Suddenly surprised or alarmed by something unexpected, causing a quick, involuntary reaction. This involves both emotional and physical responses to sudden stimuli.
From Middle English 'stertlen' meaning 'to rush about' or 'struggle,' related to 'start' in the sense of sudden movement. The word captures both the emotional shock and the physical jolt of surprise.
The startle response is one of our most primitive reflexes, occurring within 200 milliseconds of a stimulus and involving the same neural circuits across all mammals. It's actually a measure of emotional regulation—people with anxiety disorders often have heightened startle responses.
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