Last update: September 15, 2022
By BrainMatters

Classical conditioning is a form of learning in which the pairing of two stimuli causes the response to one of them to change. This form of learning was first described by a Russian researcher named Ivan Pavlov. While doing research on digestion in dogs, Pavlov discovered that the production of saliva in the dogs occurred even before he fed the dogs. He then did further research on this fact to see if he could subconsciously teach the dogs to produce saliva.

He did this by ringing a bell 5 seconds before he would feed the dogs. When this is repeated a few times, the dogs appear to link the bell to food. This means that the dogs produce saliva when the bell starts to ring, without food being present.

Of course, classical conditioning does not only work on dogs that want to eat. Therefore, there is a general model that describes how classical conditioning works. It begins with the presentation of a conditioned stimulus (CS), which elicits no response in the subject. After this, the CS is followed by an unconditioned stimulus (US), which elicits an automatic response. This automatic response is called the unconditioned response (UR). When the CS and the US are combined a number of times, a conditioned response (CR) is produced in the subject. The CR is often the same as the UR, but now appears automatically after the CS is presented.

In addition to classical conditioning, there is operant conditioning, in which the reaction of a person or animal is followed by a punishment or reward. When an action is followed by a reward the chance that this action will be performed again increases. When the action is followed by a punishment, the probability that the action will be repeated actually decreases.

With the discovery of classical and operant conditioning, an important step was made in understanding how humans and animals learn. After this discovery, the focus of research became the neural mechanisms underlying these forms of learning. Pavlov himself suggested that classical conditioning reflects a reinforced connection between a CS area and an US area. If activity then takes place in the CS area, it automatically flows to the US area as well, thus eliciting the response. Subsequently, this idea of Pavlov was tested by other researchers. This was done by first teaching rats something and then cutting the connections in the brain. The idea behind this was that if learning took place by strengthening connections, the learned phenomenon should disappear when these connections were removed. However, no sites were found in the cortex where these connections might be located. That is, when cutting into the cortex, or removing different parts of the cortex, the learned response still remained intact after conditioning.

At this point, however, it is clear that the above research is not only cumbersome, but also useless. For starters, learning does not take place primarily in the cortex. In addition, there are so many neurons, and therefore connections between neurons, in the brain that it is unlikely that you will cut away just the one specific learned connection during a study. New studies have therefore been conducted, looking at the activity of individual neurons during classical conditioning. In the process, neurons in a certain part of the cerebellum were found to be essential. These neurons, in the lateral interpositus nucleus (LIP), were not at all active during the first offers of the CR, but after learning the CR, these cells actually became active when offering the CR. Indeed, other studies have confirmed that the LIP must be intact for learning to occur. In addition, the LIP must also still be fully functioning for the CR to take place after the learning phase. So you could say that the LIP is also important for retrieval of this information.

Author: Myrthe Princen (translated by Thomas von Rein)

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