A parent gives their child milk every day before bedtime. Eventually, the child begins to get sleepy every time she drinks milk. What do the milk and the sleepiness represent, respectively?
a) Unconditioned stimulus and unconditioned response
b) Conditioned stimulus and unconditioned response
c) Unconditioned stimulus and conditioned response
d) Conditioned stimulus and conditioned response
D is correct. Conditioned stimulus and conditioned response. The milk is the conditioned stimulus because it initially has no effect on the child (it is originally a neutral stimulus). The sleepiness begins as the unconditioned response to bedtime, because it is a natural reaction to going to bed. However, after the milk stimulus is paired with bedtime, the sleepiness becomes the conditioned response. Answer choice A is incorrect because milk is the conditioned stimulus, not the unconditioned stimulus. The unconditioned stimulus is likely “bedtime.” Answer choice B is incorrect because, in association to milk, sleepiness is the conditioned response, not the unconditioned response. Answer choice C is incorrect because milk is the conditioned stimulus, not the unconditioned stimulus.
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You’ve almost certainly heard of Pavlov’s dogs, who were trained to drool as if they smelled meat whenever they heard the sound of their dinner bell. This occurred through classical conditioning, which is the learning process by which an unconditioned stimulus (the meat) is paired with a conditioned or neutral stimulus (the bell) until the conditioned stimulus begins to elicit a conditioned response (drooling). This MCAT post will cover all these terms in-depth, and will also explain the concept of classical conditioning itself.
An unconditioned stimulus acts on an animal to produce an unconditioned response. Being unconditioned, you can think of this as a component of a biological need or reflex. This does not have to be learned – for example, if you deliver an air puff to the eye of an animal, the animal will automatically blink. The physical disturbance to the eye is an unconditioned stimulus, and the blink is the unconditioned response, occurring without any prior training.
A conditioned or neutral stimulus, is a stimulus that initially has no effect on the animal. In other words, the stimulus does not trigger an innate response. Because there is no default response associated with a conditioned stimulus, researchers can pair the unconditioned stimulus with the conditioned/neutral stimulus, and train the animal to produce a conditioned response to the conditioned stimulus. After training, presenting the conditioned stimulus alone will produce a conditioned response. The process by which the conditioned response is learned is classical conditioning.
An unconditioned response, being biologically engrained, is fairly permanent. However, a conditioned response is less permanent than the unconditioned response. It’s possible to eliminate the conditioned response over time through a process called extinction. If a conditioned response has undergone extinction, exposing the organism to the conditioned stimulus will not produce any effect on the animal, as it initially did not.
Figure 1 illustrates the Pavlov’s dog experiment, which demonstrated classical conditioning in animals. The unconditioned stimulus is food, because when food is presented to a dog, the dog will naturally salivate as the unconditioned response. The neutral or the conditioned stimulus is something we can choose – here it’s the ringing of a bell. The dog initially has no response to the neutral stimulus, so when the bell rings the dog is not affected. In a process called pairing, the dog is presented food and the bell is rung simultaneously, and this is done multiple times over a period of time. After a while, the dog begins to associate the bell with food. Now, when the bell is rung, he will begin to expect food and salivate.
Next let’s consider a few additional terms: acquisition, extinction, and spontaneous recovery.
Acquisition refers to the process by which a conditioned/neutral stimulus is paired with the unconditioned stimulus. The more often you pair the stimuli together, the stronger the conditioned response in the animal will be (Figure 2).
Extinction occurs when you present the conditioned stimulus alone without the unconditioned stimulus. Over time, the animal learns that the unconditioned stimulus is not associated with the conditioned stimulus, and so it learns to not produce the conditioned response.
Spontaneous recovery occurs after an extinction event, where the conditioned stimulus was no longer able to produce a conditioned response. If an animal is given a break or rest period, presenting the conditioned stimulus will suddenly produce the conditioned response again, in a process called spontaneous recovery. The strength of the conditioned response after a spontaneous recovery event is generally weaker than after the initial period of conditioning.
Generalization refers to the idea that if a conditioned animal is presented with stimuli that are similar to the conditioned stimulus, the similar stimuli may also elicit the conditioned response. Consider fear-conditioning in rodents, in which the unconditioned stimulus is a foot shock and the unconditioned response is freezing behavior. A conditioned stimulus would be something like a 10 kHz tone, which if paired with the foot shock, would cause the 10 kHz tone to induce freezing behavior. If you then present a similar stimulus such as a 16 kHz tone, the mouse may also experience freezing behavior.
Discrimination is the opposite process to generalization, occurring when only the conditioned stimulus can elicit the conditioned response and similar stimuli cannot. In the case of fear conditioning in rodents, if the 16 kHz tone did not induce freezing behavior, then the rodents are able to discriminate between these two stimuli.
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