Disturbed cerebellar input affects consecutive movement performance : Comparison of healthy subjects and patients with cerebellar ataxia

Objective The cerebellum should be involved in providing temporal computations in motor production. The inability to compute time differences would affect time-related tasks. However, the cerebellar role in proprioception to control precise movement performance is still contradictory. The aim of the study was to investigate the characteristics of performance of the rapid terminal movements in patients with cerebellar ataxia. Ataxia of these patients had been identified as "pure" spinocerebellar ataxia. Method Movement performance was compared in six patients and six healthy subjects. Movements were performed from the initial to the target position, with the movement length of 40 degree in the elbow flexion. First, motor threshold of the motor cortex was determined. TMS was then applied with the double-cone coil right of the inion, in the two experimental conditions: stimulus was applied with the strength of 5% below the established motor threshold, at the moment of computer generated tone command to start the flexion movement, or stimulus was applied 20 ms before the GO signal, with the same strength. Two additional conditions were also tested: TM stimulus was applied with the strength of 30% above the motor threshold at the moment of movement start, and at 20 ms before movement start. Results Results point toward extension of the performed movement when stimulus was applied 20 ms before the movement start. Patients show tendency to lengthen their movements, and therefore accuracy of the movements deteriorate. Length of the movements was also prolonged in the conditions where stronger stimulus was applied, but accuracy was less affected with increase in the stimulus strength. Conclusion Accuracy of the movements was affected when the stimulus was applied in the phase of the movement preparation. Disturbance of the preparatory processes to establish correct movement pattern appear to be important even in the patients with pure cerebellar ataxia.