Effect of US signal value on blocking of a CS-US association

When an unconditioned stimulus (US) signals the delivery of a second US, there is poor conditioned responding to a conditioned stimulus (CS) that signals the second US. Using an appetitive conditioning preparation with rats, 3 experiments examined the factors that contribute to this poor conditioned responding. In Experiment 1, conditioned responding was improved when US-alone presentations came before, but not after, CS training. Experiment 2 showed that the effect in Experiment 1 was not due to context extinction, and Experiment 3 showed that context-US associations do not significantly contribute to responding in the US-US training procedure. The results show that poor conditioned responding is an acquisition deficit, arising from US, rather than context, signal value.