In respondent conditioning, a type of higher-order conditioning in which a contingent relation between two stimuli precedes making one of them a CS. Sensory preconditioning is said to have occurred if the other stimulus elicits the CR solely by virtue of its relation to the first stimulus. Preconditioning procedures have also been extended to operant cases (e.g., correlating response-independent reinforcers with one stimulus that later will signal reinforcement but not with a second one that later will signal extinction sometimes facilitates acquisition of an operant discrimination between the stimuli).