Hand orientation is insufficiently compensated for in haptic spatial perception.

What humans haptically perceive as parallel is often far from physically parallel. These deviations from parallelity are highly significant and very systematic. There exists accumulating evidence, both psychophysical and neurophysiological, that what is haptically parallel is decided in a frame of reference intermediate to an allocentric and an egocentric one. The central question here concerns the nature of the egocentric frame of reference. In the literature, various kinds of egocentric reference frames are mentioned for haptic spatial tasks, such as hand-centered, arm-centered, and body-centered frames of reference. Thus far, it has not been possible to distinguish between body-centered, arm-centered, and hand-centered reference frames in our experiments, as hand and arm orientation always covaried with distance from the body-midline. In the current set of experiments the influence of body-centered and hand-centered reference frames could be dissociated. Subjects were asked to make a test bar haptically parallel to a reference bar in five different conditions, in which their hands were oriented straight ahead, rotated to the left, rotated to the right, rotated outward or rotated inward. If the reference frame is body-centered, the deviations should be independent of condition. If, on the other hand, the reference frame is hand-centered, the deviations should vary with condition. The results show that deviation size varies strongly with condition, exactly in the way predicted by the influence of a hand-centered egocentric frame of reference. Interestingly, this implies that subjects do not sufficiently take into account the orientation of their hands.