Habituation
Habituation is a form of non-associative learning in which a non-reinforced response to a stimulus decreases after repeated or prolonged presentations of that stimulus.[1] For example, organisms may habituate to repeated sudden loud noises when they learn these have no consequences.[2]
Responses that habituate include those that involve the entire organism or those that involve only biological component systems of the organism. The broad ubiquity of habituation across all forms of life has resulted in it being called "the simplest, most universal form of learning...as fundamental a characteristic of life as DNA."[3] Functionally-speaking, by diminishing the response to an inconsequential stimulus, habituation is thought to free up cognitive resources to other stimuli that are associated with biologically important events.
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.[4] Habituation is relevant in psychiatry and psychopathology, as a number of neuropsychiatric conditions, including autism, schizophrenia, migraine, and Tourette's, show reductions in habituation to a variety of stimulus-types both simple and complex.[5]
Drug habituation[edit]
There is an additional connotation to the term habituation which applies to psychological dependency on drugs, and is included in several online dictionaries.[6] A team of specialists from the World Health Organization assembled in 1957 to address the problem of drug addiction and adopted the term "drug habituation" to distinguish some drug-use behaviors from drug addiction. 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".[7] By 1964 the America Surgeon's General report on smoking and health[8] included four features that characterize drug habituation according to WHO: 1) "a desire (but not a compulsion) to continue taking the drug for the sense of improved well-being which it engenders"; 2) "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,[9] 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.[10]
Characteristics[edit]
Habituation as a form of non-associative learning can be distinguished from other behavioral changes (e.g., sensory/neural adaptation, fatigue) by considering the characteristics of habituation that have been identified over several decades of research. The characteristics first described by Thompson and Spencer[11] were updated in 2008 and 2009, to include the following:[12]
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 (increases in magnitude) when a significant amount of time (hours, days, weeks) 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 (i.e., shorter interstimulus interval) will increase the rate of habituation. 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. 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 (stimulus generalization). 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. (For example, if one was habituated to the taste of lemon, their responding would increase significantly when presented with the taste of lime). 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 is temporary and is called "dishabituation" and always occurs to the original eliciting stimulus (not to the added 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 (i.e., shows little or no spontaneous recovery). Long-term habituation can be distinguished from short-term habituation which is identified by the nine characteristics listed above.
Relevance to neuropsychiatry[edit]
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.[5] In human clinical studies, habituation is most often studied using the acoustic startle reflex; acoustic tones are delivered to participants through headphones and the subsequent eye-blink response is recorded directly by observation or by electromyography (EMG). Depending on the disorder, habituation phenomena have been implicated as a cause, symptom, or therapy.[5] Reduced habituation is the most common habituation phenotype reported across neuropsychiatric disorders although enhanced habituation has been observed in HD and ADHD.[5] It also appears that abnormal habituation is often predictive of symptom severity in several neuropsychiatric disorders, including ASD,[38] PD,[39] and HD.[40][41] Moreover, there are instances where treatments that normalise the habituation-deficit also improve other associated symptoms.[42] As a therapy, habituation processes have been hypothesized to underlie the efficacy of behavioural therapies (i.e. habit reversal training, exposure therapy) for TS and PTSD,[43] although extinction processes may be operating instead.