EMOTIONAL AND BEHAVIORAL-CORRELATES OF MEDIODORSAL THALAMIC NEURONS DURING ASSOCIATIVE LEARNING IN RATS

Neuronal activity was recorded from the mediodorsal thalamic nucleus ( MD) of behaving rats that were trained to lick a protruding spout just after a conditioned stimulus to obtain reward or to avoid shock. Cond itioned stimuli included both elemental (auditory or visual stimuli) a nd configural (simultaneous presentation of auditory and visual stimul i predicting reward outcome opposite that predicted by each stimulus p resented alone) stimuli. Of 122 MD neurons responding during the task, the activity of 13 increased just before licking only during the task , but not before spontaneous licking during the intertrial interval (c onditioned behavior related). These conditioned behavior related neuro ns were located mainly in the lateral MD, which has intimate anatomica l connections with motor-related areas such as anterior cingulate and striatum. The activity of the other 109 neurons was related to conditi oned stimulation (conditioned stimulus related). Most of these neurons responded differentially to both elemental and configural stimuli in terms of reward contingency, and also changed their responses during e xtinction and relearning trials. Conditioned stimulus-related neurons with latencies <300 msec were located mainly in the rostromedial MD, w hich receives afferents from the basolateral nucleus of the amygdala i n which sensory information from various sources coverge. Furthermore, most differential neurons that were tested responded during the delay period in a reward task in which a delay was imposed between the cond itioned stimulus and reward delivery. The present results, along with previous anatomical studies, suggest the existence of two limbic circu its: anterior cingulate-striatum-lateral MD (motor) and amygdala-media l MD-orbital prefrontal cortex (short-term memory/emotion).