Overshadowing and summation in compound stimulus conditioning of the rabbit's nictitating membrane response

The present experiments examined acquisition of the rabbit's nictitating membrane response to a light + tone simultaneous compound stimulus and its components as a function of the intensity of the tone. In Experiment 1, the tone intensity was varied across the values of 85, 89, and 93 dB, and the CS-US (conditioned stimulus-unconditioned stimulus) interval was 400 msec. In Experiment 2, the tone intensities were 73, 85, and 93 dB, and the CS-US interval was 800 msec. Experiments 3 and 4 further examined the effects of the 73-dB tone at CS-US intervals of 400 and 800 msec, respectively. All experiments included control groups, which were trained with either a light or a tone CS. In brief, the experiments revealed repeated instances of overshadowing, i.e., the impairment of conditioned response (CR) acquisition to one or both of the components of a compound. Moreover, two types of summation were obtained: within-subjects summation, in which rabbits trained with a compound showed a higher level of responding to the compound than to either of its component CSs (Experiments 2, 3, and 4), and between-groups summation, in which a group trained with a compound showed faster CR acquisition than either of its corresponding control groups trained with a single CS (Experiments 1 and 2). The results are discussed in terms of perceptual and distributive processing models of compound stimulus conditioning.