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 is 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).