overshadowing

Overshadowing is a phenomenon in classical conditioning in which a more salient conditioned stimulus (CS) interferes with the learning about a less salient CS when both are presented together with the same unconditioned stimulus (US). In typical demonstrations, two neutral stimuli, such as a light and a tone, are paired with a reflexive US like a puff of air or a mild shock. If the light is more noticeable to the subject, the learning tends to favor the light, producing a stronger conditioned response to the light and a weaker or absent response to the tone.

The mechanism behind overshadowing is often explained by cue competition for associative strength. Under theories such as the Rescorla-Wagner model, the total amount of learning that can be distributed among competing CS is limited. When two stimuli are presented together, the more salient cue captures a larger share of this limited associative strength, leaving less available for the less salient cue. Factors influencing the magnitude of overshadowing include differences in salience or perceptual prominence, similarity between cues, temporal contiguity with the US, and the overall strength of the US.

Overshadowing demonstrates that learning about a cue can be affected by other stimuli in the environment, not solely by the property of the cue itself. It is a form of cue competition related to blocking and can be influenced by training conditions and prior conditioning. While widely observed in animal studies, the extent of overshadowing can vary across procedures and species, and some results may be accommodated by alternative theories that emphasize attention or context.