Early and late acquisition of discriminative neuronal activity during differential conditioning in rabbits: specificity within the laminae of cingulate cortex and the anteroventral thalamus.

Multiple-unit activity of the cingulate cortex and the anteroventral (AV) nucleus of the thalamus was recorded during discriminative conditioning of an avoidance response (locomotion) in rabbits. The results indicated a greater unit response in cingulate cortex to the positive conditional stimulus (CS+; a tone paired with a footshock unconditional stimulus [UCS]) relative to the negative conditional stimulus (CS-; a tone randomly interspersed with the positive stimuli but never paired with the UCS). The majority of neuronal records obtained from the deep laminae (V and VI) of cingulate cortex manifested first neuronal discrimination in the session of first exposure to conditioning. However, the majority of neuronal records of the superficial laminae (I-IV) showed first discrimination at a late stage of training, during the session in which the criterion of behavioral discrimination was met. The late developing discriminative activity of the superficial laminae was coincident with the late developing discriminative activity of the AV thalamus. Once acquired, neuronal discrimination in cortex persisted throughout 600 msec after CS onset, and during six sessions of training (overtraining) beyond criterion. Analysis of individual neuronal records suggested that the persistence during overtraining resulted from replacement of early fading neuronal discriminations by late neuronal discriminations.