Movement errors can rise in the brain before the movement starts.

Churchland et al report in Neuron Magazine that variations in firing of the motor cortical neurons that plan and preceed trained skilled physical movement (like throwing a dart at a dart board) are responsible for a large fraction of the variability of the movement. This contradicts our usual assumption that something goes wrong during the movement. Recording from monkey motor and pre-motor cortex neurons, they found that variations in the velocity of trained reaches correlated with fluctuations in brain activity during the preparatory period — hundreds of milliseconds before the movement started.