Simultaneous single unit recording in the medial nucleus of the medial geniculate nucleus and amygdaloid central nucleus throughout habituation, acquisition, and extinction of the rabbit's classically conditioned heart rate.

The present study examined single neuron activity in the medial nucleus of the medial geniculate (mMG) and amygdaloid central nucleus (ACe) simultaneously across several phases of differential heart rate conditioning (habituation, acquisition, and extinction). Within the same animals, the magnitude of mMG and ACe unit responses to two tone conditioned stimuli (CS) exhibited habituation, differential acquisition, and extinction. Neurons in each area developed a differential response latency to the CSs during acquisition, suggesting that mMG and ACe may be involved in changes of synaptic efficacy. Units in both areas rapidly developed a differential response magnitude to the CSs (< 6 acquisition trials), however, mMG units responded to the CSs with a shorter latency than ACe units across all phases of training. This suggests that unlearned and learned CS information may access mMG before ACe. These results are consistent with the notion that conditioning-induced plasticity which occurs in mMG may influence the conditioning-induced plasticity that occurs further downstream in the amygdala.