Why do we freeze when startled? New study in flies points to serotonin

(The Zuckerman Institute at Columbia University) A Columbia University study in fruit flies has identified serotonin as a chemical that triggers the body's startle response, the automatic deer-in-the-headlights reflex that freezes the body momentarily in response to a potential threat. Today's study reveals that when a fly experiences an unexpected change to its surroundings, such as a sudden vibration, release of serotonin helps to literally -- and temporarily -- stop the fly in its tracks.