The role of the striatum in adaptation learning: a computational model.

To investigate the functional role of the striatum in visuo-motor adaptation, we extend the DIRECT-model for visuo-motor reaching movements formulated by Bullock et al.(J Cogn Neurosci 5:408-435,1993) through two parallel loops, each modeling a distinct contribution of the cortico-cerebellar-thalamo-cortical and the cortico-striato-thalamo-cortical networks to visuo-motor adaptation. Based on evidence of Robertson and Miall(Neuroreport 10(5): 1029-1034, 1999), we implement the function of the cortico-cerebellar-thalamo-cortical loop as a module that gradually adapts to small changes in sensorimotor relationships. The cortico-striato-thalamo-cortical loop on the other hand is hypothesized to act as an adaptive search element, guessing new sensorimotor-transformations and reinforcing successful guesses while punishing unsuccessful ones. In a first step, we show that the model reproduces trajectories and error curves of healthy subjects in a two dimensional center-out reaching task with rotated screen cursor visual feedback. In a second step, we disable learning processes in the cortico-striato- thalamo-cortical loop to simulate subjects with Parkinson's disease (PD), and show that this leads to error curves typical of subjects with PD. We conclude that the results support our hypothesis, i.e., that the role of the cortico-striato-thalamo-cortical loop in visuo-motor adaptation is that of an adaptive search element.