A step and a hop on the Müller-Lyer: illusion effects on lower-limb movements.

Past studies have shown that the effects of visual illusions on upper-limb movements can be modulated by the availability of current or recent visual information. The generality of this finding across body movements was examined by measuring the impact of the Müller-Lyer illusion on two types of lower limb actions. Subjects stood at one end of a Müller-Lyer figure, then either stepped or hopped to the other end. Visual feedback was manipulated through the use of visual closed-loop, open-loop, and open-loop three second delay conditions. For both stepping and hopping, effects of the illusion on movement accuracy were small in the closed-loop, moderate in the open-loop, and large in the three-second open-loop delay condition. These results were similar to those found in studies involving the Müller-Lyer illusion and upper-limb movements. The similar effects across different body movements suggests that a common visuomotor system subserves both upper and lower-limb movements, and that egocentric and allocentric reference frames make similar contributions for the two types of movements.