CS-US contiguity and CS intensity in conditioning of the rabbit's nictitating membrane response to serial compound stimuli.

The present experiments examined acquisition of the rabbit's nictitating membrane response to serial compound stimuli. Each compound consisted of two distinctive conditioned stimuli (CSA and CSB) that were followed by a shock unconditioned stimulus (US). In Experiment 1, CSA was a highly salient 93-dB tone, and CSB was a moderately salient flashing light. The CSA-US interval was 800 msec, and the CSB-US interval was varied across 400, 600, and 800 msec. In Experiment 2, the flashing light served as CSA, and CSB was either a 73-, a 85-, or a 93-dB tone. The CSA-US interval remained 800 msec, and the CSB-US interval was fixed at 400 msec. The experiments revealed that the CSB-US interval and CSB intensity determined the rate of conditioned response (CR) acquisition to the compound. Yet, CR acquisition to CSB as measured on test trials showed impairments relative to the level of responding in single CS-control conditions. However, the impairment in CR acquisition to CSB was attenuated by increasing CSB-US contiguity, that is, by decreasing the CSB-US interval (Experiment 1) or by increasing CSB intensity (Experiment 2). In Experiment 1, the impairment in acquisition to the light CSB appeared to be primarily a consequence of the tone CSA's greater salience. However, in Experiment 2, impairments in CR acquisition to CSB appeared even when CSB had the combined advantages of CS-US contiguity and great salience relative to CSA. Thus, the results indicated a role for CSA's temporal primacy in determining CR acquisition to the components of a serial compound. The results are discussed in terms of selective-attention, generalization-decrement, and information hypotheses.