<P> The stimulus - model comparator theory emerged from the research of Sokolov who used the orienting response as the cornerstone of his studies, and operationally defining the orienting response as EEG activity . Orienting responses are heightened sensitivity experienced by an organism when exposed to a new or changing stimulus . Orienting responses can result in overt, observable behaviors as well as psychophysiological responses such as EEG activity and undergo habituation with repeated presentation of the eliciting stimulus . The Sokolov model assumes that when a stimulus is experienced several times the nervous system creates a model of the expected stimulus (a stimulus model). With additional presentations of the stimulus the experienced stimulus is compared with the stimulus model . If the experienced stimulus matches the stimulus model responding is inhibited . At first the stimulus model is not a very good representation of the presented stimulus and therefore there is a mismatch and responding continues, but with additional presentations the stimulus model is improved; consequently there is no longer a mismatch and responding is inhibited causing habituation . However, if the stimulus is changed so that it no longer matches the stimulus model the inhibition of the orienting response is weakened, and an orienting response returns . Sokolov places the location of the Stimulus - Model within the cerebral cortex of the brain . </P> <P> The Groves and Thompson dual - process theory of habituation posits that two separate processes exist in the central nervous system that interacts to produce habituation . The two distinct processes are a habituation process and a sensitization process . The dual - process theory argues that all noticeable stimuli will elicit both of these processes and that the behavioral output will reflect a summation of both processes . The habituation process is decremental, whereas the sensitization process is incremental enhancing the tendency to respond . Thus when the habituation process exceeds the sensitization process behavior shows habituation, but if the sensitization process exceeds the habituation process, then behavior shows sensitization . Groves and Thompson hypothesize the existence of two neural pathways: an "S-R pathway" involved with the habituation process, and a "state pathway" involved with sensitization . The state system is seen as equivalent to a general state of arousal . </P> <P> Habituation has been observed in an enormously wide range of species from motile single - celled organisms such as the amoeba and Stentor coeruleus to sea slugs to humans . Habituation processes are adaptive, allowing animals to adjust their innate behaviors to changes in their natural world . A natural animal instinct, for example, is to protect themselves and their territory from any danger and potential predators . It is obvious that an animal needs to respond quickly to the sudden appearance of a predator . What may be less obvious is the importance of defensive responses to the sudden appearance of any new, unfamiliar stimulus, whether it is dangerous or not . An initial defensive response to a new stimulus is important because if an animal fails to respond to a potentially dangerous unknown stimulus, the results could be deadly . Despite this initial, innate defensive response to an unfamiliar stimulus, the response becomes habituated if the stimulus repeatedly occurs but causes no harm . An example of this is the prairie dog habituating to humans . Prairie dogs give alarm calls when they detect a potentially dangerous stimulus . This defensive call occurs when any mammal, snake, or large bird approaches them . However, they habituate to noises, such as human footsteps, that occur repeatedly but result in no harm to them . If prairie dogs never habituate to nonthreatening stimuli, they would be constantly sending out alarm calls and wasting their time and energy . However, the habituation process in prairie dogs may depend on several factors including the particular defensive response . In one study that measured several different responses to the repeated presence of humans, the alarm calls of prairie dogs showed habituation whereas the behavior of escaping into their burrows showed sensitization . </P> <P> Another example of the importance of habituation in the animal world is provided by a study with harbor seals . In one study researchers measured the responses of harbor seals to underwater calls of different types of killer whales . The seals showed a strong response when they heard the calls of mammal - eating killer whales . However, they did not respond strongly when hearing familiar calls of the local fish - eating population . The seals, therefore, are capable of habituating to the calls of harmless predators, in this case harmless killer whales . While some researchers prefer to simply describe the adaptive value of observable habituated behavior others find it useful to infer psychological processes from the observed behavior change . For example, habituation of aggressive responses in male bullfrogs has been explained as "an attentional or learning process that allows animals to form enduring mental representations of the physical properties of a repeated stimulus and to shift their focus of attention away from sources of irrelevant or unimportant stimulation". </P>

One type of physiological process that is associated with habituation is