startle reflex

n. A bodily reflexive jerk, without cognitive mediation, to a sudden, unexpected stimulus. This reflex is present even in neonates. The stimulus may be a touch, noise, bright light, or other sudden change in the environment. The startle reflex may be accompanied by an array of sympathetic nervous system responses such as the release of adrenaline and an increase in heart rate.

Researchers use a heightened or prolonged startle reflex to measure fear/anxiety levels associated with increased activity in the amygdala, a regulator of fear/anxiety. Treatment with benzodiazepines such as diazepam (Valium) or alcohol reduces exaggerated anxiety startle responses, leading to the possibility of abuse of either drug.