a phenomenon of stimulus control in which previous learning restricts or prevents conditioning of a response to a new stimulus. For example, a light paired with an unconditioned stimulus for several trials results in some conditioning for the light. Adding a tone at this point, to form a compound stimulus, would result in the tone being less effective as an elicitor than it would if it had been present from the beginning. Also called Kamin blocking effect. [Leon J. Kamin (1927– ), U.S. psychologist]