**7.3 An implied direction of calibration?**

The idea that haptic scaling of perceived orientation during dynamic touch might be based on the ratio between the vertical extent of (finger-tip) travel and the total extent of (fingertip) travel is intriguing. Gravitational forces are highly relevant to haptics. The coding of orientation in this form would serve to express a useful ratio. When there is no vertical movement, the surface is horizontal. When the vertical component equals the total extent of travel, the surface is vertical. When the vertical extent is half the total extent of travel (i.e., sin(30°) = 0.5), the surface is "half-way" between horizontal and vertical (i.e., the perceived bisection point). These facts fit our data well and seem appropriate for manual haptics. It is much less clear why the visual perception of the slant of manually-reachable surfaces should be similarly coded, given that neither the vertical extent of orientation change nor the overall surface extent are directly given in vision. However, if vision is calibrated to haptic experience, this would be sufficient to suggest why the visual and haptic codings are aligned, but also why the haptic experience of individuals who are congenitally blind are distorted in a similar manner. Insofar as it depends on manual haptic exploration, this hypothesis would apply specifically to manually-reachable surfaces. An implied direction of calibration does not require that calibration always go in this direction, but only supposes that there is a natural basis for sinusoidal scaling of manual haptic slant perception and that this basis could then drive the visual scaling.
