Effect of a stimulus intervening between CS and US in autoshaping.

Five experiments explored the effect of placing a stimulus between the termination of a key light CS and the onset of a food US in an autoshaping preparation with pigeon subjects. Whether that stimulus was itself a key light evoking pecking or an auditory event failing to evoke pecking, it facilitated performance to the key light CS. The use of a within-subjects design made differential conditioning of background cues and stimulus generalization unlikely accounts. Experiment 3 found that extinction of the intervening stimulus did not remove the facilitative effect. Experiment 4 found that pairing the CS with food and the intervening stimulus did not produce the effect unless both occurred on the same trial. Together, these experiments rule out an account in terms of second-order conditioning of the CS by the intervening stimulus. Experiment 5 used an intervening stimulus in a second-order conditioning design to demonstrate that the stimulus acted to improve the association between the CS and the reinforcer. That experiment found the intervening stimulus to make responding to the CS more vulnerable to subsequent extinction of its reinforcer. Together, these experiments document a kind of catalytic effect of the intervening stimulus on the CS-reinforcer association.