Sensorimotor adaptation error signals are derived from realistic predictions of movement 1 outcomes 2 3 Aaron

20 Neural systems that control movement maintain accuracy by adaptively altering motor 21 commands in response to errors. It is often assumed that the error signal that drives adaptation is 22 equivalent to the sensory error observed at the conclusion of a movement; for saccades, this is 23 typically the visual (retinal) error. However, we instead propose that the adaptation error signal 24 is derived as the difference between the observed visual error and a realistic prediction of 25 movement outcome. Using a modified saccade-adaptation task in human subjects, we precisely 26 controlled the amount of error experienced at the conclusion of a movement by back-stepping the 27 target so that the saccade is hypometric (positive retinal error), but less hypometric than if the 28 target had not moved (smaller retinal error than expected). This separates prediction error from 29 both visual errors and motor corrections. Despite positive visual errors and forward-directed 30 motor corrections, we find an adaptive decrease in saccade amplitudes – a finding that is well31 explained by the employment of a prediction-based error signal. Furthermore, adaptive changes 32 in movement size are linearly correlated to the disparity between the predicted and observed 33 movement outcomes, in agreement with the forward-model hypothesis of motor learning which 34 states that adaptation error signals incorporate predictions of motor outcomes computed using a 35 copy of the motor command (efference copy). 36