The visual and haptic contributions to hand perception.

Previous research has found that the perception of our hands is distorted. The characteristics of this distortion are an overestimation of hand width and an underestimation of finger length. The present study examined the role that different sensory modalities (vision and/or haptics) play in the perception of our hands. Participants pointed to their concealed hand in one of three groups: Vision+Haptics, Vision-only, or Haptics-only. Participants in the Vision+Haptics group had vision (non-informative) of the experimental setup and of the pointing hand, but no vision of the hand being estimated. They also experienced haptic feedback as the palm of the hand was in contact with the undersurface of a tabletop, where the estimations were made. Participants in the Vision-only group, instead of placing the hand to be estimated underneath the tabletop, they placed it behind their backs. Participants in this group were asked to imagine as if the hand was under the table when making their estimations. In the Haptics-only group, participants completed the task with the hand underneath the tabletop (as in the Vision+Haptics group) but did so while wearing a blindfold (no vision). All participants estimated the position of ten landmarks on the hand: the fingertip and the metacarpophalangeal joint of each digit. Hand maps were constructed using a 3D motion capture system. Participants in the Haptics-only group produced the most accurate hand maps. We discuss the possibility that vision interferes with somatosensory processing.