[5] There is an additional connotation to the term habituation which applies to psychological dependency on drugs, and is included in several online dictionaries.
According to the WHO lexicon of alcohol and drug terms, habituation is defined as "becoming accustomed to any behavior or condition, including psychoactive substance use".
This is not to be confused with true habituation to drugs, wherein repeated doses have an increasingly diminished effect, as is often seen in addicts or persons taking painkillers frequently.
Habituation is also proclaimed to be a form of implicit learning, which is commonly the case with continually repeated stimuli.
Furthermore, continued exposure to the stimulus after the habituated response has plateaued (i.e., show no further decrement) may have additional effects on subsequent tests of behavior such as delaying spontaneous recovery.
Stimulus discrimination can be used to rule out sensory adaptation and fatigue as an alternative explanation of the habituation process.
Researchers also use evidence of dishabituation to rule out sensory adaptation and fatigue as alternative explanations of the habituation process.
The changes in synaptic transmission that occur during habituation have been well-characterized in the Aplysia gill and siphon withdrawal reflex.
[14] The experimental investigation of simple organisms such as the large protozoan Stentor coeruleus provides an understanding of the cellular mechanisms that are involved in the habituation process.
[17] Their results showed that the human amygdala responds and rapidly habituates preferentially to fearful facial expressions over neutral ones.
[21][22][20] Eventually, however, more research from the medical and scientific communities concluded that stimulus-dependent variability reflexes is clinically normal.
Stimulus-specificity stipulates that the response-decline is not general (due to motor fatigue) but occurs only to the original stimulus that was repeated.
If a response-decline shows 1) dishabituation, 2) spontaneous recovery that is inversely correlated with the extent of decline, and/or 3) stimulus-specificity, then habituation learning is supported.
Various models have been proposed to account for habituation including the Stimulus-Model Comparator theory formulated by Evgeny Sokolov,[25] the Groves and Thompson dual-process theory,[26] and the SOP (Standard Operating Procedures/Sometimes Opponent Process) model formulated by Allan Wagner[27] 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.
With additional presentations the stimulus model is improved, there is no longer a mismatch, and responding is inhibited causing habituation.
[26] Habituation has been observed in an enormously wide range of species from motile single-celled organisms such as the amoeba[28] and Stentor coeruleus[15] to sea slugs[29] to humans.
[30] Habituation processes are adaptive, allowing animals to adjust their innate behaviors to changes in their natural world.
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.
If prairie dogs never habituate to nonthreatening stimuli, they would be constantly sending out alarm calls and wasting their time and energy.
In one study researchers measured the responses of harbor seals to underwater calls of different types of killer whales.
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".
Eating less during a meal is usually interpreted as reaching satiety or "getting full", but experiments suggest that habituation also plays an important role.
[35] Food variety also slows the rate of habituation in children and may be an important contributing factor to the recent increases in obesity.
[36] Richard Solomon and John Corbit (1974) proposed the opponent-process theory, arguing that habituation is also found in emotional responses.
[37] Habituation abnormalities have been repeatedly observed in a variety of neuropsychiatric conditions including autism spectrum disorder (ASD), fragile X syndrome, schizophrenia, Parkinson's disease (PD), Huntington's disease (HD), attention deficit hyperactivity disorder (ADHD), Tourette's syndrome (TS), and migraine.
In another study, Mitumba chimpanzees in the Gombe National Park were habituated for at least four years before the introduction of systematic data collection.
[45] Researchers also use habituation and dishabituation procedures in the laboratory to study the perceptual and cognitive capabilities of human infants.
A recent fMRI study revealed that the presentation of a dishabituating stimulus has an observable, physical effect upon the brain.
Habituation is also found to be influenced by unchangeable factors such as infant age, gender, and complexity of the stimulus.
In previous theories of habituation, an infant's dishabituation was thought to represent their own realization of the remembered stimulus of stimuli.