about the biological world, Bullock saw that if we were to understand the evolution of nervous systems— and understand evolution via the study of nervous systems—it was incumbent on us to cast our net as widely as possible. For Bullock, this simply underlined the communal nature of science, as no one individual could collect sufficient data about enough different species to evaluate appropriately hypotheses in comparative biology. Given the example he set over his career, one wonders whether Bullock himself was the exception to this rule.

BIBLIOGRAPHY

The personal and scientific papers of Theodore H. Bullock, comprising over 33 cubic feet of material from his entire career, can be found the Scripps Institution of Oceanography Archives located in the library of that institution. Starred references (*) below are reprinted in How Do Brains Work? Papers of a Comparative Neurophysiologist, Contemporary Neuroscientists. Boston: Birkhauser, 1993.

WORKS BY BULLOCK

*With Raymond B. Cowles. “Physiology of an Infrared Receptor: The Facial Pit of Pit Vipers.” Science 115, no. 2994 (1952): 541–543.

With F. P. J. Diecke. “Properties of an Infra-red Receptor.” Journal of Physiology 134 (1956): 47–87.

* “Neuron Doctrine and Electrophysiology.” Science 129, no. 3355 (1959): 997–1002.

*With Akira Watanabe. “Modulation of Activity of One Neuron by Subthreshold Slow Potentials in Another in Lobster Cardiac Ganglion.” Journal of General Physiology 43, no. 6 (1960): 1031–1045. *With S. Hagiwara, K. Kusano, and K. Negishi. “Evidence for a Category of Electroreceptors in the Lateral Line of Gymnotid Fishes.” Science 134 (1961): 1426–1427.

*With Shiko Chichibu. “Further Analysis of Sensory Coding in Electroreceptors of Electric Fish.” Proceedings of the National Academy of Sciences of the United States of America 54, no. 2 (1965): 422–429.

With G. Adrian Horridge. Structure and Function in the Nervous Systems of Invertebrates. 2 vols. San Francisco: W.H. Freeman, 1965.

* “An Essay on the Discovery of Sensory Receptors and the Assignment of Their Functions Together with an Introduction to Electroreceptors.” In Handbook of Sensory Physiology, edited by A. Fessard. Berlin: Springer-Verlag, 1974.

With Richard Orkand and Alan Grinnell. Introduction to Nervous Systems. San Francisco: W.H. Freeman, 1977.

With Walter F. Heiligenberg, eds. Electroreception. New York: John Wiley, 1986.

How Do Brains Work? Papers of a Comparative Neurophysiologist, Contemporary Neuroscientists. Boston: Birkhauser, 1993. “Theodore H. Bullock.” In The History of Neuroscience in Autobiography, edited by L. R. Squire. Washington, DC: Society for Neuroscience, 1996.

With Carl D. Hopkins, Arthur N. Popper, and Richard R. Fay, eds.Electroreception. Edited by R. R. Fay and A. N. Popper. Springer Handbook of Auditory Research, vol. 21, New York: Springer, 2005.

OTHER SOURCES

Josephson, Robert K. “Theodore Holmes Bullock (1915–2005).” Biological Bulletin 210 (2006): 169–170.

Keeley, Brian L. “Fixing Content and Function in Neurobiological Systems: The Neuroethology of Electroreception.” Biology & Philosophy 14 (1999): 395–430.

Pearce, Jeremy. “Theodore H. Bullock, 90; Studied How Animals Function.” New York Times, 9 January 2006, p. B7. Ramón y Cajal, Santiago. Neuron Theory or Reticular Theory?Objective Evidence of the Anatomical Unity of Nerve Cells. Translated by M. Ubeda Purkiss and Clement A. Fox. Madrid: Consejo Superior de Investigaciones Cientificas, 1954.

Zupanc, G. K. “Obituary: Theodore H. Bullock (1915–2005).” Nature 439, no. 7074 (2006): 280.

Brian L. Keeley