Latent Inhibition

Latent inhibition is a term used in classical conditioning, also known as Pavlovian conditioning, which was discovered by Russian physiologist Ivan Pavlov (1849–1936) and his assistant in 1901.

Pavlov was interested in natural responses, which are responses that occur reflexively as a reaction to a stimulus. As an example, a dog will salivate when it smells food. Pavlov found that he could trigger such a natural response to an introduced stimulus. For instance, by sounding a buzzer at the time the food is provided to the dog, the dog will eventually associate the buzzer with the subsequent delivery of food, and after a period of time, the animal will salivate when it hears the buzzer, even if no food is forthcoming. Salivation, therefore, becomes a learned response to the introduced stimulus and is called a classically conditioned response.

In 1959, psychology researchers Robert Lubow and A. Ulrich Moore were interested in classical conditioning and published a paper describing their experiments on sheep and goats. They focused the natural reaction of a flexion (bending) of the leg in response to a mild shock. For the experiment, they exposed half of the animals to either a flashing light or a turning rotor, and they exposed the other half to neither. Afterward, they displayed the flashing light or the turning rotor to both groups, and paired it with the shock. They then observed how quickly the sheep and goats acquired a leg flexion when the light flashed or the rotor turned. Their work demonstrated a retarded development of a classically conditioned response among the animals if the stimulus—known as a conditioned stimulus (CS)—was one with which they were already familiar. In other words, prior exposure to the stimulus inhibited learning.

Other scientists had noted this effect before Lubow and Moore, but it was the two psychology researchers who coined the term latent inhibition. Their 1959 research on sheep and goats also prompted many more studies into latent inhibition. The phenomenon has since been demonstrated in other animals and associated with a wide range of behaviors. A rabbit study is a well-known example. Normally, a puff of air in a rabbit's eyes will cause it to blink: The stimulus of the air puff results in the natural response of blinking. For this study, the rabbits were split into two groups: one that had received prior experience with the CS, in this case a tone, and the other that had no prior experience with the CS. The experiment then paired the CS with the puff of air. The unexposed group of rabbits learned to blink in response to the tone more quickly than did those rabbits that were previously exposed. The previously exposed rabbits, therefore, exhibited latent inhibition.

Further studies established that latent inhibition occurs in humans, too, but only when the CS is masked, or included within another task, so that the subject is distracted from focusing mainly on the CS. Further research has shown that, depending on the experimental design, latent inhibition can sometimes occur in humans without masking.

Although latent inhibition has been known since the middle of the twentieth century, research continued to fully understand this effect and its applications as of 2015.

See also Schizophrenia .