Ivan Pavlov

Source

• Born: 14 September 1849
• Birthplace: Ryazan, Russia
• Died: 1936
• Best Known As: Author of Conditioned Reflexes (1926)

Ivan Petrovich Pavlov studied medicine in Russia and Germany, accepting posts in St. Petersburg as a professor in pharmacology and physiology. In 1889 Pavlov began experiments with dogs that proved their reflexes could be conditioned by external stimuli. Specifically, after they were conditioned by the ringing of a bell at feeding time, they would reflexively salivate upon hearing the bell, whether or not food was present. In 1904 Pavlov won the Nobel Prize for his work on digestive physiology, but he is most widely known today as an early influence on behavioral psychology.

For more information on Ivan Petrovich Pavlov, visit Britannica.com.

Russian physiologist (1849–1936)

Born at Ryazan in Russia, Pavlov studied medicine and general science at the University of St. Petersburg and the Military Medical Academy. He subsequently carried out research in Breslau (now Wrocław, in Poland) and Leipzig (1883–86). Returning to St. Petersburg, he became professor of physiology at the Medical Academy and director of the physiology department of the Institute of Experimental Medicine.

Pavlov's early research lay in the physiology of mammalian digestion, showing, for example, that the secretion of digestive juices in the stomach is prompted by the sight of food and nerve stimulation via the brain. For this work Pavlov received the Nobel Prize for physiology or medicine (1904). He then went on to study the way that dogs and other animals may be induced to salivate and show signs of anticipation of food by actions, such as the ringing of a bell or even a powerful electric shock, that they have learned to associate with the appearance of food. Pavlov's work on conditional or acquired reflexes, which he believed to be associated with different areas of the brain cortex, has led to a new psychologically-oriented school of physiology and has stimulated ideas as to the probability of many aspects of human behavior being a result of ‘conditioning’.

Pavlov openly criticized communism and the Soviet government. In 1922 he requested and was refused permission to move his laboratory abroad. Following the expulsion of priests' sons from the Medical Academy, Pavlov, himself the son of a priest, resigned from the chair of physiology in protest. Despite such actions his work continued to be supported by state funds and Pavlovian psychology remained popular in the Soviet Union.

Ivan Petrovich Pavlov (1849-1936) was a Russian physiologist who won the Nobel Prize in Physiology or Medicine in 1904 for his work on the physiology of gastric secretion. However he is more famous for his subsequent studies on reflexes and for laying the foundations of the field of behavioural psychology.

He was the eldest of eleven children of a Russian orthodox priest, and entered the theological seminary in his home town of Ryazan, in provincial Russia, with the intention of following his father's profession. In later years he recalled to colleagues that it was seeing an illustration of the gastrointestinal tract in a book by the English writer George Henry Lewes, The Physiology of Common Life, that persuaded him to leave the religious life to study the natural sciences. This drawing of the alimentary system was based on the physiological research work of Claude Bernard, and the complexity of it challenged Pavlov to explore its intricacies further.

Pavlov entered the university of St Petersburg in 1879 to study medicine, and, after graduating, obtained his doctorate in 1883 from the Medical Military Academy, also in St Petersburg. As a student he had been particularly influenced by, and collaborated with, the physiologist Elie Cyon, and on Cyon's advice he studied the nerves to the pancreas, and identified those which stimulated the secretion of its digestive juices; for this he was awarded the University's gold medal. It was thus a natural progression for him to travel abroad to study with two of the greatest living physiologists, Carl Ludwig in Leipzig and Rudolph Heidenhain in Breslau. Soon after returning to St Petersburg he was appointed Professor of Physiology at the Medical Military Academy, in 1890. He remained there until 1924, when the newly-created Soviet Academy of Sciences established a special Institute of Physiology for him.

Pavlov's work can be divided into two distinct phases: earlier work on digestion, and later work on the conditioned reflex. He was a noted experimenter and renowned surgeon, and, using anaesthetized experimental animals, usually dogs, he created several ‘windows’ in the body through which the secretions of the stomach, salivary glands, pancreas, and intestine could be collected. Of particular value was his ability to bring the stomach out through the body wall with its nerve and blood supply intact, so that he could observe its functions in a conscious dog, behaving normally. He also made an artificial hole in the oesophagus (gullet) so that food taken into the mouth escaped before it reached the stomach. Together, these allowed him to show that gastric juices were secreted in anticipation of receiving food from the mouth, and also to collect and study the secretions uncontaminated by food particles. His techniques led to the discovery and identification of several key enzymes and mechanisms that occur in normal digestion. This work had commercial spin-offs: from 1898 onwards Pavlov's lab contained a ‘gastric acid factory’, the acid produced by the dogs being collected and sold as a remedy for dyspepsia, and by 1904 the project was contributing over 65% of the laboratory's budget.

Perhaps the most influential observation he made was in the early years of the twentieth century, when he noticed that even the mere sight and smell of food could stimulate the anticipatory production of salivary and gastric secretions in his experimental dogs. Further systematic experiments on this phenomenon revealed that if the appearance of food was repeatedly preceded by the ringing of a bell, then eventually the dog would produce secretions after hearing the bell, and before or without the appearance of food. This encouraged Pavlov and his co-workers to turn their attention to the activities of the higher nervous system and to the further study of such responses, which he originally called a ‘conditional reflex’, because ‘their inclusion as reflexes had for him a conditional character’. To English speakers the expression has now become known as ‘conditioned reflex’, although the French still refer to ‘le réflexe conditionnel’. His work from then onwards focused on links between nervous activity and behaviour, extending into observational and theoretical work on human behaviour. Many of his views were rapidly absorbed into psychological and psychiatric practice and teaching.

By the time of the Russian and Bolshevik revolutions in 1917, Pavlov was a world-renowned scientist. He was subsequently courted by the new regime, which wanted to build up Soviet science. In the years immediately after the revolutions Pavlov frequently denounced the Bolsheviks and their ideology, and at one period considered emigrating. He was, however, offered privileges for himself and his colleagues that permitted him to continue working, and by the 1930s had apparently reconciled himself to living in Soviet Russia, particularly through his friendship with Nikolai Bukharin. He continued nonetheless to be a critic of the government, and was subject to secret police surveillance for many years up to his death in 1936.

— E. M. Tansey

Bibliography

• Brown, E. M. (1990). Ivan Petrovich Pavlov. In Nobel Laureates in Medicine or Physiology: a biographical dictionary, (ed. D. M. Fox, M. Meldrum, and I. Rezak). Garland Publishing, New York.
• Todes, D. P. (1995). Pavlov and the Bolsheviks. History and Philosophy of the Life Sciences, 17, 379-418.
• Todes, D. P. (1997). Pavlov's physiology factory. Isis, 88, 205-46.
• Todes, D. P. (1997). From the machine to the ghost within: Pavlov's transition from digestive physiology to conditional reflexes. American Psychologist, 52, 947-55

See also conditioning.

(1849-1936) Russian physiologist; discovered conditioned reflexes, e.g. dogs salivating in anticipation of food; Nobel Prize 1904 for his work on the physiology of digestion.

The Russian physiologist Ivan Petrovich Pavlov (1849-1936) pioneered in the study of circulation, digestion, and conditioned reflexes. He believed that he clearly established the physiological nature of psychological phenomena.

Ivan Pavlov was born in Ryazan on Sept. 26, 1849, the son of a poor parish priest, from whom Pavlov acquired a lifelong love for physical labor and for learning. At the age of 9 or 10, Pavlov suffered from a fall which affected his general health and delayed his formal education. When he was 11, he entered the second grade of the church school at Ryazan. In 1864 he went to the Theological Seminary of Ryazan, studying religion, classical languages, and philosophy and developing an interest in science.

Making of a Physiologist

In 1870 Pavlov gained admission to the University of St. Petersburg (Leningrad), electing animal physiology as his major field and chemistry as his minor. There he studied inorganic chemistry under Dmitrii Mendeleev and organic chemistry under Aleksandr Butlerov, but the deepest impression was made by the lectures and the skilled experimental techniques of Ilya Tsion. It was in Tsion's laboratory that Pavlov was exposed to scientific investigations, resulting in his paper "On the Nerves Controlling the Pancreatic Gland."

After graduating, Pavlov entered the third course of the Medico-Chirurgical Academy (renamed in 1881 the Military Medical Academy), working as a laboratory assistant (1876-1878). In 1877 he published his first work, Experimental Data Concerning the Accommodating Mechanism of the Blood Vessels, dealing with the reflex regulation of the circulation of blood. Two years later he completed his course at the academy, and on the basis of a competitive examination he was awarded a scholarship for postgraduate study at the academy.

Pavlov spent the next decade in Sergei Botkins laboratory at the academy. In 1883 Pavlov completed his thesis, The Centrifugal Nerves of the Heart, and received the degree of doctor of medicine. The following year he was appointed lecturer in physiology at the academy, won the Wylie fellowship, and then spent the next 2 years in Germany. During the 1880s Pavlov perfected his experimental techniques which made possible his later important discoveries.

In 1881 Pavlov married Serafima Karchevskaia, a woman with profound spiritual feeling, a deep love for literature, and strong affection for her husband. In 1890 he was appointed to the vacant chair of pharmacology at the academy, and a year later he assumed the directorship of the department of physiology of the Institute of Experimental Medicine. Five years later he accepted the chair of physiology at the academy, which he held until 1925. For the next 45 years Pavlov pursued his studies on the digestive glands and conditioned reflexes.

Scientific Contributions

During the first phase of his scientific activity (1874-1888), Pavlov developed operative-surgical techniques that enabled him to perform experiments on unanesthetized animals without inflicting much pain. He studied the circulatory system, particularly the oscillation of blood pressure under various controlled conditions and the regulation of cardiac activity. He noted that the blood pressure of his dogs hardly varied despite the feeding of dry food or excessive amounts of meat broth. In his examination of cardiac activity he was able to observe the special nerve fibers that controlled the rhythm and the strength of the heartbeat. His theory was that the heart is regulated by four specific nerve fibers; it is now generally accepted that the vagus and sympathetic nerves produce the effects on the heart that Pavlov noticed.

In the course of his second phase of scientific work (1888-1902), Pavlov concentrated on the nerves directing the digestive glands and the functions of the alimentary canal under normal conditions. He discovered the secretory nerves of the pancreas in 1888 and the following year the nerves controlling the secretory activity of the gastric glands. Pavlov and his pupils also produced a considerable amount of accurate data on the workings of the gastrointestinal tract, which served as a basis for Pavlov's Lectures on the Work of the Principal Digestive Glands (published in Russia in 1897). For this work Pavlov received in 1904 the Nobel Prize in physiology or medicine.

The final phase of Pavlov's scientific career (1902-1936) was primarily concerned with ascertaining the functions of the cerebral cortex by means of conditioned reflexes. Prior to 1900, Pavlov observed that his dogs would secrete saliva and gastric juices before the meat was actually given to them. The sight, odor, or even the footsteps of the attendant were sufficient to trigger the flow of saliva. Pavlov realized that the dogs were responding to activity associated with their feeding, and in 1901 he termed such a response a "conditioned reflex," which was acquired, or learned, as opposed to the unconditioned, or inherited, reflex. He faced a dilemma: could he embark on the study of conditioned reflexes by applying physiological methods to what was generally viewed as psychic phenomena? He opted to follow Ivan Sechenov, who considered that, in theory, psychic phenomena are essentially reflexes and therefore subject to physiological analysis.

The important lectures, papers, and speeches of Pavlov dealing with conditioned reflexes and the cerebral cortex are presented in Twenty Years of Objective Study of the Higher Nervous Activity (Behavior) of Animals: Conditioned Reflexes (1923) and Lectures on the Work of the Cerebral Hemispheres (1927). He not only concerned himself with the formation of conditioned responses but noted that they were subject to various kinds of manipulation. He discovered that conditioned responses can be extinguished - at least temporarily - if not reinforced; that one conditioned stimulus can replace another and yet produce identical conditioned responses; and that there are several orders of conditioning. In time Pavlov developed a purely physiological theory of cortical excitation and inhibition which considered, among other things, the process of sleep identical with internal inhibition. However magnificent his experiments were in revealing the responses of animals to conditioning stimuli, he encountered difficulty in experimentally proving his assertion that conditioned responses are due to temporary neuronal connections in the cortex.

In 1918 Pavlov had an opportunity to study several cases of mental illness and thought that a physiological approach to psychiatric phenomena might prove useful. He noted that he could induce "experimental neuroses" in animals by overstraining the excitatory process or the inhibitory process, or by quickly alternating excitation and inhibition. Pavlov then drew an analogy between the functional disorders in animals with those observed in humans. In examining the catatonic manifestations of schizophrenia, he characterized this psychopathological state as actually being "chronic hypnosis" - chiefly as a consequence of weak cortical cells - which functions as a protective mechanism, preserving the nerve cells from further weakening or destruction.

In Pavlov's last scientific article, "The Conditioned Reflex" (1934), written for the Great Medical Encyclopedia, he discussed his theory of the two signaling systems which differentiated the animal nervous system from that of man. The first signaling system, possessed both by humans and animals, receives stimulations and impressions of the external world through sense organs. The second signaling system in man deals with the signals of the first system, involving words, thoughts, abstractions, and generalizations. Conditioned reflexes play a significant role in both signal systems. Pavlov declared that "the conditioned reflex has become the central phenomenon in physiology"; he saw in the conditioned reflex the principal mechanism of adaptation to the environment by the living organism.

Philosophy and Outlook

Pavlov's endeavor to give the conditioned reflex widest application in animal and human behavior tended to color his philosophical view of psychology. Although he did not go so far as to deny psychology the right to exist, in his own work and in his demands upon his collaborators he insisted that the language of physiology be employed exclusively to describe psychic activity. Ultimately he envisioned a time when psychology would be completely subsumed into physiology. Respecting the Cartesian duality of mind and matter, Pavlov saw no need for it inasmuch as he believed all mental processes can be explained physiologically.

Politically, most of his life Pavlov was opposed to the extremist positions of the right and left. He did not welcome the Russian February Revolution of 1917 with any enthusiasm. As for the Bolshevik program for creating a Communist society, Pavlov publically stated, "If that which the Bolsheviks are doing with Russia is an experiment, for such an experiment I should regret giving even a frog." Despite his early hostility to the Communist regime, in 1921 a decree of the Soviet of People's Commissars, signed by Lenin himself, assured Pavlov of continuing support for his scientific work and special privileges. Undoubtedly, Soviet authorities viewed Pavlov's approach to psychology as confirmation of Marxist materialism as well as a method of restructuring society. By 1935 Pavlov became reconciled to the Soviet Communist system, declaring that the "government, too, is an experimenter but in an immeasurably higher category."

Pavlov became seriously ill in 1935 but recovered sufficiently to participate at the Fifteenth International Physiological Congress, and later he attended the Neurological Congress at London. On Feb. 27, 1936, he died.

Further Reading

Still the finest biographical study of Pavlov is the one produced by his senior surviving student, Boris P. Babkin, Pavlov: A Biography (1949). Also useful are Ezras A. Asratian, I. P. Pavlov: His Life and Work (1953), and Harry K. Wells, Ivan P. Pavlov: Toward a Scientific Psychology and Psychiatry (1956). For the influence of Pavlov on Soviet psychology see Raymond A. Bauer, The New Man in Soviet Psychology (1952), and A Handbook of Contemporary Soviet Psychology, edited by Michael Cole and Irving Maltzman (1969). An early history of Russian physiology is in Alexander S. Vucinich, Science in Russian Culture: A History to 1860 (1963).

(1849 - 1936), Russian physiologist and Nobel Prize winner.

Ivan Pavlov was born in Ryazan. His father, a local priest, wanted him to attend the theological seminary, but Pavlov's interest in natural sciences led him to enroll in St. Petersburg University in 1870. In 1883 he completed his doctoral dissertation and in 1890 became professor and head of the physiology division of the St. Petersburg Institute of Experimental Medicine, where he remained until 1925. Pavlov's work on the functioning of the digestive system earned him the Nobel Prize in 1904. His originality lay in his approach to physiology, which considered the coordinated functioning of the organism as a whole, as well as his innovative surgical technique, which allowed him to observe digestion in live animals.

Pavlov's most well known research involved the study of conditioned reflexes. In his famous experiment, he placed a dog in a room free of all distractions. He found that the dog, accustomed to hearing a bell ring when being fed, would eventually salivate at the sound of the bell alone. Pavlov also applied his findings to the human nervous system. His work advanced the understanding of physiology and influenced international developments in medicine, psychology, and pedagogy.

Pavlov did not support the Bolshevik Revolution and in 1920 asked for permission to leave with his family. Vladimir Lenin, aware of the international prestige Pavlov brought to science in the Soviet Union, personally intervened to guarantee the resources for Pavlov to continue his research. In 1935, the International Congress of Physiologists awarded Pavlov the distinction of world senior physiologist. He died of pneumonia in Leningrad at the age of eighty-seven.

Bibliography

Joravsky, David. (1989). Russian Psychology: A Critical History. Cambridge, MA: Blackwell Publishers.

Porter, Roy, ed. (1994). The Biographical Dictionary of Scientists, 2nd ed. New York: Oxford University Press.

—SHARON A. KOWALSKY

Ivan Pavlov (1849-1936) was a Russian physiologist who became famous for his experiments with dogs in which the animals performed a specific behavior upon being confronted with a certain stimulus. In these well-known investigations, minor surgery was performed on a dog so that its saliva could be measured. The dog was deprived of food, a bell was sounded, and meat powder was placed in the dog's mouth. The meat powder caused the hungry dog to salivate; this is an example of an unconditioned reflex. However, eventually, after many trials, the dog would salivate at the sound of the bell, without meat powder being offered. This is an example of a conditioned reflex or classical Pavlovian conditioning. Although he never thought much of the then-fledgling science of psychology, Pavlov's work on conditioned reflexes has been far reaching, from elementary education to adult training programs. Pavlov was awarded the Nobel Prize in Physiology or Medicine (1904) for his study of the physiology of digestion.

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(pav-lawfs, pav-lawvz)

The dogs used in conditioned response experiments by a Russian scientist of the late nineteenth century, Ivan Pavlov. In these experiments, Pavlov sounded a bell while presenting food to a dog, thereby stimulating the natural flow of saliva in the dog's mouth. After the procedure was repeated several times, the dog would salivate at the sound of the bell, even when no food was presented.

Someone who reacts instinctively rather than reflectively to a situation is said to be engaging in a Pavlovian reaction.

(1849–1936). Russian physiologist. His importance for the study of animal behaviour is mainly due to his work on conditional reflexes; this work provided the basis for much of the subsequent research on learning. Pavlov also conducted significant research on the physiology of digestion and on neurosis.

1. Brief life
2. Physiology of circulation and digestion
3. Conditional reflexes
4. Experimental neurosis and personality

1. Brief life

Pavlov was born in Ryazan, Russia, and attended the religious school and seminary there, where he studied natural science. He did not complete his studies, but entered St Petersburg University in 1870, where he continued to study natural science, and decided to make his career as a physiologist. After graduation in 1875, he went to the Military Medical Academy to pursue his research. He completed his doctorate there in 1883, and then went to Germany (1884–6), where he studied in Leipzig with Carl Ludwig, and in Breslau. In 1890 he was appointed professor in the department of pharmacology in the Military Medical Academy, and in 1895 he moved to the department of physiology. In 1904 he received the Nobel Prize for his work on the physiology of digestion. He remained professor of physiology until 1925, when he resigned in protest against the expulsion of sons of priests from the Academy (he himself was the son of a priest, but would not have been expelled). Initially Pavlov was outspokenly opposed to the Bolsheviks, even though they supported his research. In 1922 he asked Lenin's permission to transfer his research abroad, but was refused: Lenin wanted prestigious scientists. However, during the last few years before his death (in Leningrad) Pavlov increasingly accepted and approved of the Bolsheviks. From 1925 to 1936 he worked mainly in three laboratories: the Institute of Physiology of the Soviet Academy of Sciences (which is now named after him), the Institute of Experimental Medicine, and the biological laboratory at Koltushy (now Pavlovo), near St Petersburg.

2. Physiology of circulation and digestion

Pavlov held that physiologists should study 'the actual course of particular physiological processes in a whole and normal organism'. He also held that the main problems for experimental research were the mutual interactions of organs within the body, and the relation of the organism to its environment. The method of working on the whole, healthy body of an animal contrasted with the mainstream of physiology in the latter half of the 19th century; most investigations then were on isolated organs and prepared specimens.

Pavlov's work on the physiology of circulation (c.1874–88) was mainly concerned with the mechanisms that regulate blood pressure. His experimental animal for this, and for most subsequent research, was the dog. He was mainly interested in nervous mechanisms. He discovered, for instance, that the vagus nerve controls blood pressure, and that there are four nerves controlling the heartbeat, which can vary the heartbeat's rhythm and intensity (work on the nervous control of the heart had formed his doctorate).

Pavlov's work on the physiology of digestion began in about 1879, and culminated in his book The Work of the Digestive Glands (1902, a translation of Lektsii o rabotie glavnych pishchevaritel'nykh zhelez, 1897). He investigated the nervous mechanisms controlling the secretions of the various digestive glands, and how these nervous mechanisms were stimulated by food. He had to expose the structures of interest surgically, and work on them in a healthy animal, so it was crucial to his success that he was also a brilliant surgeon. (Similar experiments had been attempted in the laboratory in Breslau that he visited in the mid-1880s, but had failed because the experimenters lacked Pavlov's surgical skill.) Once he had exposed part of the gut, Pavlov could directly insert food or chemicals, and observe the effects on the activity of the digestive glands. The method of sham feeding was a related development. A slit is made in the animal's throat so that food entering through the mouth falls out through the neck before reaching the stomach. The animal can be fed through a second opening made into the stomach. By sham feeding, Pavlov could observe the effect of food in the mouth on the secretion of digestive juices elsewhere in the gut; he found that the taste of food in the mouth causes the release of gastric juices in the stomach. A smaller quantity of juice is released if food is put directly into the stomach. Sham feeding has been used and developed extensively by later workers.

Pavlov's own theory for the control of digestive secretions postulated control exclusively by nervous mechanisms. Subsequent research has shown this theory to be incomplete: control by hormones also occurs. He made many other important discoveries while working on digestion. Two of the most important were the enzyme enterokinase, which controls the activity of another digestive enzyme, and the connection between the properties of the saliva and the type of food being eaten (the Pavlovian curves of salivary secretion). N. P. Shepovalnikov and Pavlov were co-discoverers of enterokinase.

Pavlov's work on the physiology of digestion is important for understanding his work on animal behaviour, for his explanations of animal behaviour are similar to those of the control of digestion. In addition, his methods were similar in all his studies.

3. Conditional reflexes

While at the seminary in Ryazan, Pavlov had read I. M. Sechenov's Refleksy golovnago mozga (Reflexes of the Brain), which argued that mental events were reflexes. Then, while working on the physiology of digestion, he had noticed 'psychic' salivation: when the dog was confronted by a stimulus that customarily preceded feeding, it salivated even before being fed. This psychic salivation could be induced, for example, by the animal's food container, or by the presence of the attendant who normally fed the animal, or even by the sound of the attendant's approach. Armed with these incidental observations and with the reflexology of Sechenov, and stimulated by Charles Darwin's evolutionary arguments about animal behaviour (which encouraged materialistic analyses of mental events), Pavlov set out to investigate the psychic salivation of dogs. This was a simple extension of his earlier studies on the control of digestive secretions, and once again it was the nervous reflex that he looked to for his explanation. He worked on conditional reflexes from about 1902 until his death.

A typical Pavlovian experiment on psychic salivation would be as follows. On several occasions a bell is rung just before the dog is fed, and the dog salivates on receiving its food. Then the bell is rung without presentation of food. It is observed that the dog salivates in response to the bell's ringing. Pavlov termed the food the unconditional stimulus, the sound of the bell the conditional stimulus, the salivation to the food the unconditional reflex, and the salivation to the bell alone the conditional reflex. ('Conditional' is what Pavlov actually wrote, but the early mistranslation of 'conditioned' is now widespread in the psychological literature.)

Many of the fine details of the conditional reflex were studied by Pavlov and his collaborators. First, there is the temporal sequence of stimuli. Pavlov found that it is much easier to form a conditional reflex if the unconditional stimulus (food) follows the conditional one (bell) than if they are simultaneous, or if the conditional stimulus follows the unconditional. Second, there is the time delay between stimuli. Here he found that a discrete conditional stimulus is more effective in forming a conditional reflex if it occurs near in time to the food than if it occurs a long time before. However, if the conditional stimulus starts a long time before, but continues right up to when the unconditional stimulus is presented, then it is as effective as a conditional stimulus which starts just before the food is given. Third, there is the intensity of the stimuli. A dog salivates more if it is trained on bigger pieces of food, and similarly, it salivates more to a louder bell. Fourth, Pavlov studied generalization of the conditional stimulus. If the animal has been trained on a stimulus of one pitch, it can then be tested for a response to a stimulus of another pitch. This leads to a method of investigating the animal's powers of sensory discrimination, which again was originated and developed by Pavlov.

Pavlov was not only interested in how conditional reflexes were gained; he also studied how they were lost. He classified factors causing loss of a conditional reflex into cases of either external inhibition or internal inhibition. If an animal, conditioned in one way, is moved into a new environment, or is exposed to new stimuli before being fed, it loses its original conditional reflex; this is called external inhibition. There are several forms of internal inhibition. The most straightforward is the gradual loss of the conditional reflex if the food is withheld after the conditional stimulus; the conditional reflex requires regular reinforcement (to use Pavlov's term) by the unconditional stimulus.

Pavlov thought of the conditional reflex as similar to any other kind of reflex. The flow of digestive juices is stimulated by the mechanical and chemical properties of food, through the mediation of a nervous (unconditional) reflex. Similarly, salivation could be induced by some environmental indicator of food, again by a nervous (conditional, in this case) reflex. The conditional reflex, however, is easily modifiable by the environment, according to whatever the local indicators of food happen to be. So, Pavlov regarded the formation of conditional reflexes as an adaptation whereby the animal could survive better in a changing environment.

Pavlov also speculated on the fine details of the formation of a conditional reflex. He suggested that the cells of the central nervous system must change structurally and chemically when a conditional reflex is formed: 'the locking in, the formation of new connections, we attribute to the functioning of the separating membrane, should it exist, or simply to the branching between neurones.' This idea has subsequently been confirmed.

Although nearly all his research was on dogs, Pavlov also showed that conditional reflexes can be formed in mice and monkeys, and he was in no doubt that they occur in man, and in all other animals. He wrote: 'A temporary nervous connection is a universal physiological phenomenon in the animal world and exists in us ourselves.' He also showed that more or less any environmental factor can act as a conditional stimulus (though this conclusion has been slightly modified by later research). Pavlov was aiming at truly universal laws of learning, and that is why his discoveries are so fundamental to modern theories of associative learning. One branch of psychology — behaviourism — went so far as to attribute all human behaviour to conditioning and reinforcement. Pavlov, however, made no such extravagant claims for the conditional reflex, and ridiculed the claims of behaviourism to scientific status. The most famous expression in English of Pavlov's work on conditioning is his book Conditioned Reflexes (1927). In psychiatry, conditioning is used in behaviour therapy.

4. Experimental neurosis and personality

In a famous experiment (1921) by Shenger-Krestovnika a circle was used as a conditional stimulus before feeding, and the dog was also trained to associate an ellipse with not being fed. By small steps the ellipse was then made more and more like a circle. When the ellipse was almost round, initially the dog could usually distinguish it from a circle. But after a few weeks the dog became neurotic: it ceased to be able to recognize obvious ellipses and circles, became very excited, and was no longer calm during experiments. Pavlov termed the animal's abnormal condition experimental neurosis, and he attributed it to a disturbance of the balance between excitatory and inhibitory processes in the nervous system. This explanation of experimental neurosis is grounded in Pavlov's theory of personality. He explained personality by variation in the excitation of the nervous system. He did not, however, attribute neurosis solely to external factors, such as contradictory stimuli. His experiments on experimental neuroses showed that dogs with different 'personalities' were differentially susceptible to the treatment: the same treatment on different dogs could produce quite different neuroses. In the 1930s Pavlov decided to work on the genetics of behaviour, and his government built him the biological station at Koltushy for this research.

There is an underlying unity to all Pavlov's work. In his earliest work on blood circulation he established his method (intervention in unanaesthetized, whole dogs) and his paradigm explanation: nervous control. The same procedure was used in his work on the control of digestive secretion, and he started research on conditional reflexes as just another kind of nervous control of digestion. Late in his life, he saw how conditioning could be used to analyse personality and neurosis, and once again he was able to carry over his theoretical framework into a new field.

(Published 1987)

— Mark Ridley

Bibliography
• Babkin, B. P. (1951). Pavlov: A Biography.
• Gray, J. A. (1979). Pavlov.
• Grigorian, N. A. (1974). Pavlov, Ivan Petrovich.

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