Dissociation of Associative and Nonassociative Concommitants of Classical Fear Conditioning in the Freely Behaving Rat

An acoustic stimulus previously paired with footshock elicits stereotyped increases in arterial pressure and heart rate and induces freezing behavior in freely behaving rats. Although the arterial pressure and freezing responses differ between groups given paired and random presentations of the tone and shock, the increases in heart rate do not. These observations, if taken at face value, suggest that the arterial pressure and freezing responses reflect associative learning but that the heart rate change is a nonassociative or a pseudoconditioned response. In this article we describe three experiments aimed at determining why the CS elicits similar increases in heart rate in groups given paired and random training. The first study demonstrates that regardless of the pseudoconditioning control procedure used (random, backwards, shock-alone, or naive), the same pattern of results is obtained: the increases in arterial pressure are greater in the paired than in each control group, but the heart rate rises to the same extent in all groups. The second study determined that the context in which the responses are tested (conditioning apparatus vs. novel test chamber) does not affect the general pattern of results obtained. The third study demonstrates that the superficially similar increases in heart rate in conditioned and pseudoconditioned rats are achieved by different physiological mechanisms: coactivation of the sympathethic and parasympathetic nervous systems in conditioned rats and sympathetic excitation alone in pseudoconditioned rats. Thus, the heart is influenced by associative emotional processes, but heart rate is not, under these conditions, a particularly useful index of those influences.