Size of error affects cerebellar contributions to motor learning

SE Criscimagna-Hemminger, AJ Bastian, and R Shadmehr (2010) Journal of Neurophysiology

Abstract Small errors may affect the process of learning in a fundamentally different way than large errors. For example, adapting reaching movements in response to a small perturbation produces generalization patterns that are different than large perturbations. Are distinct neural mechanisms engaged in response to large vs. small errors? Here, we examined the motor learning process in patients with severe degeneration of the cerebellum. Consistent with earlier reports, we found that the patients were profoundly impaired in adapting their motor commands during reaching movements in response to large, sudden perturbations. However, when the same magnitude perturbation was imposed gradually over many trials, the patients showed marked improvements, uncovering a latent ability to learn from errors. Upon sudden removal of the perturbation, the patients exhibited after-effects that persisted much longer than in healthy controls. That is, despite cerebellar damage, the brain maintained the ability to learn from small errors, and the motor memory that resulted from this learning was strongly resistant to change. Of note was the fact that upon completion of learning, the motor output of the cerebellar patients remained distinct from healthy controls in terms of its temporal characteristics. Therefore, cerebellar degeneration impaired the ability to learn from large magnitude errors, but had a lesser impact on learning from small errors. The neural basis of motor learning in response to small and large errors appears to be distinct.