Cerebellar posterior interpositus nucleus as an enhancer of classically conditioned eyelid responses in alert cats

Cerebellar posterior interpositus neurons were recorded in cats during dela yed and trace conditioning of eyeblinks. Type A neurons increased their fir ing in the time interval between conditioned and unconditioned stimulus pre sentations for both paradigms, while type B neurons decreased it. The disch arge of different type A neurons recorded across successive conditioning se ssions increased, with slopes of 0.061-0.078 spikes/ s/trial. Both types of neurons modified their firing several trials in advance of the appearance of eyelid conditioned responses, but for each conditioned stimulus presenta tion their response started after conditioned response onset. Interpositus microstimulation evoked eyelid responses similar in amplitude and profiles to conditioned responses, and microinjection of muscimol decreased conditio ned response amplitude. It is proposed that the interpositus nucleus is an enhancer, but not the initiator, of eyelid conditioned responses.