Visuo-motor recalibration alters depth perception

Relative depth from retinal disparities is overestimated at distances within the peripersonal space (where objects can be reached) and underestimated beyond the peripersonal space. This finding suggests that the extent of the peripersonal space may constrain the interpretation of retinal disparity. If this is the case then modifying the peripersonal space through visuo-motor recalibration should also modulate perceived depth from retinal disparities. Participants performed a manual depth estimation task before and after visuo-motor recalibration. Visuo-motor recalibration was induced by displacing the visual feedback of the index finger in depth (15 cm). During the recalibration phase participants were required to repeatedly reach-to-point without haptic feedback to a vertical rod positioned at various distances within and beyond the peripersonal space. In the test phase, participants viewed a display composed of three vertical rods: one rod was positioned midway and in front of two flanking rods. Three depth separations were used between the central rod and the flanker rods, and this arrangement of rods was presented at three distances. Three-dimensional information was provided in a virtual environment by binocular disparities with consistent vergence and accommodation cues. We found that visuo-motor recalibration strongly increased the estimates in the manual depth estimation task, coherently with the idea that the perceived depth is rescaled in accordance with an extended peripersonal space. Contrary to this result, the conflict between proprioceptive and visual cues in the recalibration phase should decrease perceived absolute distance. This would lead to an underestimation of the relative depth from retinal disparities. To test whether the effect we found in the manual depth estimation task is truly perceptual, participants also performed an open-loop reach-to-grasp task. Interestingly, in this motor task neither the grip aperture nor the final hand position were affected by recalibration revealing also a possible dissociation between reach-to-point and reach-to-grasp movements.