habituation

habituation You are sitting quietly, reading a book. Suddenly, a tap on the window startles you. Your heart rate rises momentarily and you glance at the window to see if someone is there. But you see that the wind has strengthened and the branch of a tree has touched the glass. As your interest returns to the book, repeated taps of the branch on the window evoke progressively smaller reactions, until they hardly intrude into your reading at all.

The gradual reduction of existing responses to repeated presentations of a stimulus is ‘habituation’. At first sight, it might seem that habituation is nothing more than some sort of fatigue process in the relevant sensory or motor neural pathways. But habituation has several key characteristics that identify it as an active process that is biologically useful. For example, once the response to a familiar stimulus has habituated, another intense stimulus can cause the response to the familiar stimulus to return immediately, by a process of dishabituation. Furthermore, habituation is relatively stimulus-specific, so that responses to the repeated stimulus are reduced but responses to different, novel stimuli are not. Neither of these characteristics is consistent with a fatigue mechanism for habituation. Instead, they indicate an active, stimulus-specific form of learning. More complex forms of learning, such as conditioning, involve an association between two or more stimuli or events. Habituation does not, so it is regarded as a non-associative form of learning.

Habituation is ubiquitous in the animal kingdom. It has been observed in the gill-withdrawal reflex of marine molluscs, in limb-withdrawal reflexes mediated by the spinal cord of mammals, and in the auditory startle response discussed above. Just like more complex forms of memory, habituation initially depends upon short-term mechanisms that last between minutes and hours. With more stimulus presentations occurring over hours or days, long-term mechanisms take over and these can support habituated responses over much longer periods.

Habituation is present from an early stage of development and can be seen in infants as young as two months, who like to fixate and inspect novel visual stimuli. When presented with pairs of pictures that always include both a familiar and a new scene, the infants will fixate the new picture — indicating a habituation of the fixation response to the familiar scene. Habituation to the previously presented stimulus maximizes input from the new stimulus. Other novel stimuli may be of great significance because they may signal danger. Reflex responses to such stimuli provide appropriate defensive behaviours. But if the stimuli are not intense and no damage is done, then repeated presentation leads to habituation.

Habituation allows the nervous system to optimize sensory–motor processing by eliminating unnecessary responses. It allows us to adapt to the familiar in order to preserve our ability to react rapidly and appropriately to the new.

Christopher Yeo