Live near a train line long enough, and the sound of passing trains eventually stops registering consciously at all. Hear a loud, sudden bang in an unfamiliar, tense situation, though, and each subsequent unexpected noise can make you flinch harder than the last, not less. Both are examples of how repeated exposure to a stimulus changes a response over time — but they're opposite effects, called habituation and sensitisation, and which one occurs depends heavily on the nature and intensity of the stimulus itself.
Habituation: the brain learning a stimulus is safe to ignore
Habituation is a decrease in response to a stimulus that occurs after repeated, harmless exposure to it. It's one of the simplest and most widespread forms of learning across the animal kingdom, and it serves an obviously useful function: a nervous system that responded with full alertness to every single repeated, ultimately inconsequential stimulus — a ticking clock, a hum of traffic, a familiar smell — would waste enormous cognitive and physiological resources on things that have already proven, through repetition, not to require a response. Habituation lets an organism selectively tune out the predictable and harmless, freeing attention for stimuli that are actually novel or potentially significant.
Sensitisation: the opposite response, and when it takes over
Sensitisation, the reverse process, increases responsiveness to a stimulus after repeated exposure, and it tends to occur specifically with intense, painful, or threatening stimuli rather than mild, neutral ones. A single strong or alarming stimulus can temporarily lower the threshold for a broad range of subsequent responses, meaning even unrelated stimuli that follow can provoke a stronger-than-normal reaction for a period afterward. This makes adaptive sense in its own right: an organism that has just encountered something genuinely threatening benefits from heightened vigilance immediately afterward, even at the cost of some false alarms, rather than complacently tuning the danger out the way it would with a harmless, repeated stimulus. Whether repeated exposure to a given stimulus produces habituation or sensitisation ultimately depends on factors like the stimulus's intensity and the context in which it occurs, rather than either response being a fixed, universal rule.
What we're still unsure about
The basic phenomena of habituation and sensitisation, and their opposite directional effects, are well documented across a wide range of species and stimuli, including at the level of individual neurons in simpler organisms studied in detail. What remains more genuinely complex is predicting, for a specific stimulus and organism, exactly which effect will dominate and how strongly, since the two processes aren't mutually exclusive and can operate simultaneously on different underlying pathways — a phenomenon called dual-process theory, which holds that both habituation and sensitisation are always occurring to some degree, with the net observed response reflecting whichever process currently has the upper hand, a balance that's often difficult to predict precisely in advance for any specific real-world case.
This sits inside Habituation & Sensitisation, one of seven topics in Behavioral Psychology, one of four domains in Psychology, one of seventeen subjects the app can quiz you on.