Multiple-unit activity of the prefrontal cortex and mediodorsal thalamic nucleus during reversal learning of discriminative avoidance behavior in rabbits.

Multiple-unit activity of the prefrontal cortex (PFC) and the mediodorsal (MD) thalamic nucleus was recorded during reversal training following differential conditioning of a locomotory (wheel rotation) avoidance response in rabbits. The positive and negative conditional stimuli (CS+ and CS-) were pure tones and the unconditional stimulus (UCS) was footshock. A major objective was to compare the activity in the PFC and MD nucleus with that in the neighboring cingulate cortical and anteroventral (AV) thalamic system, studied previously. During the first session of reversal training the rostral-sulcal subfield of the PFC manifested significant discriminative activity appropriate to the original habit, i.e., a greater discharge to the original CS+ than to the original CS-. The difference between the CS+ and CS- elicited discharges diminished as reversal learning progressed, but discriminative activity appropriate to the reversal problem did not develop. The medial subdivision of the MD nucleus which is interconnected reciprocally with the rostral-sulcal PFC subfield did not manifest discriminative activity during reversal training (nor had it done so during original learning). The caudal subfield of the PFC (which had shown rapidly developing acquisition of the discriminative discharges followed by diminution of the discriminative discharges during original training) manifested an original discriminative effect in the initial session of reversal training followed by the reverse discriminative discharge in the criterial session. The lateral subdivision of the MD nucleus which is interconnected reciprocally with the caudal PFC subfield, also manifested the original discriminative effect in the initial session of reversal training, followed by the reverse discriminative effect during the subsequent (precriterial and criterial) sessions. Thus, as in the AV nucleus, the lateral MD subdivision showed earlier and more robust development of discriminative activity appropriate to the reversal problem, than its cortical counterpart. The present results also suggested that cerebral cortical areas such as the caudal PFC, which transfer discriminative activity to their respective thalamic nuclei during discriminative acquisition, are subsequently more likely to manifest the reverse discriminative activity, than are cortical areas such as the rostral-sulcal subfields of the PFC, which do not transfer their discriminative activity.