sensitization

Meaning: Sensitization is nonassociative learning: after an intense, noxious or threatening stimulus, an organism responds more strongly to later stimuli. No link between two stimuli is learned. The increase often spreads to other, even mild, stimuli, so a person who has just heard a loud crash may jump at a quiet sound. In the classic sea slug (Aplysia) gill-withdrawal studies, a strong tail shock enhances transmission at the sensory-to-motor synapse, partly through serotonin released by facilitating interneurons. In biology, immune sensitization is a different usage: a first allergen exposure primes the body for a stronger later response.

On the MCAT: Passages give a series of stimuli and a measure of response. A response that falls with repetition of a harmless stimulus is habituation. A response that rises after a strong stimulus is sensitization. Expect questions that ask you to classify the example as nonassociative rather than classical or operant conditioning. You may also be asked to predict what happens when a new strong stimulus follows habituation (dishabituation, the return of the response).

Common confusion: Sensitization is not classical conditioning, because no neutral stimulus is paired with an unconditioned stimulus. Dishabituation also raises a response, but it specifically restores one that had been habituated. Sensitization raises responsiveness more generally.