Phenomena observed::Classical conditioning


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Phenomena observed


During acquisition the CS and US are paired as described above. The extent of conditioning may be tracked by test trials. In these test trials, the CS is presented alone and the CR is measured. A single CS-US pairing may suffice to yield a CR on a test, but usually a number of pairings are necessary. This repeated number of trials increase the strength and/or frequency of the CR gradually. The speed of conditioning depends on a number of factors, such as the nature and strength of both the CS and the US, previous experience and the animal's motivational state<ref name="Shet"/><ref name="Bouton"/> Acquisition may occur with a single pairing of the CS and US, but usually, there is a gradual increase in the conditioned response to the CS. This slows down the process as it nears completion.<ref name="Schacter 2009 267">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>


In order to make a learned behavior disappear, the experimenter must present a CS alone, without the presence of the US. Once this process is repeated continuously, eventually, the CS will stop eliciting a CR. This means that the CR has been "extinguished".<ref name = Shet/>

Classical conditioning - extinction.svg

External inhibition

External inhibition may be observed if a strong or unfamiliar stimulus is presented just before, or at the same time as, the CS. This causes a reduction in the conditioned response to the CS.

Recovery from extinction

Several procedures lead to the recovery of an extinguished CR. The following examples assume that the CS has first been conditioned and that this has been followed by extinction of the CR as described above. These procedures illustrate that the extinction procedure does not completely eliminate the effect of conditioning<ref name = "Bouton"/>

  • Reacquisition:

If the CS is again paired with the US, a CR is again acquired, but this second acquisition usually happens much faster than the first one.

  • Spontaneous recovery:

Spontaneous recovery is defined as the reappearance of the conditioned response after a rest period. That is, if the CS is tested at a later time (for example an hour or a day) after conditioning it will again elicit a CR. This renewed CR is usually much weaker than the CR observed prior to extinction.

  • Disinhibition:

If the CS is tested just after extinction and an intense but associatively neutral stimulus has occurred, there may be a temporary recovery of the conditioned response to the CS

  • Reinstatement:

If the US used in conditioning is presented to a subject in the same place where conditioning and extinction occurred, but without the CS being present, the CS often elicits a response when it is tested later.

  • Renewal:

Renewal is a reemergence of a conditioned response following extinction when an animal is returned to the environment in which the conditioned response was acquired.

Stimulus generalization

Stimulus generalization is said to occur if, after a particular CS has come to elicit a CR, another similar stimulus will elicit the same CR. Usually the more similar are the CS and the test stimulus the stronger is the CR to the test stimulus.<ref name = Shet/> The more the test stimulus differs from the CS the more the conditioned response will differ from that previously observed.

Stimulus discrimination

One observes stimulus discrimination when one stimulus ("CS1") elicits one CR and another stimulus ("CS2") elicits either another CR or no CR at all. This can be brought about by, for example, pairing CS1 with an effective US and presenting CS2 with no US.<ref name = Shet/>

Latent inhibition

{{#invoke:main|main}} In latent inhibition, an exposure to a stimulus of little or no consequence will prevent a conditioned association with the stimulus being formed. This process will inhibit the formation of memory by preventing learning of the observed stimuli. This process is thought to prevent information overload.<ref name = Shet/>

Conditioned suppression

This is one of the most common ways to measure the strength of learning in classical conditioning. A typical example of this procedure is as follows: a rat first learns to press a lever through operant conditioning. Then, in a series of trials, the rat is exposed to a CS, a light or a noise, followed by the US, a mild electric shock. An association between the CS and US develops, and the rat slows or stops its lever pressing when the CS comes on. The rate of pressing during the CS measures the strength of classical conditioning; that is, the slower the rat presses, the stronger the association of the CS and the US. (Slow pressing indicates a "fear" conditioned response, and it is an example of a conditioned emotional response, see section below.)

Conditioned inhibition

Three phases of conditioning are typically used:

Phase 1
A CS (CS+) is paired with a US until asymptotic CR levels are reached.
Phase 2
CS+/US trials are continued, but these are interspersed with trials on which the CS+ is paired with a second CS, (the CS-) but not with the US (i.e. CS+/CS- trials). Typically, organisms show CRs on CS+/US trials, but stop responding on CS+/CS− trials.
Phase 3
  • Summation test for conditioned inhibition: The CS- from phase 2 is presented together with a new CS+ that was conditioned as in phase 1. Conditioned inhibition is found if the response is less to the CS+/CS- pair than it is to the CS+ alone.
  • Retardation test for conditioned inhibition: The CS- from phase 2 is paired with the US. If conditioned inhibition has occurred, the rate of acquisition to the previous CS− should be less than the rate of acquisition that would be found without the phase 2 treatment.


{{#invoke:main|main}} This form of classical conditioning involves two phases.

Phase 1

A CS (CS1) is paired with a US.

Phase 2

A compound CS (CS1+CS2) is paired with a US.


A separate test for each CS (CS1 and CS2) is performed. The blocking effect is observed in a lack of conditional response to CS2, suggesting that the first phase of training blocked the acquisition of the second CS.

Classical conditioning - blocking.svg

Classical conditioning sections
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Phenomena observed
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