Bilateral electrolytic lesions of the anteroventral (AV) nucleus of the thalamus given after training impaired retention performance (extinction and reacquisition) of rabbits in a differential avoidance conditioning task. In addition, the lesions abolished the excitatory, discriminative multiple-unit discharges that had developed in the cingulate and retrosplenial cortices to the auditory conditional stimuli (CSs) during the course of behavioral acquisition, prior to the induction of the lesions. The excitatory discharges were supplanted in the subjects with lesions by CS-elicited reduction of neuronal firing to levels below the prestimulus baseline. Lesions given before training did not disrupt behavioral acquisition, but they did eliminate the excitatory tone-elicited neuronal discharges that normally occur in the cortex before and during training. The CS-elicited reduction of neuronal firing did not occur at the beginning of training in the subjects given lesions before training, but it developed during the course of training. The lesions did not eliminate the excitatory and discriminative neuronal activity of the prefrontal cortex. These results demonstrate that excitatory and discriminative neuronal discharges in the cingulate and retrosplenial cortices are critically dependent on the connections of these areas with the anterior thalamic nuclei. Also the lesion-induced disruption of performance during extinction and reacquisition but not during original learning confirms a prediction from past electrophysiological studies, that the AV thalamic nucleus is involved in the mediation of the maintenance and retention of the conditioned avoidance behavior, but not in its original acquisition.

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 ol 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.A major strategy for gaining knowledge (CS) paired with a footshock unconditional about the neural substrates of conditioning stimulus (UCS) during instrumental avoidand learning involves study of the neuronal ance conditioning. The neuronal response correlates of classical and instrumental did not occur at a comparable magnitude to conditioning in intact, behaving animals generalization stimuli presented after be-

In three experiments, the nictitating membrane response of rabbits was conditioned for 10 daily sessions at interstimulus intervals (ISIs) ranging from 48 to 125 msec, followed by a shift to 250 msec for 5 days. At tested ISIs shorter than 67 msec, there was no evidence of conditioning, and postshift performance revealed neither facilitation nor interference as a result of the first 10 conditioning sessions. Postshift performance of groups conditioned at preshift ISIs of 67 msec or longer revealed a gradient of increasing savings with increasing lSI. One of the groups in Experiment 1, initially conditioned at 250 msec lSI and then shifted to 48 msec, exhibited extinction of the previously well-conditioned response. Analysis of CR-onset latencies substantiated the absence of associative effects at very short ISIs. It was concluded that there is a temporal limit below which classical conditioning of the nictitating membrane response of rabbits employing forward CS-US pairing does not occur.The detailed interstimulus interval (lSI) function for the rabbit nictitating response (NMR), established by Smith, Coleman and Gormezano (1969), indicates that the minimum lSI required for conditioning is between 50 and 100 msec. Smith et al. found no evidence of conditioning at ISIs of -50, 0 or 50 msec, but increasingly rapid acquisition at ISIs of 100 msec and higher. Recent studies, however, have seemed to show substantial conditioning at 50-msec lSI (Patterson, 1970) as well as simultaneous and backward conditioning in rats and rabbits (e.g

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