A stimulus applied in response to or opposition to another stimulus, especially in behavioral or physiological experiments.
From 'counter-' plus 'stimulus' (from Latin 'stimulare' meaning to prick or urge). Emerges in experimental psychology and physiology during the 20th century.
In Pavlovian conditioning, researchers discovered that a counterstimulus can completely override learned responses—ring a bell before a dog has learned to associate it with food, and suddenly the conditioning process resets, showing how powerful timing and opposition can be.
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