Pär Svensson

Learned movements elicited by direct stimulation of cerebellar mossy fiber afferents.

Definitive evidence is presented that the conditioned stimulus (CS) in classical conditioning reaches the cerebellum via the mossy fiber system. Decerebrate ferrets received paired forelimb and periocular stimulation until they responded with blinks to the forelimb stimulus. When direct mossy fiber stimulation was then given, the animals responded with conditioned blinks immediately, that is, without ever having been trained to the mossy fiber stimulation. Antidromic activation was prevented by blocking mossy fibers with lignocaine ventral to the stimulation site. It could be excluded that cerebellar output functioned as the CS. Analysis of latencies suggests that conditioned responses (CRs) are not generated by mossy fiber collaterals to the deep nuclei. Hence, the memory trace is probably located in the cerebellar cortex.

Feedback control of Purkinje cell activity by the cerebello-olivary pathway.

The pathway from the deep cerebellar nuclei to the inferior olive, the source of the climbing fibre input to the cerebellum, inhibits olivary transmission. As climbing fibre activity can depress the background firing of the Purkinje cells, it was suggested that nucleo‐olivary (N–O) inhibition is a negative feedback mechanism for regulating Purkinje cell excitability. This suggestion was investigated, in a set‐up with decerebrate ferrets, both by blocking and by stimulating cerebellar output while recording Purkinje cell activity. Blocking the N–O pathway was followed by an increased climbing fibre activity and a dramatic reduction in simple spike firing. Stimulation of the N–O fibres depressed climbing fibre responses and caused an increase in simple spike firing. These results are taken as support for the feedback hypothesis.

Number of spikes in climbing fibers determines the direction of cerebellar learning.

Cerebellar learning requires context information from mossy fibers and a teaching signal through the climbing fibers from the inferior olive. Although the inferior olive fires in bursts, virtually all studies have used a teaching signal consisting of a single pulse. Following a number of failed attempts to induce cerebellar learning in decerebrate ferrets with a nonburst signal, we tested the effect of varying the number of pulses in the climbing fiber teaching signal. The results show that training with a single pulse in a conditioning paradigm in vivo does not result in learning, but rather causes extinction of a previously learned response.

Effect of Conditioned Stimulus Parameters on Timing of Conditioned Purkinje Cell Responses.

Pavlovian eyeblink conditioning is a useful experimental model for studying adaptive timing, an important aspect of skilled movements. The conditioned response (CR) is precisely timed to occur just before the onset of the expected unconditioned stimulus (US). The timing can be changed immediately, however, by varying parameters of the conditioned stimulus (CS). It has previously been shown that increasing the intensity of a peripheral CS or the frequency of a CS consisting of a train of stimuli to the mossy fibers shortens the latency of the CR. The adaptive timing of behavioral CRs probably reflects the timing of an underlying learned inhibitory response in cerebellar Purkinje cells. It is not known how the latency of this Purkinje cell CR is controlled. We have recorded form Purkinje cells in conditioned decerebrate ferrets while increasing the intensity of a peripheral CS or the frequency of a mossy fiber CS. We observe changes in the timing of the Purkinje cell CR that match the behavioral effects. The results are consistent with the effect of CS parameters on behavioral CR latency being caused by corresponding changes in Purkinje cell CRs. They suggest that synaptic temporal summation may be one of several mechanisms underlying adaptive timing of movements.

Extinction of conditioned blink responses by cerebello-olivary pathway stimulation.

Learning of classically conditioned eyeblink responses depends on mechanisms within the cerebellum. It has been suggested that climbing fibres from the inferior olive transmit the unconditioned stimulus signal to the cerebellum. We have previously shown that the pathway from the deep cerebellar nuclei to the inferior olive inhibits olivary activity. It is known that repeated presentation of the conditioned stimulus on its own leads to extinction of the conditioned response. If the unconditioned stimulus signal is transmitted to the cerebellum via the inferior olive – climbing fibre system then stimulation of the nucleo-olivary pathway just before the unconditioned stimulus in a trained animal should lead to extinction. The results from this investigation confirm this.

Bilateral disruption of conditioned responses after unilateral blockade of cerebellar output in the decerebrate ferret

1 Lesions of the cerebellar cortex can abolish classically conditioned eyeblink responses, but some recovery with retraining has been observed. It has been suggested that the recovered responses are generated by the intact contralateral cerebellar hemisphere. In order to investigate this suggestion, bilaterally acquired conditioned responses were studied after the unilateral blockade of cerebellar output. 2 Decerebrate ferrets were trained with ipsilateral electrical forelimb stimulation (300 ms, 50 Hz, 1 mA) as the conditioned stimulus and bilaterally applied peri‐orbital stimulation (40 ms, 50 Hz, 3 mA) as the unconditioned stimulus. The conditioned and unconditioned eyeblink responses were monitored by EMG recordings from the orbicularis oculi muscle. The output from one cerebellar hemisphere was blocked either by injecting small amounts of lignocaine (lidocaine; 0.5‐1.0μl)) into the brachium conjunctivum, or by a restricted mechanical lesion of the brainstem rostral to the cerebellum. 3 As described by previous investigators, the unilateral blockade of cerebellar output abolished ipsilateral conditioned responses. 4 More importantly, such blockade also abolished or strongly depressed contralateral conditioned responses. When mechanical lesions of the brachium conjunctivum were made, contralateral responses, in contrast to ipsilateral responses, recovered within 1–2.5 h. 5 When the unconditioned stimulus was removed on one side, causing extinction of conditioned responses on this side, conditioned responses were temporarily depressed on the trained side as well. 6 Unilateral interruption of cerebellar output had no clear effect on contralateral unconditioned reflex responses. 7 The results demonstrate that one cerebellar hemisphere in ferrets exerts a marked control of contralateral conditioned eyeblink responses, probably via premotor neurones involved specifically in conditioned, and not in unconditioned, responses.

Golgi Cell Activity During Eyeblink Conditioning in Decerebrate Ferrets.

Golgi cells have a central position in the cerebellar cortical network and are indirectly connected to Purkinje cells, which are important for the acquisition of learned responses in classical conditioning. In order to clarify the role of Golgi cells in classical conditioning, we made extracellular Golgi cell recordings during different stages of conditioning, using four different conditional stimuli. Our results show that forelimb and superior colliculus stimulation, but not mossy fiber stimulation, evokes a short latency increase in Golgi cell firing. These results suggest that Golgi cells are involved in modulating input to the cerebellar cortex. There were however no differences in Golgi cell activity between naïve and trained animals, which suggests that Golgi cells are not intimately involved in the plastic changes that occur during classical conditioning. The absence of long latency effects of the conditional stimulus also questions whether Golgi cells contribute to the generation of a temporal code in the granule cells.