Blocking of conditioning to a cocaine-paired stimulus: testing the hypothesis that cocaine perpetually produces a signal of larger-than-expected reward.

According to a recent account of addiction, dopaminergic effects of drugs like cocaine mimic the neuronal signal that occurs when a natural reward has a larger value than expected. Consequently, the drug's expected reward value increases with each administration, leading to an over-selection of drug-seeking behavior. One prediction of this hypothesis is that the blocking effect, a cornerstone of contemporary learning theory, should not occur with drug reinforcers. To test this prediction, two groups of rats were trained to self-administer cocaine with a nose-poking response. For 5 sessions, a tone was paired with each self-administered injection (blocking group), or no stimulus was paired with injection (non-blocking group). Then, in both groups, the tone and a light were both paired with each injection for 5 sessions. In subsequent testing, the light functioned as a conditioned reinforcer for a new response (lever-pressing) in the non-blocking group, but not the blocking group. Thus, contrary to prediction, pre-training with the tone blocked conditioning to the light. Although these results fail to support a potentially powerful explanation of addiction, they are consistent with the fact that most conditioning and learning phenomena that occur with non-drug reinforcers can also be demonstrated with drug reinforcers.