The Cerebellum and Red Nucleus Are Not Required forIn VitroClassical Conditioning of the Turtle Abducens Nerve Response

Abstract

The role of the cerebellum during motor learning is a controversial issue. Many authors have suggested that the cerebellum and its connections with the red nucleus are essential for the acquisition of the conditioned eye blink reflex. Although there is little argument that the cerebellum is an important component to the generation of the conditioned response (CR), a number of studies have suggested that the cerebellum is not essential for conditioning. Using an in vitro model of the classically conditioned turtle abducens nerve response, we investigated the effect of cerebellar and red nucleus lesions on the acquisition, extinction, and reacquisition of CRs. Neural discharge was recorded from the abducens nerve after a single shock unconditioned stimulus (US) was applied to the ipsilateral trigeminal nerve. When the US was paired with a conditioned stimulus (CS) applied to the posterior eighth, or auditory, nerve, a positive slope of CR acquisition was recorded in the abducens nerve. After extinction stimuli in which the CS and US were alternated, the number of CRs decreased to near zero. When the CS and US were once again paired, reacquisition at a faster rate was recorded. The CRs showed unusual timing features compared with preparations in which the cerebellum was intact; they had significantly shorter latencies and showed burst-like responses. These data demonstrate that it is possible to classically condition this in vitro preparation in the absence of the cerebellum and red nucleus. However, the latencies of CRs were found to be dramatically altered in the cerebellar-lesioned preparations, suggesting that the cerebellum does play a role in the timing of the CR.