At first glance, habituation and sensitization seem to be opposite behavioral responses—one a decrease in responsiveness and the other an increase— but, in fact, they are physiologically different processes, each with its own set of unique characteristics.
At the physiological level, the two responses are determined by contrasting neurological processes that take place in different parts of the nervous system. Habituation is thought to take place primarily in the reflex arc (or SR) system, which consists of short neuronal circuits between sense organs and muscles. In contrast, sensitization is assumed to occur in the state system, or that part of the nervous system that regulates an organism's state of responsiveness. The SR system controls specific responses, whereas the state system determines an organism's general level of readiness to respond. The interaction between habituation and sensitization and these systems determines the exact outcome of a response. At the cellular level, habituated sensory neurons produce fewer neurotransmitters on the postsynaptic membrane, while sensitized neurons are stimulated by other neurons to increase neurotransmitter production and hence responsiveness of the nerves. Thus, while their ultimate neurological effects are somewhat opposite, the mechanisms by which such effects are achieved are quite different.
Other important differences between habituation and sensitization include contrasting recovery times, opposite patterns of stimulus specificity, and differences in responsiveness to stimulus intensity. Sensitization is generally characterized by a short-term or spontaneous recovery, as are some cases of habituation. In certain situations, however, recovery from habitua-tion may take several days, and even then it may result in incomplete or less intense responses.
In comparison to sensitization, habituation is usually elicited by very specific sign stimuli such as certain colors, shapes, or sounds. Thus, even after complete habituation to one stimulus, the organism will still respond fully to a second stimulus. Sensitization, on the other hand, can be characterized as a more generalized response, one in which a single stimulus will result in complete sensitization to a variety of stimuli. Such fundamental differences between these two learning processes reflect differences in their function and survival value. It is a clear advantage to an organism to increase its general awareness to a variety of stimuli (such as occurs in sensitization) once it is alarmed. A similar generalized pattern of habituation, however, would shut down the organism's sensitivity to many important stimuli and possibly put the organism in danger.
A final important difference between habituation and sensitization is the manner in which the two processes are affected by stimulus strength. Habituation is more likely to occur if the repeated stimulus is weak, and sensitization will occur when the stimulus is strong.
These various characteristics have important survival implications, especially for species that rely on stereotypic responses to avoid predation and other life-threatening situations. They ensure that the response is elicited in a timely fashion, that the animal is returned to a normal state in a relatively short period of time, and that the animal is not overwhelmed with sensory input.
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