Ivan Petrovich Pavlov, the son of a village priest, was born in Russia in 1849, and received his early education in a local seminary. In 1870 he entered the University of St. Petersburg in the natural history section with animal physiology as his specialty. After obtaining his degree in 1875, he enrolled as an advanced student in the medical school, not with any thought of a career as a practicing physician but as further preparation for a research post in physiology. His academic success was such that on completion of his thesis, he won a scholarship to Germany where he worked with prominent physiologists for two years. Nevertheless, it was not until the age of forty-one, in 1890, that he was made professor of pharmacology (later physiology) at the St. Petersburg Military Medical Academy and head of the physiology department of the Institute of Experimental Medicine. For many years he devoted his research attention to the processes of digestion. In fact, half of his career was devoted to work on digestion, for which he received the Nobel Prize in 1904. Only after the age of fifty did he study what became known as conditioning, which study covered a span of another thirty years.
The specific impetus for the study of conditioning was a phase of his work on the digestive glands. Using the dog as the experimental animal, Pavlov's general method was a surgical arrangement so that digestive secretions flowed to the surface of the body for collection and measurement. One aspect of his work was on the functioning of saliva in digestion. By operation, a salivary duct could be diverted so that the saliva stimulated by meat in the mouth of the dog flowed through a fistula to the outside of the body where it was collected. Prior to 1900 and before working with the conditioned response, he had noticed that a dog secreted saliva before the meat was given to him. Further observation showed this occurred not only when the dog saw the meat, but even on hearing the footsteps of the attendant.
The secretory reflex with the innate response of salivation to ingested food had now become "conditioned," to the sight of the food or the sound of the footsteps of the attendant. Pavlov realized that this happened because this sight or sound had been so often associated with ingestion of food. This is association by frequency of contiguity, as it would be called in associationistic terms. The term, "conditioned reflex," was first applied to this phenomenon in 1901.
Should he follow up this new lead into an area which many physiologists would view with disdain, since it was psychic in nature? Some leading physiologists, in point of fact, on hearing of his dilemma, advised his against embarking on the work. On the other hand, he had the example of Sechenov before him. After a long struggle with himself, he resolved to go ahead and to make it a physiological problem by maintaining the role of the external observer with no consideration of introspective findings. This was in 1900-1901.
He absorbed himself in his new task. The basic procedures, with the exception of the selection of the stimulus to bring on salivary flow, had already been standardized through his work on digestion. His already extensive laboratory resources were directed to this new problem. When the Soviet government came to power, his research faculties were expanded. An increasing number of associates and assistants joined him. This collaborative effort was an example of coordination of research involving more researchers and extending over more years than anything since Wundt. Over the years, some two hundred collaborators worked with him on problems in conditioning.
The basic model for Pavlov's work was the presentation of two kinds of stimuli, one, "appropriate," biologically "adequate," or "unconditioned;" the other, "psychic," "conditioned," or "learned." Each reflex has an appropriate (unconditioned) stimulus which brings it on. If the patellar tendon is struck, the knee jerks; if food is placed on the tongue, saliva flows; if the finger is pricked, it is jerked back. In his studies, Pavlov tended to depend upon the food powder leading to the salivary response, although other forms were also used. When a response such as salivation becomes attached to a stimulus that formerly did not arouse it, it is said to be conditioned.
Practically any stimulus, Pavlov found, can act as a conditioning stimulus to produce a conditioned response. The salivation to the sight of the food or the sound of the footsteps of the attendant are stimuli for conditioned responses. Sight of food or the sound of footsteps had come to serve as signals, and they now brought about a response formerly elicited by food in contact with the tongue.
Pavlov originally referred to "psychic reflexes," that is, reflexes aroused not by the adequate stimulus of meat in contact with the tongue but by some other form of stimulation which had been presented along with the meat. He almost immediately dropped this term, in favor of "conditioned" responses. Over the years, Pavlov preferred to use as conditioned stimuli, the sound of a tuning fork, a bell, and a light flash. These were the stimuli which acquired a new reaction, namely a flow of saliva.
A specific instance might be given: A hungry dog would be led into the experimental room and placed in a restraining harness (to which he was already accustomed). After a few minutes a metronome would tick for thirty seconds, and then food powder would be mechanically introduced into the dog's mouth. Saliva would begin to flow. Every fifteen minutes the same sequence--thirty seconds ticking, then food introduced--would flow. Before many repetitions, saliva would begin to flow while the metronome was ticking and before the food was introduced. A conditioned response had been established. A previously inadequate stimulus (inadequate, that is, to produce saliva) would now produce a response formerly elicited only by the adequate stimulus of food powder.
In addition to the study of their formation, conditioned responses were open to all sorts of other quantitative manipulation. Among other phenomena studied by Pavlov are what have been called extinction, reinforcement, spontaneous recovery, generalization and discrimination, and higher order conditioning. "Extinction" has been studied in the following fashion: after the conditioned response has been formed, the stimulus, say the bell, is continued on the trials that follow, but the natural or adequate stimulus of meat powder is no longer introduced. This results in the conditioned stimulus losing its capacity to elicit the conditioned response. When the adequate stimulus for the conditioned response is not given, it is said to be non-reinforced. Generally speaking, repetition without reinforcement through the meat powder results in a decrease of amount of salivation from trial to trial until it ceases, that is, shows extinction. A conditioned response extinguishes, unless periodically reinforced. Extinction, however, may not be permanent. After a rest, the application of the conditioned stimulus may again produce a flow of saliva. This is an instance of so-called, spontaneous recovery.
Pavlov and his collaborators also conducted experiments on generalization and discrimination. When a stimulus different from the one to which conditioning has already been developed is introduced and, nevertheless, produces the conditioned response, generalization has occurred. A "spread" or generalization has taken place. Suppose a sound of a particular pitch has been used to begin to form a conditioned response. Now a sound of somewhat lower or higher pitch is substituted. The conditioned secretion occurs as it did to the original pitch. This is generalization. However, with continued training, the conditioned reflex acquires a certain degree of specificity, that is, it can no longer be elicited by stimuli which differ too greatly from the training stimulus. This is discrimination. This degree of discrimination is obtained without particular effort. However, the procedure used, which consists of merely continuing to present the same stimuli while occasionally interposing stimuli of different pitch to see if it will elicit the response, still leaves a relatively wide band of pitch differences which the dog will treat as the same by salivating when they are presented.
The degree of generalization remaining can be cut still further by training of a specific kind. The task now is to test the limits of the dog's discriminability, that is, to refine still further his discrimination to the point where, despite an actual difference in pitch, no difference can be detected from the dog's behavior. This training in discrimination is done by differential reinforcement; one pitch serves to lead to food, another slightly different pitch does not lead to food. Say, food is not presented to the dog after a pitch of 825 vibrations per second, but it is presented to a pitch of 812 vibrations. The dog, after repeated trials, will discriminate between the two pitches by salivating to the latter and not to the former. The pitches selected for illustration are the limits of discrimination for this particular dog, because trying to train him to a pitch difference less than that of thirteen vibrations per second (825 minus 812) led to failure to learn. In this connection, Pavlov discovered that some dogs, pushed beyond their limits of discrimination, break down, lose whatever discriminatory ability they have already gained, and become agitated. This opened up still another facet of research, that of the study of experimental neuroses, in which a dog was found to show a variety of forms of abnormal behavior by being driven beyond his capacities.
An already established conditioned response may be treated as if it were an unconditioned one in order to bring about still further or so-called higher order conditioning. Suppose a dog has been conditioned to the sound of the tuning fork vibrating at a certain rate paired with the meat powder so that it evokes salivation. The sound of the tuning fork (without the meat powder) is now paired with a sight of a black square, which prior to the experiment produced no secretory response. After paired stimulation of sound and square, the black square will now produce salivation. (Of course, if this were kept up too long, without reinforcement, extinction would occur.) This is second order conditioning. Pavlov also found it possible to go on to third order conditioning but not beyond it.
Pavlov considered conditioning a cortical, not a subcortical affair, as is the case with a reflex. As his findings accumulated, Pavlov worked through a theory of cortical excitation and inhibition to account for his results in purely physiological terms. Although his method was seized on avidly, his theory has received little attention outside of the USSR; so no attempt is made to present details.
It will be remembered that following Sechenov, Pavlov saw this work as a problem in physiology, not psychology. He never wavered in his view. It colored his attitude toward what the world called psychology. It can only be described as one of pessimistic skepticism. He believed psychology never could be an independent science and its position to be "completely hopeless." Moreover, in his own work he believed he had completely excluded it. Thus Pavlov wished everyone in his laboratory to use physiological terms exclusively; a worker was fined if he used psychological terminology.
Pavlov's attitude toward the Soviet regime is a complicated one and defies adequate summarization in a sentence or two. For many years he was outspokenly blunt and critical, once even being called in by the secret police. On the other hand, he received generous government support for his research, and there is no suggestion of an pressure being put upon him to carry on work along the lines selected by the government. His work was always of his own choosing. Pressure on those who came after him is a different matter.
In 1935 Pavlov gave voice to a changed attitude. In a speech before the Fifteenth International Congress of Physiologists, he spoke of the very favorable position that science occupied in his country and, as an experimenter, saw his country as an experimenter of an "incomparably higher category." Babkin, a co-worker, who left the USSR for Canada, believes his change of heart came about due not only to the support of science that the USSR gave but also to the fact that he was very much alive to the threat of Hitler, having expressed forebodings on several occasions before this date.
He died in 1936.
Source:
Robert I. Watson, The Great Psychologists: From Aristotle to Freud - Second Edition: (Philadelphia and New York: J. B. Lippincott Company, 1968,1963), 408-413.