Pavlovian heart rate and jaw movement conditioning in the rabbit: Effects of medial prefrontal lesions

An experiment was conducted in which daw movements (JM) and heart rate (HR) were concomitantly assessed in rabbits during simple Pavlovian conditionin g. A 2-s 1200-Hz tone was the conditioned stimulus (CS) and an intraoral 1- cc pulse of 0.5 M sucrose-water solution was the unconditioned stimulus (US ). Sham and medial prefrontal (mPFC)-lesioned animals received paired CS/US training with a 70- to 75-dB CS and were compared with sham- and mPFC-lesi oned animals that received explicitly unpaired CS/US presentations. The per centages of JM CRs were significantly greater in the paired than the unpair ed groups, but mPFC lesions had no effect on this measure. Conditioned HR d ecelerations occurred only in the paired groups and then only during the fi rst session of training. Moreover, these CS-evoked cardiac decelerations we re somewhat attenuated by the mPFC lesion. CS-evoked HR accelerations, whic h were significantly greater in unpaired than in paired animals, occurred d uring the four subsequent sessions. These results suggest that a CS-evoked cardioinhibitory process, mediated by the mPFC, is engendered by Pavlovian appetitive conditioning, as has been previously demonstrated for aversive c onditioning. However, during JM conditioning these inhibitory changes are q uickly replaced by tachycardia, possibly related to increased nonspecific s omatomotor activity, since the tachycardia was somewhat greater in the unpa ired animals.