Partitioning the components of visuomotor adaptation to prism-altered distance.

While the mechanisms of short-term adaptation to prism-altered apparent visual direction have been widely investigated, the processes underlying adaptation to prism-altered perceived distance are less well known. This study used a hand-pointing paradigm and exposure with base-out prisms to evaluate the relative contributions of sensory (visual and proprioceptive) and motor components of adaptation to perceived-distance alteration. A main experiment was designed to elicit adaptation at the sensory and motor levels, by giving subjects altered visual feedback. A control experiment without visual feedback allowed the effects of eye muscle potentiation (EMP) induced by sustained fixation through the prisms to be uncovered. In the main experiment, the aftereffects were partitioned into two-thirds visual and one-third motor, with no significant proprioceptive component. These results differ from the classical pattern of short-term adaptation to prism-altered apparent visual direction, which includes mainly proprioceptive/motor adaptive components, with a smaller visual component. This difference can be attributed to differences in accuracy between proprioception and vision for localization in depth or in lateral directions. In addition, a comparison of the visual aftereffects in the main and control experiments revealed two sub-components with equal contributions: a recalibration of the mapping between the vergence signal and perceived distance, and an EMP-related aftereffect. These findings indicate that "visual" adaptation actually involves a multiplicity of processes.