Joseph E. Steinmetz

Lesions of the inferior olivary complex cause extinction of the classically conditioned eyeblink response

The dentate-interpositus nuclei of the cerebellum are known to be critically involved in the production of the classically conditioned eyeblink response in the rabbit. The rostro-medial portions of the inferior olivary complex (rmIO) project to these nuclei as well as receive projections from the fifth sensory nuclei. Lesions of the rmIO caused a previously classically conditioned eyeblink response to slowly decrease in amplitude and frequency with continued paired conditioned stimulus-unconditioned stimulus (CS-UCS) training in a manner which was identical to extinction of the learned response in control animals. The lesion had no effect on the performance of the unconditioned response. Lesions of the rmIO before training prevented learning from occurring. Lesions of other portions of the IO or of the reticular formation did not specifically affect the learned response. We conclude that the rmIO is critically involved in the learning and maintenance of the classically conditioned eyeblink response. We propose that the rmIO may serve as a pathway for information from the unconditioned stimulus to reach the cerebellum, and as such may in fact be the essential reinforcing or teaching input for the learning of classically conditioned responses. These results support the hypothesis that the cerebellum contains neuronal changes which are a critical portion of the memory trace for the conditioned response.

Classical conditioning of the rabbit eyelid response with a mossy-fiber stimulation CS: I. Pontine nuclei and middle cerebellar peduncle stimulation.

The nictitating membrane/eyelid responses of 18 rabbits were classically conditioned using cerebellar mossy-fiber stimulation as a conditioned stimulus (CS) and air puff as an unconditioned stimulus (US). The dorsolateral, lateral, and medial pontine nuclei and the middle cerebellar peduncle were effective stimulation-CS sites for training. In one group of rabbits, robust conditioned eyelid responses were produced with paired trials and subsequently extinguished with CS-alone and explicitly unpaired presentation of the CS and US. In a second group of rabbits, no conditioned responses were evident for 4 days of unpaired CS and US presentations. Conditioned responses did develop, however, after paired training was begun. Lesions of the interpositus nucleus of the cerebellum completely abolished the conditioned responses of a third group of rabbits overtrained with the mossy-fiber CS and air-puff US. These results support previous studies which have demonstrated that the cerebellum is critically involved in acquisition and retention of simple learned responses. In addition, the present results support previous theories of cerebellar function which have proposed that mossy fibers supply critical learning input to the cerebellum for acquisition and retention of motor skills.

Reacquisition of classical conditioning after removal of cerebellar cortex

SummaryWe trained rabbits with white noise and light conditioned stimuli and a face shock unconditioned stimulus for classical conditioning of the nictitating membrane response and then removed the ipsilateral cerebellar cortex by aspiration. Larsells hemisphere lobule VI was completely removed in 5 rabbits; the pattern of degeneration in the inferior olive matches the projections to HVI reported in experiments with horseradish peroxidase. All rabbits showed an initial abolition of conditioned responses but then relearned within two days. This indicates that cerebellar cortical lobule HVI normally is involved but is not necessary for classical conditioning.

Classical conditioning of the rabbit eyelid response with mossy fiber stimulation as the conditioned stimulus

The nictitating membrane responses of 12 rabbits were classically conditioned using mossy fiber stimulation as a conditioned stimulus (CS) and airpuff as an unconditioned stimulus. The dorsolateral pontine nucleus and the lateral reticular nucleus were effective stimulation-CS sites for learning. Robust conditioned responses were obtained with relatively few paired trials and extinguished with subsequent CS-alone presentations. In addition, explicitly unpaired presentations of stimulation and airpuff failed to produce response increments. These results support previous theories of cerebellar function that have proposed that mossy fibers supply “learning” input to the cerebellum for motor learning.

Classical conditioning does not occur when direct stimulation of the red nucleus or cerebellar nuclei is the unconditioned stimulus

The involvement of the cerebellum and the red nucleus in the classically conditioned nictitating membrane/eyeblink response in the rabbit is investigated using direct stimulation of the interpositus or the red nucleus as the unconditioned stimulus. Stimulation of either of these structures produced eye closure in naive animals, and this eye closure was paired with a tone in the standard Pavlovian conditioning paradigm. The results indicate that eyelid closure due to stimulation of either the red nucleus or the interpositus nucleus is not sufficient for the development of conditioned responses to the tone. Animals which had received interpositus stimulation as the unconditioned stimulus acquired the conditioned response to tone significantly faster following the substitution of air puff for stimulation than did those animals that had received red nucleus stimulation, or controls that did not receive any stimulation. However, animals that had been trained to tone-air puff could not retain the conditioned response after being switched to tone-interpositus stimulation. Lesions of the interpositus and the red nucleus through the stimulating electrodes were effective in impairing or abolishing conditioned responses. The results are interpreted to indicate that the red nucleus and interpositus are elements of the circuit that carries out the expression of the conditioned response. In addition the interpositus, but not the red nucleus, may be critical in the formation of the memory trace for the conditioned stimulus-unconditioned stimulus association, by virtue of the greatly accelerated learning that results from its stimulation.

The role of the middle cerebellar peduncle in acquisition and retention of the rabbit’s classically conditioned nictitating membrane response

In the present study, the middle cerebellar peduncles of 20 rabbits were lesioned before or after the left nictitating membrane/eyelid response was classically conditioned. Naive animals with completely destructed peduncles were unable to learn; in trained animals, the CRs were abolished using tones, lights, tactile stimulation, or pontine stimulation as conditioned stimuli. These results support the hypothesis that conditioned stimulus information is projected to the cerebellum via mossy fibers in the middle cerebellar peduncle during classical conditioning of discrete, somatic responses.

Rapid transfer of training occurs when direct mossy fiber stimulation is used as a conditioned stimulus for classical eyelid conditioning

Fifteen rabbits were classically conditioned using stimulation of the right pontine nuclei as a conditioned stimulus and a corneal airpuff as an unconditioned stimulus. After conditioning criterion was reached, the stimulation conditioned stimulus was transferred to the left pontine nuclei and the rate of conditioned response acquisition observed. Our results indicate that when electrodes were placed symmetrically in the right and left pontine nuclei, extremely rapid transfer of training occurred. Together with previous data, the present data supply further indirect evidence that the site of neural plasticity which underlies acquisition and retention of classically conditioned skeletal muscle responses is located efferent to the pontine nuclei, namely in the cerebellum.

Classical conditioning of the rabbit eyelid response with a mossy-fiber stimulation CS: II. Lateral reticular nucleus stimulation.

Stimulation of mossy fibers arising from the pontine nuclei can be used as a conditioned stimulus (CS) during classical conditioning of the eyelid/nictitating membrane response (NM). In the present experiment we stimulated another source of mossy fibers, the lateral reticular nucleus (LRN), as a CS for NM conditioning. LRN stimulation was an effective CS, resulting in learning, and the conditioned response to LRN stimulation showed normal extinction. Unpaired presentation of CS and UCS did not result in pseudo-conditioning. Lesions of the cerebellar dentate-interpositus region abolished the conditioned response but left the unconditioned reflex response intact. We suggest that mossy fibers may normally carry CS information to the cerebellum.