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Conditioned Stimulus in Psychology

| Pavlov’s classic experiment | Conditioned stimulus vs unconditioned stimulus | Examples | Little Albert experiment | Second-order conditioning | Fear conditioning paradigm | Extinction |

What is a conditioned stimulus

A conditioned stimulus is a neutral cue or event that produces an involuntary response after repeatedly being paired with an unconditioned stimulus that naturally elicits that behavior.

This term originated in a learning process called classical conditioning. Russian physiologist Ivan Pavlov discovered this learning experience when studying digestion in dogs and received the Nobel Prize. Therefore, classical conditioning is also known as respondent conditioning or Pavlovian conditioning​1​.

hand feeds dog eats food in a bowl

Pavlov’s classic experiment on dogs

In his iconic experiment with dogs, Pavlov noticed that his dogs would salivate when they heard him coming even before they saw the food. So he came up with an experiment to study this automatic response.

Each time he fed the dog, he ranged a bell at the exact time or right before presenting the food. Several repetitions later, he found that ringing the bell alone could produce the same salivation in the dogs without the presentation of food.

He concluded that this was associative learning. The dogs had learned to associate the ringing with food and therefore the ringing could bring about the same response.

In the experiment, the food was an unconditioned stimulus (US) that was biologically relevant. It could naturally cause the dogs to salivate and salivation was an unconditioned response (UR). 

After being paired with it repeatedly, the bell ringing, originally a neutral stimulus (NS) that had nothing to do with dog salivation, started to trigger the same response. Bell ringing became a conditioned stimulus (CS) and the salivation induced was a conditioned response (CR).

Even without the presence of the unconditioned stimuli, automatic associations allow the conditioned stimulus to produce the same response​2​.

Conditioned stimulus vs unconditioned stimulus

The main difference between a conditioned and unconditioned stimulus is that an unconditioned stimulus is a reflexive response that can naturally trigger biologically involuntary behavior while a conditioned stimulus can only do this after conditioning. 

Before conditioning, a conditioned stimulus is neutral and cannot trigger the same behavior. There is no biological connection between a neutral stimulus and the unconditioned response.

Examples of conditioned stimuli in everyday lives

Food poisoning

One of the most frequently encountered examples of conditioned stimulus is food poisoning. After eating something (US) and getting sick from it, certain characteristics of that food, such as its scent or taste (CS), can be associated with feeling sick (CR).

The next time you smell or taste it, you automatically reject that food to avoid getting sick again.

Exam anxiety

A student has failed an exam and becomes nervous about taking exams. Items in the exam room such as the desk and the pen have become conditioned stimuli. 

Even when they are not part of an exam(US), these objects (CS) have become associated with negative feelings causing nervousness and anxiety (UR, CR). 

Christmas music

During the Christmas holidays(US), we feel happiness and excitement (UR). Just listening to jolly music or seeing Christmas lights (CS) can quickly evoke happy memories and help one get into the Christmas spirit (CR).

Little Albert experiment

In 1920, American psychologist John B. Watson and his graduate student Rosalie Rayner conducted a famous and ethically problematic experiment called “Little Albert.” 

The classical conditioning process was empirically demonstrated in humans​3​

A 9-month-old child named Albert B was selected for the experiment for showing no fear of furry animals such as rats, rabbits, dogs, and monkeys and inanimate objects such as cotton and masks. Albert, however, showed a fearful reaction whenever a long steel bar was unexpectedly struck behind his back. 

The experiment started when Albert was just over 11 months old. They present him with a white rat and Albert wanted to touch it. As he was reaching for the rat, the experimenters used a steel bar to make a loud clanging sound. 

This was repeated a number of times until Albert reacted with crying and avoidance when the rat was presented without the loud noise. He has been conditioned to fear the rat.

In this experiment, the loud sound was an unconditioned stimulus that could trigger fear, an unconditioned response. After the repeated conditioned stimulus-unconditioned stimulus association, Albert came to fear the rat itself. This type of conditioning produces aversive conditioning.

Five days later, Albert was presented with the rat, a rabbit, a short-haired dog, a sealskin coat, a bearded Santa Claus mask, and other stimuli that were previously neutral stimuli to him. Albert’s fearful conditioned response has generalized to the rabbit, dog, and sealskin coats​4​.

Second-order conditioning

In higher-order conditioning, the conditional stimulus can act as an unconditioned stimulus that can condition another neutral stimulus.

For example, Ivan P. Pavlov conducted the following experiment to demonstrate the second level of conditioning​5​.

Experiment 1: A metronome (CS1) was paired with the presentation of food (US) to evoke salivation in the dogs (CR).

Experiment 2: In the second-order conditioning situation, the sound of a metronome was paired with a black square (CS2) to create a second conditioned stimulus. The black square can now elicit dog salivation by itself.

Fear conditioning paradigm

PTSD is a psychological disorder that can occur in people who have experienced or witnessed traumatic events, such as war, serious injury, or an accident (US) that produces intense fear (UR).

For these people, things that are present or associated with the traumatic event can become conditioned. Such an aversive stimulus can trigger intense bodily reactions long after the event. These contextual stimuli can be sound, smell, object, or crime scene.

The intrusive thoughts and flashbacks (CR) from fear conditioning can be debilitating for the patients and require help from mental health professionals​6​.

Extinction

Later in his extinction experiments, Pavlov demonstrated that this form of learning can be abolished if the conditioned stimulus stops being continuously followed by the unconditioned stimulus.

There is a breakdown in the relationship between CS and US, resulting in a weakened CR. If the bell is repeatedly rung without food, the conditioned response of salivating will gradually disappear resulting in extinction​7​.

When a conditioned response is extinguished, it can still return unexpectedly, albeit weaker. It is called spontaneous recovery​8​.

References

  1. 1.
    Clark RE. The classical origins of Pavlov’s conditioning. Integr psych behav. Published online October 2004:279-294. doi:10.1007/bf02734167
  2. 2.
    Pavlov IP. Tioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex. Oxford University Press; 1927.
  3. 3.
    Watson JB, Rayner R. Conditioned emotional reactions. Journal of experimental psychology. 1920;3(1):1.
  4. 4.
    Harris B. Whatever happened to little Albert? American Psychologist. Published online February 1979:151-160. doi:10.1037/0003-066x.34.2.151
  5. 5.
    Sahley C, Rudy JW, Gelperin A. An analysis of associative learning in a terrestrial mollusc. J Comp Physiol. Published online 1981:1-8. doi:10.1007/bf00612791
  6. 6.
    Peri T, Ben-Shakhar G, Orr SP, Shalev AY. Psychophysiologic assessment of aversive conditioning in posttraumatic stress disorder. Biological Psychiatry. Published online March 2000:512-519. doi:10.1016/s0006-3223(99)00144-4
  7. 7.
    Bouton ME, Moody EW. Memory processes in classical conditioning. Neuroscience & Biobehavioral Reviews. Published online January 2004:663-674. doi:10.1016/j.neubiorev.2004.09.001
  8. 8.
    Rescorla RA. Spontaneous Recovery. Learn Mem. Published online September 2004:501-509. doi:10.1101/lm.77504

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