Vision in depth is distorted. A similar distortion can be observed for pointing to visual targets in depth. It has been suggested that pointing errors in depth reflect the visual distortion. Alternatively, pointing in depth might be guided by a prior that biases movements toward the natural grasping distance at which object manipulation is usually performed. To dissociate whether pointing is guided by distorted vision only or whether it takes into account a natural grasping distance prior, we adapted pointing movements. Participants received visual feedback about the success of their pointing once the movement was finished. We distorted the feedback to signal either that pointing was not far enough or in separate sessions that pointing was too far. Participants adapted to this artificial error by either extending or shortening their pointing movements. The generalization of pointing adaptation revealed a bias in movement planning that is inconsistent with pointing being guided only by distorted vision but with the involvement of knowledge about the natural grasping distance. Adaptation was strongest for pointing movements to a middle position that corresponds to the natural grasping distance and it was weakest for movements leading away from it. It has been demonstrated that pointing adaptation in depth changes visual perception (Volcic R, Fantoni C, Caudek C, Assad JA, Domini F. J Neurosci 33: 17081-17088, 2013). We also wondered how effects of pointing adaptation on visual space would generalize in depth. We found that adaptation changed visual space, but that this change was independent of the adaptation direction.NEW & NOTEWORTHY Which information guides pointing in 3D space? Inaccuracies of vision in depth generate the need for the sensorimotor system to rely on other information sources to optimally plan movement trajectories. Here, we implemented pointing adaptation experiments that could dissociate if the generalization of adaptation follows visual distortions or if it is informed by a "natural grasping distance" prior. Adaptation was strongest for movements toward the natural grasping distance, suggesting the latter hypothesis to be true.
Keywords: 3D vision; adaptation; pointing.