Cerebellar activation during classical conditioning of the human flexion reflex: a PET study

The present study investigated the involvement of the cerebellum in classical conditioning of the cutaneomuscular flexion reflex in four normal volunteers using positron emission tomography (PET). The flexion reflex was elicited by electrical pulses applied to the medial plantar nerve (unconditioned stimulus, US). A tone was presented as the conditioning stimulus, which co-terminated with the US. The incidence of conditioned responses was correlated with changes in rCBI during the acquisition process of flexion reflex conditioning. Blood flow was significantly increased in an area extending from the ipsilateral cerebellum and hippocampus to bilateral frontal regions (p = 0.009). These data provide support for an involvement of the cerebellum as well as hippocampus among other neural systems in classical flexion reflex conditioning.