The effect of habituation is to eliminate unnecessary responses, but the main function of learning has usually been thought to be the production of new responses. Traditional psychological theories of learning have assumed that the learning of new patterns of behaviour comes about through the association of a new response with a particular stimulus. Consequently, psychologists usually have either ignored the possibility that nonassociative processes might be sufficient to increase the probability of a new response or regarded it as a nuisance that interferes with the measurement of associative changes. They have rarely treated it as a subject worthy of study in its own right.

This is unfortunate, for the nonassociative phenomenon of sensitization is probably fairly widespread, and it provides a simple means of acquiring adaptive behaviour. Sensitization is said to occur when the repeated presentation of a particular significant stimulus (such as food or electric shock) lowers the threshold for the elicitation of appropriate behaviour to the point where a second stimulus, not normally capable of calling forth that behaviour, now does so. A typical example is provided by the behaviour of the marine worm Nereis. If the worm is kept in a small tube and fed at regular intervals, it becomes progressively more likely to respond to any novel stimulus, such as a change in illumination, by exploratory, food-seeking movements toward the open end of the tube. If, on the other hand, the worm receives mild electric shocks at regular intervals, it becomes progressively more likely to respond to a novel stimulus by withdrawal.

The first point to note about sensitization is its relationship to habituation. Habituation refers to a decline in the probability of responding to a repeatedly presented stimulus. Sensitization, by contrast, refers to an increase in the probability that behaviour appropriate to a repeatedly presented stimulus will occur, even in response to another stimulus. Although these two outcomes cannot be observed simultaneously, it is quite possible that the same operation—repeated presentation of a stimulus—can simultaneously engage two different processes, one causing a decline in the probability of responding to that stimulus, the other causing an increase. Experimental analysis suggests that both processes are real and may be engaged in the same experiment, so that the observed change in behaviour actually results from a mixture of the two. Typically, the process of habituation wins out, and what is observed is an overall decline in responding. But a common finding in habituation experiments is that responding initially increases before declining; the implication is that the initial presentations of a stimulus result in more sensitization than habituation, while further presentations produce more habituation than sensitization. A second factor influencing the relative importance of the two processes is the intensity or significance of the stimulus. A weak stimulus, or one with little intrinsic biological significance, will show relatively rapid habituation and little or no initial sensitization. A stronger stimulus, especially one, such as food or shock, that has substantial significance to the animal, may show marked sensitization and relatively little habituation.

The second point about sensitization is that it may mimic the effect of associative learning or conditioning. As has been mentioned, in a classical conditioning experiment a neutral stimulus, such as a change in illumination, is paired with the delivery of a significant stimulus, such as food or shock. Repeated pairing causes the neutral stimulus to elicit responses initially called forth by the significant stimulus; for example, a change in illumination that has been associated with an electric shock would come to elicit retreat or withdrawal. But in the case of the worm Nereis, experiments demonstrate that the light would come to elicit this change in behaviour whether or not it had been paired with shock: all that is needed is sufficient exposure to the shock. To attribute the change in behaviour toward the light to its association with food or shock, one must show that this change is greater than that which would have resulted from sensitization alone.

The physiological processes underlying sensitization, like those underlying habituation, have been analyzed in experiments on such invertebrate species as Aplysia. Not surprisingly, the mechanisms involved appear to mirror one another. Whereas habituation is correlated with a decline in postsynaptic potentials, sensitization is correlated with an increase in the magnitude of postsynaptic potentials at the same locus.

Although sensitization has often been treated as a nuisance whose effects must be controlled in studies of habituation or associative learning, it remains a process worthy of study in its own right, for the behavioral changes it produces can have significant adaptive value. Without requiring the presumably more complex neural machinery necessary to subserve associative learning, sensitization enables animals to respond to local variations in the occurrence of significant events. If an animal’s sources of food tend to occur together (that is, they are not distributed randomly in time or space), then it pays that animal, having once found food, to continue to behave in a food-gathering manner. Conversely, the animal that is increasingly wary after exposure to danger will have a better chance of evading a lurking predator. Sensitization thus enables an animal to take advantage of statistical regularities in the occurrence of significant events, without requiring it to detect other events that predict the significant ones. No doubt, further advantage accrues to the animal that can perform such calculations, for associative learning provides a powerful means of predicting the future. But there can be equally little doubt that such a process requires a more elaborate nervous system.