Enhanced unblocking from sustained post-trial surprise.

Two appetitive conditioning experiments with rats investigated the mechanisms and properties of unblocking that results from the surprising omission of an expected post-trial unconditioned stimulus (US). Experiment 1 demonstrated unblocking under circumstances in which differences in the contribution of generalization decrement and within-compound associations are equated across experimental and control groups. Following Stage 1 training in which a conditioned stimulus (CS) A was followed by a US and a post-trial US, in Experiment 2 we arranged for the post-trial US to be present on some trials with AX but not others. Under these circumstances an enhancement of unblocking to X was observed, relative to a group who received standard unblocking by US omission. The implications of these results for attentional and US-processing theories of associative learning are discussed.