**5. Experiment 1: Numeric estimation of real surface orientation in depth assessed by dynamic touch with the tip of the index finger**

The main question of the present experiment is whether the haptic perception of surface orientation (geographical slant in the pitch axis relative to the observer) by dynamic touch will show the same kinds of spatial bias documented in vision by Durgin, Li and Hajnal (2010). Whereas Durgin, Li and Hajnal reported evidence of similar bias in perceived surface orientation based on static contact with the palm of the hand, Hajnal et al. (2011) have argued that there is very little bias evident in dynamic touch. However, as noted above, it is not clear that the linear fit they plotted is better justified by their data than a cubic fit, like that shown in Figure 7. Moreover, examination of the raw data of Hajnal et al. suggested that participants relied nearly exclusively on angular estimates that were multiples of 5. This may have contributed to distorting the lower end of the range. Finally, because Hajnal et al. did not constrain their participants' exploratory strategies, it is possible that the observed function was less exaggerated in some places because of a tendency for oblique paths of travel along the slanted surface. In the present study we used real surfaces and provided a ridge along the main axis of the surface to ensure that the steepest direction of inclination was felt. In addition we asked that participants be as precise as possible in their responses.
