**5.1 Method**

All experimental procedures were conducted in accord with the ethical standards of the American Psychological Association and approved by a local institutional review board. The general method is similar to those employed by Hajnal et al. (2011) and by Durgin, Li and Hajnal (2010). Participants made numeric estimates of the slant of surfaces explored haptically.

Spatial Biases and the Haptic Experience of Surface Orientation 87

Participants were shown the apparatus with the surface in the horizontal position and the procedure was explained to them prior to signing an informed consent form. Participants were shown where to stand (directly in front of the apparatus) and then asked put on the blindfold. Before each trial, the surfaces were set to the intended orientation manually using pre-set positions by the experimenter who then told the participant to explore the surface. The participants were to run the tip of their right index finger alongside the elevated ridge formed by a wire attached to the surface. No time limit was specified for exploration. When satisfied with their haptic observation the participant was to indicate the orientation of the surface in deg. Half were instructed that vertical was 0° and horizontal was 90°. The other half were told to consider horizontal to be 0° and vertical to be 90°. Participants were encouraged to be as precise as possible in their estimates by estimating orientation to the nearest 1° (even with such instruction, there is a strong bias toward values divisible by 5).

Fig. 9. Mean haptic slant estimates by reference condition in Experiment 1 with error bars

Mean estimates were computed for each presented orientation by condition. Figure 9 shows the estimates for each condition. It can be seen that the spatial bias was in the same direction in each condition inasmuch as participants overestimated deviations from horizontal and

To represent the spatial bias function, we recalculated each estimate in the vertical referent condition with respect to horizontal and then averaged all estimates with respect to horizontal. The resulting function is plotted in Figure 10, superimposed on the similarlyderived spatial function for visual slant perception from Durgin, Li and Hajnal (2010, Experiment 1), plotted earlier in Figure 1. The functions are strikingly similar, as predicted

representing ± 1 SEM. Trend lines are best fitting cubic polynomials.

**5.1.4 Procedure** 

**5.2 Results** 

underestimated deviations from vertical.
