Disentangling acute motor deficits and adaptive responses evoked by the loss of cerebellar output.

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Cerebellar patients exhibit various motor impairments, but the sequence of primary and compensatory processes leading to these deficits remains unclear. To investigate this, we reversibly blocked cerebellar outflow in monkeys performing planar reaching. The block caused a spatially tuned reduction in hand velocity due to decreased muscle torque, especially in movements with high coupling torques. Examining repeated movements to the same target revealed that during multi-joint reaching movements, the reduced velocity was driven by an acute deficit superimposed on a gradually emergent strategic slowing aimed at minimizing passive inter-joint interactions. However, the reduced velocity did not explain the decomposed and variable trajectories observed during the cerebellar block. Our findings suggest that loss of cerebellar signals leads to motor impairments through insufficient muscle torques and an altered control strategy to compensate for the impaired control of limb dynamics. However, impaired feedforward control also increases motor noise, which cannot be strategically eliminated.