Habituation · Lobeworks/17

Habituation is the simplest form of learning, a non-associative one: the response of the nervous system to a stimulus falls as the stimulus is repeated without consequence, and the fall cannot be explained by the adaptation of the receptor or by fatigue of the muscle.

Habituation. Habituation is the simplest form of learning, a non-associative one: the response of the nervous system to a stimulus falls as the stimulus is repeated without consequence, and the fall cannot be explained by the adaptation of the receptor or by fatigue of the muscle.

That exclusion is what defines it. In sensory adaptation the receptor itself sends less; in habituation the receptor can go on signalling as before while a circuit further in answers less. The classic case is the sea slug Aplysia: touched again and again on its siphon, it withdraws its gill less each time, and Castellucci and Kandel showed in 1974 that in the short term the sensory neurons release fewer quanta of transmitter per spike onto the motor neurons, with the receiving side unchanged. Longer-lasting habituation involves other, partly postsynaptic changes.

A fan stops being heard. Studying beside a fan, its sound fades within minutes; asked whether it is on, you hear it again at once, because the ear never stopped sending it.

A new stimulus undoes it. A different or stronger stimulus restores the response to the old one (dishabituation), a sign that the circuit has not lost the ability to answer.

It coexists with adaptation and attention. Not noticing something constant can be any of the three, or all at once: the receptor sends less, the circuit answers less, attention looks elsewhere.

In adaptation the signal coming in changes; in habituation what the nervous system does with it changes.

The sound of the fan still reaches the brain; the brain has learned it does not matter.

Questions: You stop hearing a fan while you study: is that adaptation or habituation? Mostly habituation, and attention. In sensory adaptation the receptor itself sends less, so the signal entering the nervous system changes; in habituation the receptor goes on signalling and a central circuit learns to answer less to a repeated stimulus that has no consequence. The test is that the fan comes back at once when someone mentions it: the ear never stopped sending it.