Ivan Petrovich Pavlov (1849-1936), world famous Russian physiologist became second to Freud as the most influential psychologist of our century. Pavlov was a follower of "The father of Russian psychology," Sechenov, whose theory was that the reflex is the essence of all reactions. Pavlov went further and discovered that the conditional reflex is a response, which is not directly connected with the stimulus, but caused by association with a previous experience.
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The conditioned reflex was discovered in the following experiment on a dog. A bell rung just before a dog was fed; the dog salivated on receiving its food. After repeated trials, the dog would salivate whenever the bell sounded, even if no meat powder was being presented.
The dog salivated in response to the bell ring. Pavlov decided that the food was an unconditioned stimulus, the salivation in response to the food was an unconditioned reflex,while the sound of the bell was the conditioned stimulus. Only the salivation to stimulus of the bell alone was the conditioned reflex. More than that, Pavlov found that the conditioned reflex was formed easier when the unconditioned stimulus followed the conditioned one;the conditioned reflex was formed easier, if the conditioned stimulus occurred very close in time to the unconditioned stimulus, if the bell louder, or if the dog was trained on larger pieces of meat the amount of salivation would be larger.
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This discovery was very important because the conditioned reflex was basic to the understanding of the mechanism of change in human and animal behavior.Thus scientists came to understand that behavior is no longer limited by a fixed, inherited reflex, but can be modified by experience and exposure to an unlimited number of stimuli.
Aristotle had already talked about it, but Pavlov was the first scientist who demonstrated this system of associations. Association is "mental connection between ideas.