startles

Startle is a rapid, involuntary response to sudden, unexpected stimuli. In humans and many animals, a startle reaction is brief and outwardly visible, often involving a quick bodily movement or jump, a blinking of the eyes, and autonomic arousal such as faster heartbeat and heightened alertness. Common triggers include loud noises, bright flashes, or unexpected touch. The term is used both for the reflex itself and for the act of startling someone.

The best-studied form is the acoustic startle reflex, a rapid motor response generated by a neural pathway that begins with auditory input and culminates in a motor output via brainstem circuits. In humans, the pathway involves cochlear root neurons projecting to the caudal pontine reticular nucleus and then through reticulospinal circuits to facial muscles and the limbs. The amygdala and other limbic structures modulate the magnitude of the startle, especially when the stimulus carries emotional significance. Startle magnitude can be influenced by context, state of arousal, and prior experience.

Startle responses can be modulated in several ways. Habituation reduces response with repeated exposure to the same stimulus. Fear-potentiated startle increases the response when a subject is conditioned to fear the environment or cue. Prepulse inhibition, where a weaker stimulus precedes the startling one, typically reduces the startle; this phenomenon is used to study sensory processing and psychiatric conditions.

Clinical relevance includes exaggerated startle responses in conditions such as PTSD and certain startle syndromes, as well as diminished or delayed responses in some neurodegenerative diseases. Measuring startle, often via electromyography of the blink muscle, provides a tool for research into sensory processing, emotion, and neurology.