Experimental tasks and protocol.

Both groups first performed 95 trials of a precision pointing task from a seated position. The control group wore goggles with 0-diopter (0-D) lenses that did not alter visual perception. The prism group wore goggles with varying strength, rightward-shifting prism lenses (mean strength = 18-diopters, 18-D; distribution of lens strengths shown) for 60 trials after pointing with 0-diopter lenses for 20 trials. This group pointed for an additional 15 trials while wearing 0-diopters lenses. Participants in both groups had vision of the target and hand during the pointing movements in all phases (i.e., closed-loop pointing). Both groups then performed 25 trials of a tracing task in which we reversed the relationship between the stylus and cursor motion in the left-right direction, such that a rightward movement of the stylus would cause the cursor on the computer screen to move leftward. The following day, both groups repeated the tracing task. At right are examples of tracing performance (green) from one participant on trial 1 of day 1 (left) and trial 1 of day 2 (right). Top, screen view seen by the participant (colors have been changed here for clarity) with counterclockwise cursor path superimposed on the target shape (166 mm wide x 152 mm tall; path = 5 mm wide). Bottom, clockwise stylus movement. Orange circle at top of shape is the tracing start and end point.