Habituation

Habituation is a form of non-associative learning in which an innate (non-reinforced) response to a stimulus decreases after repeated or prolonged presentations of that stimulus. Responses that habituate include those that involve the intact organism (e.g., full-body startle response) or those that involve only components of the organism (e.g., habituation of neurotransmitter release from in vitro Aplysia sensory neurons). The broad ubiquity of habituation across all biologic phyla has resulted in it being called 'the simplest, most universal form of learning...as fundamental a characteristic of life as DNA.' 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 (i.e., punishment/reward). For example, organisms may habituate to repeated sudden loud noises when they learn these have no consequences. 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 may also be clinically relevant, 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 (tone) and complex (faces). Habituation is a form of non-associative learning in which an innate (non-reinforced) response to a stimulus decreases after repeated or prolonged presentations of that stimulus. Responses that habituate include those that involve the intact organism (e.g., full-body startle response) or those that involve only components of the organism (e.g., habituation of neurotransmitter release from in vitro Aplysia sensory neurons). The broad ubiquity of habituation across all biologic phyla has resulted in it being called 'the simplest, most universal form of learning...as fundamental a characteristic of life as DNA.' 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 (i.e., punishment/reward). For example, organisms may habituate to repeated sudden loud noises when they learn these have no consequences. 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 may also be clinically relevant, 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 (tone) and complex (faces). There is an additional connotation to the term habituation which applies to psychological dependency on drugs, and is included in several online dictionaries. A team of specialist 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'. By 1964 the America Surgeon's General report on smoking and health 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, 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. 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 have recently been updated and include the following: