Title: Individuals with Cerebellar Degeneration Show Similar Adaptation Deficits with Large and Small 1 Visuomotor Errors. 2 3 4 5

31 32 The cerebellum has long been recognized to play an important role in motor adaptation. Individuals 33 with cerebellar ataxia exhibit impaired learning in visuomotor adaptation tasks such as prism 34 adaptation and force field learning. Both types of tasks involve the adjustment of an internal model in 35 order to compensate for an external perturbation. This updating process is error-driven, with the error 36 signal based on the difference between anticipated and actual sensory information. This process may 37 entail a credit assignment problem, with a distinction made between error arising from faulty 38 representation of the environment and error arising from noise in the controller. We hypothesized that 39 people with ataxia may perform poorly at visuomotor adaptation because they attribute a greater 40 proportion of their error to their motor control difficulties. We tested this hypothesis using a 41 computational model based on a Kalman filter. We imposed a 20-degree visuomotor rotation in either a 42 single large step or in a series of smaller 5-degree steps. The ataxic group exhibited a comparable 43 deficit in both conditions. The computational analyses indicate that the patients’ deficit cannot be 44 accounted for simply by their increased motor variability. Rather, the patients’ deficit in learning may 45 be related to difficulty in estimating the instability in the environment or variability in their motor 46 system. 47 48