How and why neural and motor variation are related

•Errors in sensory estimates are a major source of motor variation.•Correlated sensory noise causes shared variation in neural population responses.•Shared variation propagates through the sensory-motor circuit.•Shared variation causes neuron-neuron and neuron-behavior correlations.•Averaging eliminates independent variation in neural firing.

Movements are variable. Recent findings in smooth pursuit eye movements provide an explanation for motor variation in terms of the organization of the brain's sensory-motor pathways. Variation in sensory estimation is propagated through sensory-motor circuits and ultimately causes motor variation. The sensory origin of motor variation creates trial-by-trial correlations among the responses of neurons at each level of the sensory motor circuit, and between neural and behavioral responses. We suggest that motor variation is a compromise between multiple competing constraints. The brain strives for motor behavior that is 'good enough' in the face of constraints that tend to promote variation.