Purkinje cell activity during learning a new timing in classical eyeblink conditioning.

During classical eyeblink conditioning, animals acquire adaptive timing of the conditioned response (CR) to the interstimulus interval (ISI) between the conditioned stimulus (CS) and the unconditioned stimulus (US). To investigate this coding of the timing by the cerebellum, we analyzed Purkinje cell activities during acquisition of new timing after we shifted the ISI. Decerebrate guinea pigs were conditioned to an asymptotic level of learning using a delay paradigm with a 250-ms ISI. A 350-ms tone and a 100-ms electrical shock were used as the CS and US, respectively. As reported previously in other species, Purkinje cells in the simplex lobe exhibited three types of responses to the CS: excitatory, inhibitory, or a combination of the two. After we increased the ISI to 400 ms, the frequency of the CR stayed at an asymptotic level, but the latency of the CR peak became gradually longer. Two types of cells were observed, based on changes in the nature of their response to the CS; one changed its type of response in parallel with learning the new timing, while the other did not. There was no correlation between the type of response before and after we changed the ISI. In some cells, the peak latency of activities became longer or shorter, while the type of response did not change. These results suggest that some Purkinje cells code the timing of the CR, but do not play a consistent role in shaping the CR over a range of ISIs.