Nonequivalent roles for motor and visual feedback in the Müller-Lyer and horizontal-vertical illusions

The relative contributions of motor and visual feedback to illusion decrement for the Müller-Lyer (ML) and horizontal-vertical (HV) illusions were estimated after subjects judged the perceived equality of standard and comparison lines with or without visual feedback and after the subject or the experimenter adjusted the comparison line (motor feedback). Visual feedback in the absence of motor feedback reduced magnitude of illusion by 38% for the ML illusion and by 45% for the HV illusion. Motor feedback in the absence of visual feedback reduced magnitude of illusion by 17% for the ML illusion and by 27% for the HV illusion. Estimates of “strategic” factors, commonly interpreted as the correction of inaccurate routine problem-solving and judgmental factors, also contain components of motor learning.