Habituation
Habituation is a form of non-associative learning in which an organism’s non-reinforced response to an inconsequential stimulus decreases after repeated or prolonged presentations of that stimulus. For example, organisms may habituate to repeated sudden loud noises when they learn that these have no consequences.
Habituation can occur in responses that habituate include those that involve an entire organism or specific biological component systems of an organism. The broad ubiquity of habituation across all forms of life has led to it being called "the simplest, most universal form of learning...as fundamental a characteristic of life as DNA." Functionally, habituation is thought to free up cognitive resources for other stimuli that are associated with biologically important events by diminishing the response to inconsequential stimuli.
A progressive decline of a behavior in a habituation procedure may also reflect nonspecific effects such as fatigue, which must be ruled out when the interest is in habituation. Habituation is relevant in psychiatry and psychopathology, as several neuropsychiatric conditions including autism, schizophrenia, migraine, and Tourette syndrome show reduced habituation to a variety of stimulus-types both simple and complex.
Drug habituation
There is an additional connotation to the term habituation which applies to psychological dependence on drugs, and is included in several online dictionaries. A team of specialists from the World Health Organization "a desire "little or no tendency to increase the dose"; 3) "some degree of psychic dependence on the effect of the drug, but absence of physical dependence and hence of an abstinence syndrome"; 4) "detrimental effects, if any, primarily on the individual". However, also in 1964, a committee from the World Health Organization once again convened and decided the definitions of drug habituation and drug addiction were insufficient, replacing the two terms with "drug dependence". Substance dependence is the preferred term today when describing drug-related disorders, whereas the use of the term drug habituation has declined substantially. 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.Characteristics
Habituation as a form of non-associative learning can be distinguished from other behavioral changes by considering the characteristics of habituation that have been identified over several decades of research. The characteristics first described by Thompson and Spencer were updated in 2008 and 2009, to include the following:Repeated presentation of a stimulus will cause a decrease in reaction to the stimulus. Habituation is also proclaimed to be a form of implicit learning, which is commonly the case with continually repeated stimuli. This characteristic is consistent with the definition of habituation as a procedure, but to confirm habituation as a process, additional characteristics must be demonstrated. Also observed is spontaneous recovery. That is, a habituated response to a stimulus recovers when a significant amount of time passes between stimulus presentations.
Potentiation of habituation is observed when tests of spontaneous recovery are given repeatedly. In this phenomenon, the decrease in responding that follows spontaneous recovery becomes more rapid with each test of spontaneous recovery. Also noted was that an increase in the frequency of stimulus presentation will increase the rate of habituation. Furthermore, continued exposure to the stimulus after the habituated response has plateaued may have additional effects on subsequent tests of behavior such as delaying spontaneous recovery. The concepts of stimulus generalization and stimulus discrimination will be observed. Habituation to an original stimulus will also occur to other stimuli that are similar to the original stimulus. The more similar the new stimulus is to the original stimulus, the greater the habituation that will be observed. When a subject shows habituation to a new stimulus that is similar to the original stimulus but not to a stimulus that is different from the original stimulus, then the subject is showing stimulus discrimination.. Stimulus discrimination can be used to rule out sensory adaptation and fatigue as an alternative explanation of the habituation process.
Another observation mentioned is when a single introduction of a different stimulus late in the habituation procedure when responding to the eliciting stimulus has declined can cause an increase in the habituated response. This increase in responding, or dishabituation, is temporary and always occurs to the original eliciting stimulus. Researchers also use evidence of dishabituation to rule out sensory adaptation and fatigue as alternative explanations of the habituation process. Habituation of dishabituation can occur. The amount of dishabituation that occurs as a result of the introduction of a different stimulus can decrease after repeated presentation of the "dishabituating" stimulus.
Some habituation procedures appear to result in a habituation process that last days or weeks. This is considered long-term habituation. It persists over long durations of time. Long-term habituation can be distinguished from short-term habituation which is identified by the nine characteristics listed above.
Biological mechanisms
The changes in synaptic transmission that occur during habituation have been well-characterized in the Aplysia gill and siphon withdrawal reflex. Habituation has been shown in essentially every species of animal and at least, in one species of plants, in isolated neuronally-differentiated cell-lines, as well as in quantum perovskite. 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.Neuroimaging
Within psychology, habituation has been studied through different forms of neuroimaging like PET scan and fMRI. Within fMRI, the response that habituates is the blood oxygen level-dependent signals triggered by stimuli. Decreases of the BOLD signal are interpreted as habituation.The amygdala is one of the most-studied areas of the brain in relation to habituation. A common approach is to observe the visual processing of facial expressions. A study by Breiter and colleagues used fMRI scans to identify which areas of the brain habituate and at what rate. Their results showed that the human amygdala responds and rapidly habituates preferentially to fearful facial expressions over neutral ones. They also observed significant amygdala signal changes in response to happy faces over neutral faces.
Blackford, Allen, Cowan, and Avery compared the effect of an extremely inhibited temperament and an extremely uninhibited temperament on habituation. Their study found that over repeated presentations individuals with an uninhibited temperament demonstrated habituation in both the amygdala and hippocampus, whereas participants with an inhibited temperament demonstrated habituation in neither brain region. The researchers suggest that this failure to habituate reflects a social learning deficit in individuals with an extremely inhibited temperament, which is a possible mechanism for a higher risk of social anxiety.
Debate about learning-status
Although habituation has been regarded as a learning process by some as early as 1887, its learning status remained controversial up until the 1920s - 1930s. While conceding that reflexes may "relax" or otherwise decrease with repeated stimulation, the "invariance doctrine" stipulated that reflexes should not remain constant and that variable reflexes were a pathological manifestation. Indeed, air pilots who showed habituation of post-rotational nystagmus reflex were sometimes ejected from or not recruited for service for World War I: on the grounds that a variable reflex response indicated either a defective vestibular apparatus or a lack of vigilance. Eventually, however, more research from the medical and scientific communities concluded that stimulus-dependent variability reflexes is clinically normal. The opposition to the considering habituation a form of learning was also based on the assumption that learning processes must produce novel behavioral responses and must occur in the cerebral cortex. Non-associative forms of learning such as habituation do not produce novel responses but rather diminish a pre-existing responses and often are shown to depend on peripheral synaptic changes in the sensory-motor pathway. Most modern learning theorists, however, consider any behavioral change that occurs as a result of experience to be learning, so long as it cannot be accounted for by motor fatigue, sensory adaptation, developmental changes or damage.Criteria for verifying a response-decline as learning
Importantly, systematic response-declines can be produced by non-learning factors such as sensory adaptation, motor fatigue, or damage. Three diagnostic criteria are used to distinguish response-declines produced by these non-learning factors and response-declines produced by habituation processes. These are:
- Recovery by Dishabituation
- Sensitivity of Spontaneous Recovery to Rate-of-Stimulation
- Stimulus-specificity
Despite the ubiquity of habituation and its modern acceptance as a genuine form of learning it has not enjoyed the same focus within research as other forms of learning. On this topic, the animal psychologist James McConnell said "...nobody cares…much about habituation"). It has been suggested that the apathy held towards habituation is due to 1) resistance from traditional learning theorists maintain memory requires reproduction of propositional/linguistic content; 2) resistance from behaviorists who maintain that "true" learning requires the development of a novel response the behavioral measure of habituation is very susceptible to confound by non-learning factors.