**4.1 Calibration between proprioception and the visual experience of slant**

The function shown in Figure 6 for free-hand manual gestures was obtained with the same set of surfaces used to obtain the function in Figure 1 for verbal estimates of slant. Nonetheless the verbal estimates show a great deal of bias (exaggeration of the deviation from horizontal), whereas the manual gestures appear to be well-calibrated. One possibility is that proprioception is calibrated to vision. That is, the perceived orientation of the hand ought to match the perceived orientation of the surface. In support of this view, Li and Durgin (submitted) have found that perceived hand orientation during free-hand gestures follows the same function as the verbal pattern shown in Figure 1.
