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Pavlov's aim was to use the salivary conditioning method to investigate the function of the brain of higher animals in their adaptation to the external environment. The salivary reflex, according to Pavlov, was of minor biological significance but a good indicator of the subtle changes in the brain under different experimental conditions. To account for conditioned reflex phenomena, Pavlov faced two alternatives: to offer an objective (physiological) or a subjective (psychological) explanation. In 1901, after a bitter conflict with his disciple A. T. Snarskiy, Pavlov chose the first alternative. During the next decades, Pavlov provided reasons for this decision: The physiological approach (a) avoids anthropomorphizing or speculations about the dogs' subjective experiences, and (b) permits the explanation of observed phenomena which the subjective method is not capable of doing. Pavlov realized that the conditioned reflex method has a limitation; it cannot be used in the study of human subjects because their thinking interferes with experimental results.
In a doctoral dissertation written in 1898 under the direction of Ivan Petrovich Pavlov (1849-1936), the physician S. G. Vul'fson described the results of a series of experiments on rates of salivary flow produced by dogs in response to a variety of food that either impinged directly upon their sensory receptors or was presented at a distance. Vulf'son found that the rate of production of saliva is greater in response to dry than to moist foods. Later, a similar reaction, albeit to a lesser degree, occurred when the identical foods were presented at a distance. In 1902 Tolotschinoff (1903), addressing the Nordic Meeting of Scientists and Physicians held in Helsingfors [Helsinki], Finland, reported experimental results indicating that repeated presentation of food held at a distance reduced the rate of saliva from the rate that had been established originally by feeding dogs with identical food.
Five years passed between Vul'fson's systematic description of acquisition and Tolotschinoff's treatise on extinction of the conditioned reflexes. During this period, Pavlov, a staunch Darwinian, recognized that by using salivary conditioned reflexes it is possible to study higher organisms' adaptation to the external environment. Pavlov held that adaptation occurs through the functioning of the nervous system, particularly the brain. Acquisition and extinction of associations between the sight of food and eating, which triggered salivation, had...