In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening stimuli, such as sudden noise or sharp movement, and is associated with negative affect. Usually the onset of the startle response is a startle reflex reaction. The startle reflex is a brainstem reflectory reaction (reflex) that serves to protect vulnerable parts, such as 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. A variety of responses may occur because of individual's emotional state,body posture,preparation for execution of a motor task,or other activities. The startle response is implicated in the formation of specific phobias.