Startle reflex is a rapid automatic defensive response to abrupt sensory stimulation. It is ubiquitous in mammalian species, and its biological role is to protect the organism from a sudden attack or injury (Lang et al., 1993). In humans, one of the most pronounced components of the startle reflex is the eyeblink that can be elicited by strong abrupt sound and quantified using the measurement of electromyographic activity of the muscle responsible for the eyeblink (m. orbicularis oculi). Startle magnitude reflects individual difference in the overall defensive reactivity, and exaggerated startle response has been associated with different pathological conditions, most notably posttraumatic stress disorder. Most of the startle research, however, was concerned with startle modulation effects, most notably, pre-pulse inhibition and emotion-modulated startle.
