Learning, Cognition, and Conditioning – Pavlov’s Experiment


This blog is aimed at studying the fundamental contribution of Pavlov.

Pavlov’s Experiment: Definition of the terms

  1. Unconditional Stimulus (US): A stimulus that biologically elicits a reflex and does that unconditionally and reliably
  2. Unconditional Response (UR): biologically elicited reflect
  3. Conditional Stimulus (CS): A stimulus when paired with an unconditional stimulus still generates an unconditional response.
  4. Conditional Response (CR): A stimulus that elicits on a conditional stimulus.

Temporal Relations Between Unconditional Stimulus and Conditional Stimulus

One of the primary law of association is contiguity, which means that any two ideas will get associated if they occur in the brain at the same time. Connecting this with Pavlov’s experiment, it simply means that a stimulus generates some activity in the brain, which he calls as “center of activity in the cerebrum.” If the centers are regularly activated together, they will become linked, so that when one center is activated, other will be automatically activated.

To understand this further, let us explore the temporal relations between US and CS.

First Relation:

From the law of contiguity, it occurs that both US and CS should occur simultaneously – exact simultaneous presentation. It that happens, there is a certain time lack before CS begins, which makes our conditioning weak (weak conditioning.)

Second Relation:

If the CS begins slightly before the US, it thus support the law of contiguity. This means, CS should precede the US. And for CS to become effective, it should act as a signal to the organism.

Third Relation: (Compound stimulus)

If CS starts before the US, say 30 seconds before and overlaps US on the onset, is results in delay conditioning. However, the result of this is astonishing. The dog salivates first at the response of the musical tone (CS), then stops for a while and again salivates till the US (food) arrives. Basically, this suggest that time itself can act as a CS eliciting the salivation (CR). In the experiment, it was a compound stimulus of the musical tone and certain passage of time, which elicited a conditional response.

Compound Conditioning

In compound conditioning, Pavlov uses 2 conditional stimulus at the same time. This experiment is based on one of the old law of association, the law of vividness. The law states that a given vivid idea automatically corresponds with the concurrent vivid ideas and overlook/neglect the weak ideas. Pavlov took two CSs – light and tone and one US – food. He studied that if both the CSs are presented with an equal intensity,  or presented alone, it will elicit a CR.

However, if one is greater in the intensity, then, only the more vivid one would elicit a response. This phenomenon, wherein one CS of the compound is much more powerful than the other is call OVERSHADOWING.

Excitation and Inhibition

The conditioning of a distinct positive response, such a salivation is called conditioning of excitation.

On returning to delayed conditioning, one can witness inhibition. Let us understand this further. In delayed conditioning, after several pairing of the CS-US, CR occurs only at the end of the CS and the beginning of the anticipated time of the US. Why the CR disappears at the start of the CS and then appears at the later stage? According to Pavlov, the answer to this was “inhibition.” He suggested that if we introduce a new CS, say a flash of light, before the CR appears, that is, during the much earlier stages of the CS, it invokes a CR, which in this case is salivation. Now, if the earlier CS and the CR had been completely disconnected, the light would not have caused a CR. “The unexpected appearance of the salivation when the light flashed during the CS indicated to Pavlov that the light interfered with the cortical process, specifically, inhibition, thus releasing the suppressed CR to the tone.

Pavlov distinguished several kinds of inhibitions, of which extinction is the most important one. When we present an established CS alone, without further reinforcement; the CR occurs for a while, fades out, and finally falls to zero. It is plausible to say here that the CS has been disassociated from the CR, even that the CS is no longer a CS but a neutral stimulus, and there is no CR—the reflex has literally extinguished. The neutral stimulus could be any event that does not result in an overt behavioral response from the organism under investigation.

Further investigations related that it is not the case. The CS-CR association is still present only inhibited by the extinction procedure. To take this further, if we remove the animal for some time, say 20 minutes, and then present the conditional stimulus again, CR (salivation) will appear. The reflex is recovered, which means that the reflex was not unlearned by extinction, but was actively inhibited. It also indicates that inhibition is a temporary state abating with time.

Let’s look at another evidence for this above argument. If the extinction goes well beyond the point at which zero salvation occurs , presenting the CS over and over again, additional unreinforced pairings will have no effect. Pavlov found that the longer extinction continued past zero, the longer it took for the CR to spontaneously recover, indicating that additional reinforced  pairing deepened and so prolonged the inhibition of the CR.

Higher Order Conditioning

If we take an established CS (CS1) and pair it with a new CS (CS2), the CR might be elicited in the right conditions. Pavlov called this as secondary conditional reflex. The building of a new CS on old CSs is called Higher Order Conditioning.

Now, if CS2 appears before CS1, not overlapping, a CR will be elicited. However, if CS2 overlaps CS1, then the conditional response is inhibited – conditional inhibition occurs. So, in this case, salvation will occur at CS1. However, no salvation will occur on the compound stimulus (CS1 + CS2). This is because the reflex is inhibited by the presence of CS2.

The question here also arises that how far we can push the higher order conditioning. HOC cannot be pushed further than secondary conditioning and defensive conditioning cannot go beyond the third-order conditioning. TILL NOW, PAVLOV’S FINDING HAS NOT BEEN OVERTURNED.

Generalization and Discrimination

Another law of association is similarity. This means similar ideas are easily associated. Conditional response not only occurs for the trained CS but also for similar CSs. For example, if the dog is trained to salivate at the tone of 1000 cycles per second, he will salivate if the similar tone is presented from near or farther (above or below). In summary, CR will occur for the related tones. Closer the CS to the original tone, CR will be elicited. Farther the CS from the original tone, CR will fade.

The extension of a CR to stimuli similar to the CS is called generalization. The gradual weakening of the CR as the test stimuli increasing differ from the CS is called generalization gradient. Having understood this, let us now look at creating a stimulus discrimination.

We can set excitation and inhibition against one another, thus generating a stimulus discrimination. Let’s taken an example here to understand this further. Suppose we condition salivation to a CS (let’s call is CS+) of a luminous circle on the screen before the dog’s eye. Now, let us begin varying the size and present ellipses varying from near circular to extreme ellipses (CS-). We will notice that generalization gradient will occur. Now, let us present extreme ellipses without reinforcing it, we will notice that inhibition starts building up for the extreme ellipses. We have created a stimulus discrimination. The dog will now respond to CS+, but will not respond to CS-. Now, if we make the successive (CS-) more and more circular, the generalization gradient of excitation and inhibition will begin to clash, thus giving way for antagonistic tendencies in the dog.

Pavlovian Conditioning of Humans:

Behaviorists eagerly adopted Pavlov’s method, because based on this experiments, they had a chance to study human behavior objectively. John. B. Watson was the first American Phsychologist who actually took Pavlov’s study and applied it to human behavior. Waston is also known as the originators of behaviorism in this respect. He began studying the behavior of children, as well, concluding that humans were simply more complicated than animals but operated on the same principles. All animals, he believed, were extremely complex machines that responded to situations according to their “wiring,” or nerve pathways that were conditioned by experience.

There are enormous data available on Watson’s research and Pavlovian connectivity. Let us for now move to the associative theory of Pavlovian Conditioning.

Associative Theory of Pavlovian Conditioning:

Stimulus Substitution Theory:

The Pavlovian theory of substitution conceives substitution as a substitution of the unconditioned stimulus (US) by the conditioned stimulus (CS) in the activation of the representation of the former.

Stimulus substitution is also known as reflect transfer account.

The theory is based on an assumption that UR and CR are identical or nearly identical responses. Per associationism, stimulus substitution holds that even if US and CS are paired, association will not happen between them. Infact association will happen with CR+UR. Now, we can replace UR with CR as they are identical or nearly identical. So, Pavlovian conditioning involves S-R learning (stimulus-response learning) between CS and CR, not S-S learning (stimulus-substitution learning) between CS and US.

This theory made five claims that have proven controversial. I will discuss each claim in my upcoming blog.

  1. Nature of the association: Stimulus substitution theory holds that the association acquired during conditioning is between the CS and the CR it evokes.
  2. Cause of association: The sole cause of association between CS and CR is the close contiguity between CS and CR.
  3. Specificity of the CS: The cue that evokes the CR is the CS alone.
  4. CR-UR equivalence: The CR is essentially the same as the UR, differing at most only in quantity, not in behavioral form.
  5. Equipotentiality: The nature of the CS is irrelevant to learning; any stimulus the animal can perceive can become CS through pairing with any US.

Reference: Learning and Cognition, Thomas Hardy Leahy and Richard Jackson Harris, and other online references.

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Two roads diverged in a yellow wood, And sorry I could not travel both And be one traveller, long I stood And looked down one as far as I could To where it bent in the undergrowth; Then took the other, as just as fair, And having perhaps the better claim, Because it was grassy and wanted wear; Though as for that the passing there Had worn them really about the same, And both that morning equally lay In leaves no step had trodden black. Oh, I kept the first for another day! Yet knowing how way leads on to way, I doubted if I should ever come back. I shall be telling this with a sigh Somewhere ages and ages hence: Two roads diverged in a wood, and I-- I took the one less traveled by, And that has made all the difference
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