Manipulation of CS-US conditional probability and of the US-US trace interval on conditioning to the CS and to a background stimulus in a CER situation

Two experiments examined the relationship between conditioning to the CS and background using a novel CER paradigm, in which a long background stimulus played the role of more conventional contextual cues. Experiment 1 manipulated the probability of US occurrence given the CS (p(US/CS)). Conditioning to the background was not a monotonically decreasing function of p(US/CS) at all shock intensities, and conditioning to the CS was remarkably insensitive to the value of p(US/CS) when assessed off the baseline. Experiment 2 manipulated the trace interval between the CS and US. Although conditioning to the CS decreased as the trace interval increased, conditioning to the background was dependent upon whether it served as the interstimulus interval (ISI; interval between the CS and US) or intertrial interval (ITI; interval between CS-US pairs) stimulus. Conditioning to the ISI background decreased as the trace interval increased, but conditioning to the ITI background at first increased, but then decreased as the trace interval was further increased. These results are discussed with respect to the adequacy of contemporary models of conditioning.