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2007-9-25
Ivan Pavlov (1870£1932), USSR physiologist and the winner of the Nobel Prize, is the first man who brought forward the concept of classical conditioned reflex. When studying digestion, he observed the salivation of a dog, a form of response to food. Then he conducted an experiment in which he showed food to the dog, and measured its saliva. It was found, in the process, that if a neutral stimulus, a kind of stimulus which wouldn't automatically cause salivation, such as bell ringing, is reinforced time and time again together with the food stimulus, the dog could "acquire" the ability of salivating even though there is only belling ring without any food. A combination of a neutral stimulus and a stimulus which can automatically arouse salivation will make the animal learn to respond to that neutral stimulus. That is basic principle in the theory of classical conditioned reflex.
Conditions for conditioned reflex involve four factors, two about stimulus, and another two responses of organism. One stimulus, called conditioned stimulus (CS), is the neutral one which doesn't cause anticipated and acquired response before a conditioned reflex is formed, such as the bell ringing in Pavlov's experiment. The other is unconditioned stimulus (UCS), which brings anticipated response before a conditioned reflex is formed. For instance, if before the formation of a conditioned reflex, once meat appears, the dog drools, the meat is an UCS. Such response of salivation without any conditioned stimulation is called unconditioned response (UCR), which will happen before any conditioned reflex is coming into being. On the other hand, response to the stimulus as a result of a conditioned reflex is called conditioned response (CR), as is described in the example of meat. When the two stimuli appear closely (in terms of both space and time) and repeatedly, a conditioned reflex will be formed. Usually, UCS should immediately follow CS. As UCS and CS show closely together several times, gradually CS will arouse salivation. By that time, the animal has formed a kind of conditioned response that once a neutral stimulus (bell ringing) appears alone, salivation takes place.
The combination of the neutral stimulus and UCS in terms of time is referred to as reinforcement. The more the combination is reinforced, the firmer the conditioned reflex will be. As CS is not confined to the acoustical stimulus, every effective stimulus from both inside and outside of the organism (including compound stimulus, relations between stimuli and time factor and etc.), once combined with UCS in time (namely reinforcement), will become CS and form a conditioned reflex. After a conditioned reflex has come into being, the secondary conditioned reflex can be formed alike as long as a new stimulus is combined with the primary conditioned reflex. So can the tertiary conditioned reflex and therefore, for human beings, multilevel conditioned can be established in the same way.
However, while CS is not reinforced by UCS, CS will encounter inhibition, mainly extinctive inhibition and differential inhibition. That means if the animal is given CS alone several times without the presence of UCS after the conditioned reflex has been established, its response to CS will be weaker and weaker till it extinguishes completely. For instance, the dog which has already formed the CR of salivation upon CS, the bell ringing, will drool less and less till none if it is not given food after the bell rings several times. This is the extinction of the conditioned reflex.
Pavlov was convinced that this extinction is caused by a transformation from excitement to inhibition, a process that CS which produces excitement gradually turns into the one that causes inhibition. The inhibition is so called extinctive inhibition. Pavlov also pointed out that it is brain that voluntarily brings the inhibition, other than the extinction or interruption of the temporary connection between CS and the correspondent response. Because an extinguished conditioned reflex can recover itself, after it is left aside for a period of time. And if CS is reinforced again later, the organism can pick up the conditioned reflex very fast. That proves the extinction is not caused by the temporary extinction of the previous connection but the temporary inhibition. Generally speaking, the firmer the conditioned reflex is, the slower the extinction takes place, and vice versa, the less firmer the conditioned reflex is, the easier the extinction will be.
In the process of forming a conditioned reflex, apart from CS, stimuli similar to the same CS may possess the effect of conditioned reflex more or less. For instance, 500 Hz pitch and food can be used together to form the conditioned reflex of salivation. And many other sounds, at the primary stage of the experiment, can also cause this conditioned reflex. But the greater the differences between them and 500 Hz pitch are, the less the effect of the conditioned reflex will be. This phenomenon is called generalization of conditioned reflex. The generalization will graduate away later if reinforcement is solely provided for CS (500 Hz pitch), other than any other similar stimulus. By doing so, the animal will only respond to the frequently reinforced stimulus and the other similar stimuli are inhibited. This is called differentiation of conditioned reflex.
As is stated above, the significance of Pavlov's work is invaluable. Not before long he published the findings, some psychologists such as the founder of the behaviorism, J. Watson, started to claim that every behavior is based on the classic conditioned reflex. Despite that his extreme allegation didn't prevail in America, theories based on classical conditioned reflex has dominated the circle of psychology for a considerable long time in Russian. Nevertheless, it is all accepted that a substantial parts of behaviors can be well explained by the theory of classical conditioned reflex.
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Organizer:
Beijing Municipal Association for Science & Technology
Undertaker: Beijing Science & Technology
Consulting Center,
Information Center of Beijing Municipal Association for
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