David G. Lavond

Effects of lesions of cerebellar nuclei on conditioned behavioral and hippocampal neuronal responses

Rabbits were overtrained using classical conditioning of the rabbit nictitating membrane (NM)/eyelid response. Unilateral electrolytic lesions were then made through electrodes previously implanted in dentate and interpositus cerebellar nuclei ipsilateral to the trained (left) eye. Lesions caused a complete or near-complete abolition of conditioned behavioral responses on the ipsilateral side, but had no effect on unconditioned responses to corneal airpuff. When training was switched to the contralateral (right) side, animals learned within the first few trials, but did not relearn when training was returned to the left (lesioned) side. Control animals in which lesions spared the deep nuclei showed no such learning deficits. Lesions of cerebellar nuclei also abolished conditioned increases in hippocampal CAl neural activity evoked by the tone conditioned stimulus in this paradigm. As with the behavior, training on the right (non-lesioned) side reinstated the conditioned neuronal response within the first few trials of training, even though behavioral responding on lesioned side showed little or no improvement. These results indicate that the cerebellum is an essential structure for the behavioral expression of learning, and plays an important role in the generation of conditioned hippocampal responses observed in this paradigm.

The engram found? Role of the cerebellum in classical conditioning of nictitating membrane and eyelid responses

Electrophysiological recording of neural unit activity during paired training trials from the ipsilateral cerebellum in rabbits well trained in short-delay classical conditioning of the nictitating membrane (NM) and eyelid responses showed CS- and UCS-evoked responses and a pattern of increased neural activity that correlates with the learned behavioral response. Large ablations of the ipsilateral cerebellum completely and permanently abolished the conditioned response in well trained animals, as did more localized stereotaxic lesions. These lesions had no effect at all on the unconditioned reflex response. In marked contrast, conditioned responses were easily trained in the eye contralateral to the cerebellar lesion. We suggest that at least a part of the “engram,” the essential neuronal plasticity that codes the learned response, may be localized to the cerebellum.

Acquisition of classical conditioning without cerebellar cortex.

The left cerebellar cortex was surgically aspirated in rabbits who were then subsequently trained for classical conditioning of the nictitating membrane. All rabbits were trained sequentially on both eyes. Rabbits with the lesion confined to the cerebellar cortex were able to learn with the eye ipsilateral to the lesion although it took many times longer than reported for either naive rabbits or for rabbits first trained on the unlesioned, contralateral side. Rabbits with lesions that included the cerebellar cortex and the cerebellar interpositus nucleus did not learn with the eye ipsilateral to the lesion. Learning with the eye contralateral to either type of lesion was always very rapid. It is now clear on the basis of this and previous studies that cerebellar cortex, unlike the cerebellar interpositus nucleus, is not essential for acquisition or relearning/retention of classical conditioning. However, cerebellar cortex normally plays an important role since acquisition of classical eyeblink conditioning is prolonged and of poor quality in its absence.

Effect of kainic acid lesions of the cerebellar interpositus nucleus on eyelid conditioning in the rabbit.

It has been shown previously that unilateral electrolytic lesions of the cerebellar dentate-interpositus nuclei selectively abolish or prevent a classically conditioned nictitating membrane/eyelid response in rabbits but do not prevent retention or learning on the contralateral side. The present study demonstrates that kainic acid lesions of the dorsal interpositus nucleus are sufficient for this effect.

Reversible lesions of the cerebellar interpositus nucleus during acquisition and retention of a classically conditioned behavior.

Previous lesion, recording, and stimulation studies have implicated the cerebellum and its associated circuitry as essentially involved in classical conditioning of discrete, somatic muscle responses. In 2 experiments, the interpositus nucleus of the cerebellum was assessed for the plasticity associated with learning and memory of the nictitating membrane (NM) response by using local cooling as a reversible lesion technique. In well-trained animals (Experiment 1), NM conditioned responses (CRs) were abolished during cooling of the interpositus but reappeared when the interpositus returned to body temperature. This cooling-warming protocol could be repeated many times. Cooling could be prolonged (one session, approximately 1 hr) with recovery of NM CRs as tested on the next day. Multiple-unit recordings related to learning were also absent in the interpositus and red nucleus during cooling. In naive animals (Experiment 2), both behavioral and unit CRs did not develop while training with cooling. There was no evidence of savings when training continued without cooling: Behavioral and unit CRs developed as if the animals were still naive. These results support the idea that the interpositus nucleus of the cerebellum is the critical locus for learning and memory of this classical CR.

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.

Concomitant classical conditioning of the rabbit nictitating membrane and eyelid responses: Correlations and implications

Simultaneous recordings of muscle unit activity from the muscles of the left and right eyelids (M. obicularis oculi) and recordings of the movement of the left nictitating membrane (NM) were taken during classical conditioning in the rabbit using a tone CS paired with an airpuff UCS to the left cornea. The unconditioned eyelid responses were found to be bilateral. The conditioned eyelid responses were also bilateral in most animals. Both the conditioned and unconditioned eyelid responses were larger on the left side. The conditioned responses of the left and right eyelids and the left NM were found to increase in magnitude and decrease in latency from the onset of the CS over training trials in almost the exactly same manner (correlations as high as .99). Behaviorally, the three responses could occur independently, suggesting that the cranial nuclei which control them (left abducens/accessory abducens, left facial nucleus, right facial nucleus) are not strongly coupled. Thus, for the learned response, it is suggested that the three nuclei are controlled by a common central system. This finding has implications for the nature of the engram--the essential neuronal circuitry encoding the learned response.

Classical conditioning in 3-, 30-, and 45-month-old rabbits: Behavioral learning and hippocampal unit activity

Evidence has accumulated to suggest that the hippocampus is impaired by aging processes. Hippocampal multiple unit activity was examined in 3-, 30-, and 45-month-old rabbits during classical conditioning in the trace paradigm. Animals were trained daily with 126 paired trials of tone and airpuff. The 3-month-olds learned to a criterion of 8/9 CRs in a mean of 3.2 days, while the 30-month-olds took a mean of 9.4 days, and the 45-month-olds took a mean of 11.75 days. Neural unit activity in the hippocampus modeled the behavioral eyelid response but took much longer to develop in the older animals. Results suggest that rabbits as young as 2 1/2 years old show behavioral and correlated neural response deficits.

Reversible lesions of the red nucleus during acquisition and retention of a classically conditioned behavior in rabbits

Previous lesion, recording, and stimulation studies implicated the cerebellum and its associated brain-stem circuitry as essentially involved in classical conditioning of discrete, somatic muscle responses. This is a companion to our study of interpositus cooling, which showed that the formation of a memory was prevented. In the present study, we assess the red nucleus for its role in the plasticity associated with learning and memory by using local cooling as a reversible lesion technique. A cooling probe was implanted lateral to the red nucleus. Recording electrodes were implanted in the right red nucleus and the left interpositus nucleus. Animals were trained for 5 days with the cooling probe activated. No behavioral conditioned responses (CR) developed, and multiple unit recordings related to learning did not develop in the red nucleus. However, a learning related model did develop in the interpositus. After 5 days of training while cooling, animals were given 5 days of normal training (cooling probe inactive) to assess retention. Substantial savings were evident when normal training was given. CRs appeared quickly on the first day of normal training, and multiple unit models were present in both red nucleus and interpositus nucleus. These results support the idea that the red nucleus is a necessary efferent for the memory trace formed in the cerebellum.

Are eyeblink responses to tone in the decerebrate, decerebellate rabbit conditioned responses?

Bloedel and associates recently claimed to have established conditioned eyeblink responses in the acute decerebrate, decerebellate rabbit. Their training procedure was extreme massed practice (mean intertrial interval of 9 s) and they used an idiosyncratic definition of the conditioned response (10% or more of the unconditioned response amplitude). They did not measure or control the excitability of their preparations and did not run any separate control groups for alpha responses, alpha conditioning or pseudoconditioning. Using normal animals we compared their training procedure with procedures standard in the field and analyzed the consequences of their scoring procedure. Our group trained at a 30-s intertrial interval (ITI) showed clear learning in the training session. In marked contrast, 3 groups trained at a 9-s ITI developed no conditioned responses. We also found that the method of scoring used by Bloedel and associates counts many spontaneous responses as conditioned responses (CRs) if unconditioned response (UR) amplitudes are low, excludes genuine CRs if UR amplitudes are high and does not control for the occurrence of spontaneous responses. It must therefore be concluded that the eyeblink responses to tone reported by Bloedel and associates to occur in the decerebrate, decerebellate rabbit are not associative CRs as they develop in the normal animal.