habituation and sensitization

Habituation and sensitization are the two simplest ways a nervous system learns: it learns to react less, or to react more, to something it keeps meeting. They are called non-associative learning, meaning the animal is not linking two different events together — it is simply adjusting its reaction to one repeated thing. In habituation, a stimulus that happens over and over without any consequence gradually stops grabbing your attention. Think of moving in next to a railway: the first night the trains jolt you awake, but after a week you sleep right through them. In sensitization, the opposite happens — after something painful or alarming, you become jumpier and respond more strongly even to mild, harmless events. After a near car crash, your heart leaps at the next honk that would normally not faze you.

These two pulls work in different directions, and which one wins depends on the situation. Habituation lets a creature ignore the harmless and the familiar so it does not waste energy reacting to every rustle of leaves; sensitization makes it extra-watchful when the world has just turned dangerous. Both are genuine learning, not mere fatigue: a habituated animal will snap back to full reaction the instant the stimulus changes or a new threat appears, which a tired-out muscle could not do. Scientists love these forms of learning because they are so basic that they can be traced in animals with only a handful of nerve cells — the sea slug Aplysia, for instance — where the very synapses that grow weaker (in habituation) or stronger (in sensitization) can be watched directly.