THE TERM "conditioned reflex" was introduced by Pavlov in 1903. He mentioned it for the first time in an oration entitled "Experimental Psychology and Psychopathology in Animals," which he delivered at the International Congress of Medicine in Madrid in April of that year.1
By a conditioned reflex Pavlov meant a response, established in an animal, to one or more unessential properties of a stimulating object. These properties, the sight or smell of food, for example, act through the appropriate receptive surface—the eye or the nose in this case—while the main receptive surface which is of concern here (the mucous membrane of the mouth cavity) remains unaffected. The conditioned response requires the participation of the central nervous system, especially that of the cerebral cortex, which may be considered the chief organ of conditioned reflexes. After the removal of the cerebral cortex in animals, only crude conditioned reflexes can be established,