Directional tuning of speed-related activation for reaching in the vertical plane in cerebellar ataxia

Abstract

The role of the cerebellum in the spatial tuning of goal-directed multi-joint movements in human is unknown. We analyzed the directional tuning of phasic EMG activities associated with upper limb reaching movements (12 targets) in the vertical plane in healthy subjects and in patients exhibiting cerebellar ataxia. Tuning of phasic EMG activities was investigated in seven muscles (brachioradialis, biceps, medial and long head of triceps, anterior and posterior deltoid, latissimus dorsi). We digitally compressed the EMG activities corresponding to slow reaches to the same targets into the time frame of the fast EMG traces. Estimates of gravity-related components were subtracted. Peaks of EMG activities in the resulting phasic traces were identified for each muscle and each target. Aberrant privileged directions of M Peak EMG (directions associated with the maximal peak of EMG amongst the 12 peaks of EMG activity in the sagittal plane) were found in all ataxic patients. Directional dominance, defined as the ratio of the M Peak EMG divided by the peak EMG in the opposite direction, was significantly higher in controls than in ataxic patients for one distal muscle (brachioradialis) and one proximal muscle (anterior deltoid). The spreading of EMG activities assessed by the global areas of the polar plots of phasic traces was broader in patients for the biceps and medial head of triceps. The distribution of densities of EMG activities (DDEMG) amongst the four quarters of the vertical plane, an index of the contrast in the intensities between quarters in polar plots, revealed increased values in control subjects for the brachioradialis, the biceps and the anterior deltoid as compared to ataxic patients. Representation of Net Vectors obtained from polar plots of peaks of EMG activities demonstrated an abnormal directional tuning in ataxic patients. In the majority of the cases, the Net Vector was outside the normal range for the following muscles: brachioradialis, biceps, anterior deltoid, posterior deltoid. This study reveals that cerebellar ataxia is associated with defective spatial properties of EMG activity during multiple joint movements. Privileged directions associated with M Peak EMG and Net Vectors are erroneous. We demonstrate that the cerebellum plays a determinant and unsuspected role in the spatial modulation of activation during speed-related action for reaching.