Conditioned Stimulus

Conditioned Stimulus Definition

A conditioned stimulus is a substitute stimulus that triggers the same response in an organism as an unconditioned stimulus. Simply put, a conditioned stimulus makes an organism react to something because it is associated with something else. Conditioned stimuli begin as neutral stimuli that do not illicit a response until conditioning has occurred via repeated stimulation. 

In other words, the response is learned over time. After repeated exposure, the neutral stimulus becomes paired with the unconditioned response and becomes a conditioned stimulus. 

The conditioned stimulus is also known as classical conditioning or Pavlovian conditioning, named for the Russian scientist Ivan Pavlov who discovered the phenomenon during his experiments with dogs. On the other side of the spectrum from the conditioned stimulus is the unconditioned stimulus. Unconditioned stimuli automatically trigger responses (natural reflexes) in organisms.

How Conditioned Stimulus Works

The period required for a neutral stimulus to become a conditioned stimulus is called the acquisition phase. During this phase, the organism learns to connect, or pair, the neutral stimulus to the unconditioned response and transform the effect into a conditioned stimulus. Timing is important to enable a conditional stimulus to develop. 

Depending on what organism is being conditioned, the interval between presentations of the stimulus can be from five seconds to several hours. On the other hand, if the conditioned stimulus no longer follows the unconditioned stimulus (meaning the pairing or learned behavior is lost), over time the conditioned response will fade in a process called extinction.

It is possible for a conditioned stimulus to, in turn, condition another stimulus. Known as higher-order conditioning or second-order conditioning, this process causes a new neutral stimulus to pair with an existing conditioned stimulus. As a result, the newly-made conditioned stimulus can elicit the same response as the original conditioned stimulus (see example #3). 

Second-order conditioning is usually the highest level of conditioning that can be achieved. The response is usually lost when attempting to propagate an effect through more than two levels of conditioning.

Examples of Conditioned Stimulus

Pavlov’s Dogs

In the mid-1920s, Russian scientist Ivan Pavlov was studying the digestive system of dogs by measuring the amount of saliva they produced in response to various foods (response to unconditioned stimulus, natural reflex). During these experiments, he noticed that the dogs started to salivate before they even tasted the food (response to conditioned stimulus). 

They were salivating at the sight of food, when they heard the footsteps of the lab personnel (meaning it was feeding time) and even at the sight of an empty food bowl. He initially termed these types of salivation “psychic secretions”. 

Pavlov went on to conduct several carefully controlled experiments to see what types of stimuli that had nothing to do with food would cause the dogs to salivate. Some of the stimuli that caused salivation were the sound of a bell, a touch on the dog’s leg and a light. Further work demonstrated that the effect of a conditional stimulus can fade over time. 

For example, over time, if a bell was rung but not followed by food, the dogs stopped salivating at the sound of the bell (extinction). Through this ground-breaking work, Pavlov had discovered the two types of responses that organisms have in response to their environment: unconditioned and conditioned.

Traffic Intersection

Imagine a person driving through an intersection at a green light and being side-swiped by another driver who ran the red light in their direction. A single experience like this can cause a traffic intersection to become a conditioned stimulus for the driver who was hit. 

From that point on, approaching an intersection, now the neutral stimulus, may cause them to have sweaty palms, grip the steering wheel harder, and have an increased heart rate and dilation of the pupils. The latter are all unconditioned responses that are unlearned and occur naturally in response to fear and anxiety.

Higher-Order or Second-Order Conditioning

The reaction of pets to the sound of a can opener is another classic example of a conditioned stimulus eliciting an unconditioned response. Second-order conditioning can be demonstrated by placing another conditioned stimulus before the sound of the can opener. Suppose the pet food is kept in a cupboard that has developed squeaky hinges. 

Over time, the animal can begin to associate the squeak with being fed, and have the same reaction as if it had heard the can opener. This is the second-order conditioning. However, there are limits to this chain reaction. If another neutral stimulus, say a bell ringing, is added before the squeak of the cupboard door, it is unlikely that the pet will begin to associate the bell with being fed, and have the same reaction as it would to the can opener. 

Recent research has confirmed that second-order conditioning involving auditory cues takes place in the amygdala portion of the rat brain. Interestingly, researchers have also discovered that if the rat amygdala is damaged after second-order conditioning has already taken place, the conditioning is enhanced and prolonged. This discovery has important implications for understanding certain neuropsychiatric disorders where the undesired behaviors are perpetuated by conditioned stimuli.

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