Limitations of PET and lesion studies in defining the role of the human cerebellum in motor learning

A possible role of the human cerebellum in conditioning of the jaw-opening reflex

The role of the human cerebellum in classical conditioning of the jaw-opening reflex was investigated using positron emission tomography (PET) in healthy subjects. The jaw-opening reflex was elicited by electrical stimulation of the right corner of the mouth (unconditioned stimulus, US). The conditioned stimulus was a tone preceding the US and coterminating with the US. Changes of regional cerebral blood flow (rCBF) were correlated with the rate of conditioning per PET scan. Conditioning effects were present in one third of all subjects. In these subjects, a significant increase of rCBF in the ipsilateral, intermediate cerebellum was shown during ongoing conditioning. Thus, the intermediate cerebellum appears to be involved in classical conditioning of the jaw-opening reflex in humans.

Non-motor associative learning in patients with isolated degenerative cerebellar disease

In recent decades it has become clear that the cerebellum is involved in associative motor learning, but its exact role in motor learning as such is still controversial. Recently, a contribution of the cerebellum to different cognitive abilities has also been considered, but it remains unclear whether the cerebellum contributes to cognitive associative learning. We compared nine patients with an isolated cerebellar degenerative disease in a cognitive associative learning task with 10 controls. Patients and controls were matched for age, sex, handedness, level of education, intelligence and capabilities of visual memory. The subjects were asked to learn the association between six pairs of colours and numerals by trial and error. Additionally, a simple reaction time and a visual scanning test were conducted in order to control for the influence of motor performance deficits in cerebellar patients. In comparison with the controls, it took the patients significantly longer to learn the correct associations between colours and numerals, and they were impaired in recognizing them later on. Two patients showed no associative learning effect at all. Neither the simple reaction time nor the visual scanning time correlated substantially with the results of associative learning. Therefore, motor-associated disabilities are unlikely to be the reason for the learning deficit in cerebellar patients. Our results suggest that the cerebellum might contribute to motor-independent processes that are generally involved in associative learning.