Two initial experiments demonstrated that direction aftereffects of potentiation in the extraocular muscles (induced through sustained versional rotation to the side) generally increase as a function of the magnitude and duration of the inducing ocular rotation and can be built up under conditions of varied as well as constant fixation. The results suggest an alternative to perceptual learning accounts of adaptation to wedge prisms, namely, that adaptive changes in perceived distance and direction are induced through a systematic bias in eye posture rather than exposure to informational discrepancy. The muscle potentiation hypothesis was tested directly against a learning hypothesis in a final adaptation experiment. Differences in the decay functions for observers engaged in five different types of interpolated activity strongly support the muscle potentiation explanation.