The instrument was purchased from SAN DIEGO Instruments. The startle response or startle reaction is a response to sudden, startling stimuli, such as sudden noise or sharp movement. Usually the onset of the startle response is reflectory. The startle reflex is a brainstem reflectory reaction that serves to protect the back of the neck (whole-body startle), or the eye (eyeblink), and also facilitates escape from sudden stimuli. It is found across the lifespan and in many species. An individual's emotional state may lead to a variety of different responses