Heart rate and somatomotor activity in rats during signalled escape and yoked classical conditioning

In the first of three studies, heart rate, movement, and EMG activity during signalled escape and regular classical conditioning were examined in unrestrained and restrained groups of rats. The direction of the HR CR was accelerative in the unrestrained groups and decelerative in the restrained groups, regardless of the presence or absence of the escape contingency. Both CRs were enhanced by the presence of directionally similar changes in movement to the CS. The decelerations of the restrained groups were correlated with decreases in movement and increases in EMG activity. Increasing the somatomotor activity associated with the US failed to modify the decelerative direction of the HR CRs of restrained rats in the second and third studies. It was suggested that HR CRs in rats may be accompaniments of naturally occurring defense behaviors such as freezing in restrained rats and aggression in unrestrained rats.