Classical conditioning is also referred to as associative learning or Pavlovian conditioning, after its primary founder, the Russian physiologist Ivan Petro-vich Pavlov (1849-1936). Pavlov's original studies involved examining digestion in dogs. The first step in digestion is salivation. Pavlov developed an apparatus that allowed him to measure the amount of saliva a dog produced when presented with food. Dogs do not need to learn to salivate when food is given to them—that is an automatic, reflexive response. However, Pavlov noticed that, with experience, the dogs began to salivate before the food was presented, suggesting that new stimuli had acquired the ability to elicit the response. In order to examine this unexpected finding, Pavlov selected specific stimuli, which he systematically presented to the dog just before food was presented. The classic example is the ringing of a bell, but there was nothing special about the bell per se. Dogs do not salivate in response to a bell ringing under normal circumstances. What made the bell special was its systematic relationship to the delivery of food. Over time, the dogs began to salivate in response to the ringing of the bell even when the food was not presented. In other words, the dogs learned to associate the bell with food so that the response (salivation) could be elicited by either stimulus.
In classical conditioning terminology, the food is the unconditioned stimulus (US). It is unconditioned (or unlearned) because the animal naturally responds to it before the experiment has begun. The sound of the bell ringing is referred to as the conditioned stimulus (CS). It is not naturally effective in eliciting salivation—for it to be so, learning on the part of the subject is required. Salivating in response to food presentation is referred to as
the unconditioned response (UR), and salivating when the bell is rung is referred to as the conditioned response (CR). Though it would seem that saliva is saliva, it is important to differentiate the conditioned from the unconditioned response, because these responses are not always identical. More important, one is a natural, unlearned response (the UR), while the other requires specific learning experiences in order to occur (the CR).
Classical conditioning is not limited to dogs and salivation. Modern researchers examine classical conditioning in a variety of ways. What is important is the specific pairing of some novel stimulus (the CS) with a stimulus that already elicits the response (the US). One common experimental procedure examines eye blink conditioning in rabbits, where a brief puff of air to the eye serves as the US, and the measured response (UR) is blinking. A tone, a light, or some other initially ineffective stimulus serves as the CS. After many pairings in which the CS precedes the air puff, the rabbit will begin to blink in response to the CS in the absence of the air puff. Another common behavior that is studied in classical conditioning research is conditioned suppression. Here a CS is paired with an aversive US, such as a mild electric shock. Presentation of the shock disrupts whatever behavior the animal is engaged in at the time, and with appropriate pairing over time, the CS comes to do so as well. A final example that many humans can relate to is taste aversion learning. Here a specific taste (CS) is paired with a drug or procedure that causes the animal to feel ill (US). In the future, the animal will avoid consuming (CR) the taste (CS) associated with illness (US). Taste aversions illustrate the fact that all forms of conditioning are not created equal. To learn a conditioned eye blink or salivation response requires many CS-US pairings, while taste aversions are often learned with only one pairing of the taste and illness.
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