An extremely rapid response to a sudden, unexpected stimulus, such as a pistol shot or a face looming up in the darkness.Analysis of the startle reaction through high speed photography (Landis and Hunt, 1939) has shown that it is a highly complex pattern which passes like a wave over the entire body. The eyes suddenly close, the mouth widens, the chin tilts upward, and the muscles of the neck are tensed. The wave response rapidly descends, and the shoulders hunch, the midsection contracts, and the knees bend. At the same time heart beat and respiration are abruptly increased as if to put the organism in a state of emergency. This entire reaction is so rapid that a person can go through it and return to his normal posture in less than one second.The startle response is found in all normal people from infancy to adulthood, and in all mammals that have been tested. The pattern is so uniform from individual to individual that it appears to be inborn. It is probably the most primitive of all emotional reactions, and appears to be an instinctive self-preservation mechanism in which the organism prepares itself to repel attack.Unlike other emotional responses, the startle reaction is highly resistant to modification by learning and experience. It can, however, be reduced in intensity if it is frequently repeated or partially anticipated. Children who make a game of frightening others find their sudden “boos” have less and less effect, and adults find they can soften reaction by warning a person to “prepare for a shock” when they have upsetting news to tell him. Other conditions increase the intensity of the reaction. It is likely to be particularly violent when we are overfatigued or under an emotional strain. One of the most common afflictions of combat veterans is an intense startle reaction to sudden loud noises such as the backfiring of a car. This reaction has been known to persist for months after a serviceman has returned to civilian life.