On the nature of the conditioned stimulus: comment on Leri and Stewart (2002).

F. Leri and J. Stewart (2002) present convincing evidence that a relapse to heroin-seeking behavior is established when rats are given a momentary lapse in which they are allowed to respond while under the influence of heroin. Although exposure to a conditioned stimulus (CS) paired with heroin self-infusion did not induce a relapse, it was able to control responding. In addition to serving as both a discriminative stimulus and a conditioned reinforcer, it is argued in this article that the CS also may have positive hedonic value independent of its association with heroin and that this property should be considered in interpreting the results of future experiments.