The startle response is a defensive response to sudden or threatening stimuli, and is associated with negative affect. Usually the onset of the startle response is a reflex reaction. The startle reflex is a brainstem reflectory reaction (reflex) that serves to protect the back of the neck (whole-body startle) and the eyes (eyeblink) and facilitates escape from sudden stimuli. It is found across the lifespan of many species. An individual's emotional state may lead to a variety of responses.