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 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. reduced habituation is the most common habituation phenotype reported across neuropsychiatric disorders although enhanced habituation has been observed in hd and adhd. it also appears that abnormal habituation is often predictive of symptom severity in several neuropsychiatric disorders, including asd, pd, and hd. moreover, there are instances where treatments that normalise the habituation-deficit also improve other associated symptoms. 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, although extinction processes may be operating instead.