habituation

Meaning: Habituation is a decrease in response to a harmless stimulus that is presented repeatedly. The organism learns to ignore it, which frees attention for new events. It is non-associative learning, because no link forms between two stimuli or between a behavior and a consequence. Its counterpart is sensitization, an increased response, often after an intense or threatening stimulus. If a new stimulus restores the response, that is dishabituation.

On the MCAT: Expect a startle response that fades with repeated sound, or an infant looking at the same picture again and again. Looking time falls, and renewed looking at a new picture suggests the infant can tell the two apart. In Aplysia, repeated touch weakens the gill-withdrawal reflex as the sensory neuron releases less neurotransmitter.

Common confusion: Habituation is not sensory adaptation. Adaptation is reduced receptor sensitivity to a constant stimulus, while habituation is a learned drop in response even though the stimulus is still detected. It is also not fatigue, since a different stimulus can restore the response. And it is not extinction, which is loss of a conditioned response when the conditioned stimulus is no longer paired with the unconditioned stimulus; that is associative learning.