In Psychology, the term ‘Blocking’ refers broadly to the classical conditioning phenomenon that has profoundly influenced thinking about associative learning. In simple words, blocking effect may refer to the failure of expression in a conditioned environment where pre-conditioned expression holds its ground beforehand.

The blocking effect we are going to study here was discovered by Leon J. Kamin. Kamin was an American psychologist known for his great contributions to the theories of learning and also for his estimates of IQ heritability. In Kamin’s blocking effect, there is a conditioning of an association between two stimuli; Conditioned Stimulus (CS) & Unconditioned Stimulus (US). The association can be sometimes impaired but always are judged over Conditioned response (CR). This happens during the conditioning process where the first CS is presented with a second CS that has already been associated with the US. In easy terms, the association fails when 2 conditioned stimuli are associated with the US; but only when one among the two of them has already been associated earlier with the US.

Through CS-US association formation, the animal in consideration acquires a behavior (conditional response, CR) to the CS that typically resembles the (unconditional response, UR; but not always), precedes the US in onset time, and reaches a maximum magnitude at about the time of US onset.


In the field of Psychology, ‘Conditioning’ is also known as ‘Pavlovian’ or ‘Respondent conditioning’. It refers to a learning procedure in which a biologically potent stimulus is paired with a neutral stimulus. The biologically potent stimulus might be food and neutral stimulus, a bell.

Conditioning also refers to the learning process that arises from this pairing. As a result of this pairing, the neutral stimulus gives the same kind of response as shown by the potent stimulus. Thus, it got stated that classical conditioning occurs when a conditioned stimulus (CS) is paired with an unconditioned stimulus (US).

In the process of pairing, a conditioned stimulus is a neutral stimulus. The one that is the unconditioned stimulus, is a biologically potent one. The unconditioned response (stated as UR) to the unconditioned stimulus can just be considered as a reflex response (just as in the case of salivation). After the pairing is repeated an organism exhibits, a conditioned response (CR) to the conditioned stimulus. It may happen when the conditioned stimulus is presented alone. A conditioned response occurs after only one pairing. The conditioned response is acquired through experience. It is also less permanent than the unconditioned response.


The complete process can be understood by an easy example. Any agent (consider a mouse) is exposed to a source of light (here, light is the first conditioned stimulus, CS1). It is exposed together with food (food here is the unconditioned stimulus, US). After repeated pairings of CS1 and the US, you will notice an effect. The agent salivates when the light comes on. We call this a conditioned response, CR.

After this, some more conditioning trials are performed. This time, the light (CS1) and a tone (CS2) were paired together with the US. When this was tested, it was seen that the agent does not salivate to the tone (CS2). The tone is taken as the second conditioning stimulus here.

In other words, an association between the tone (CS2) and the US has been “blocked”. This is because the association between CS1 and the unconditioned stimulus (US) already exists. This effect was most accurately explained by the Rescorla–Wagner model. The model states that if one CS (here, the light) is already fully associated with the US, nothing will be seen from a second CS (here, the tone).


The reverse of blocking also exists. It is often called Backward blocking. In backward blocking, the subject is exposed to the compound stimulus (CS1 and CS2 together). For the first time, both CS1 + CS2 are taken. In the second time, only CS1 alone was taken. In some human and animal studies, subjects show a reduction in the association. The effect is often seen to be weaker than the standard blocking effect, and also vanishes under some conditions. This effect was not predicted by the Rescorla–Wagner model. Still, there are other models that have been proposed that capture this effect.

In the field of psychology, the term blocking refers broadly to the failures to express knowledge or skill. It happens because of failures of learning or memory. In everyday experience, it may be seen when “blocking” of the name of a familiar face or object happens. Kamin’s blocking refers to failures of learning. It can also be the expression of conditioned responses (CRs) when a target conditioned stimulus (CS) is present.


A series of experiments were reported on three chapters in Kamin’s book published in the 1960s. Kamin showed that the earlier conditioning response as seen with CS1 interferes with the conditioned response with that of CS2. The interference happens when they both are presented together as a compound stimulus.

A related chapter by Kamin appeared in 1969. The third chapter on blocking also came up in the same year 1969. Blocking is a reliable cross-species learning effect. It has been studied mainly by using Classical Conditioning. In this, animals come to show their learned behaviors of a biological outcome; typically food, through a behavioral conditioned response. The conditioned response often resembles the unconditioned response for the outcome.


In Pavlov’s studies, the salivation as a response to food came to be aroused by a bell. The bell here is termed the conditioned stimulus (CS). This bell preceded food delivery. This anticipatory response allows us to understand the strength of the conditioned association.

It was originally seen and suggested that, if an unconditioned stimulus is already fully satisfied by one stimulus, then the addition of a new stimulus provides no new significant information about the US. Also, the unconditioned stimulus will not activate or support the learning process. This learning process is responsible for establishing a new association between Conditioned stimulus and an unconditioned stimulus. Since it was discovered, blocking has become the cornerstone of all the modern learning theories as well.

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