Cerebellar pathways and mechanisms in classical conditioning

It has been suggested that the cerebellar mossy fibre afferents transmit the conditioned stimulus (CS) information to the cerebellum. Several studies are consistent with this suggestion but alternative explanations have not been excluded. In conditioned ferrets, the forelimb CS was replaced by a direct stimulation of the cerebellar mossy fibre afferents. It was found that true conditioned responses could be elicited immediately, that is without any further training, suggesting that the CS pathway was activated. The mossy fibre stimulus must have generated the conditioned response from a cerebellar memory trace since both antidromic mossy fibre activation and post-cerebellar activation of a presumed extra-cerebellar memory trace were excluded. Demonstration of the cerebellar mossy fibre afferents as the CS pathway made it possible to address exactly what aspects of classical eye-blink conditioning the cerebellum controls. The possibility that pre-cerebellar information processing is necessary for controlling the conditioned response could be rejected. It could be shown that temporal features of the conditioned response as well as bridging of a trace interval were normal when the CS was a direct stimulation of the mossy fibres. Finally, the inhibitory nucleo-olivary pathway was characterised with respect to the temporal properties of the inhibition and the localisation of the fibres. The characterisation established a basis for future studies on the possible transmission of the unconditioned stimulus via the climbing fibres.