Stimulus-response versus stimulus-stimulus-response learning in cerebellar patients

Ample evidence exists that the cerebellum is involved in associative motor learning, particularly in eyeblink-conditioning. In visuomotor associative learning the role of the cerebellum is less clear. One open question is whether cerebellar patients' deficits in visuomotor learning are present both in a stimulus-response and a stimulus-stimulus-response association task. Twelve patients with cerebellar degeneration and 12 healthy matched control subjects participated. A magnetic resonance imaging (MRI) volumetric analysis of the cerebellum was performed to assess the degree of cerebellar atrophy. In a blocked design, subjects had to learn the association between one color square or two color squares and a right or left key press. In the latter condition, the two colors were shown one after the other in the same sequence except for two blocks at the end of the experiment. Overall, cerebellar subjects reacted significantly slower than controls. In both groups, reaction time decreased over blocks, and the learning effect was more pronounced in the stimulus-response than in the stimulus-stimulus-response condition. Post hoc analyses revealed that learning differences between conditions were significant in cerebellar patients but not control subjects. Furthermore, only healthy subjects were irritated, i.e., they significantly increased reaction times in the blocks with reversed sequence in the stimulus-stimulus-response condition. Cerebellar subjects tended to name less correct stimulus-stimulus-response associations after the experiment. Finally, cerebellar volume correlated with parameters of motor performance, but not learning. In conclusion, cerebellar patients showed deficits in stimulus-stimulus-response, but not stimulus-response learning. Future experiments are needed to differentiate between possible deficits in learning the stimulus-stimulus association, use of sequence information, and/or impaired motor performance interfering with learning.