Classically conditioned bradycardia and skeletal-motor activity in restrained rats

Classical conditioning of heart rate (HR) and skeletal-motor activity in restrained rats was examined using a single, brief (.5-msec) pulse of shock to the rat’s tail as both the CS and US. Electric shock was selected as a CS to discourage behavioral freezing and to provide an above-zero level of movement activity for evaluating the presence of conditioning. Three CS intensities and two US intensities were combined in a factorial design creating six separate experimental groups which received 50 paired trace conditioning trials at a CS-US interval of 8.1 sec. Three control groups were given 50 unpaired presentations of one of the CSs and the highest intensity US. All of the experimental groups showed the development of a bradycardia CR in the absence of a conditioned change in movement. During conditioning, HR and movement to CS onset varied together in both the experimental and control groups. Other examples of a close correspondence between HR and movement included some, but not all, of the preconditioning CS-alone responses, the URs, and shifts in baselines. The results suggested that although some nonassociatively based HR changes may have been tied to movement, the conditioned bradycardias in the experimental groups appeared to be relatively independent of movement.