**3.1 Haptic bias in the perceived orientation of locomotor surfaces**

Hajnal et al. (2011) asked participants to step onto ramps that they could not see. (They were either wearing a blindfold or an occluding collar that blocked their view of the floor.) The ramps varied in orientation from 4° to 16°. Participants were asked to provide either verbal estimates of the surface orientation or to gesture the orientation with the their hand, which was measured using digital photography. The data are reproduced in Figure 4, along with a photograph of the experimental situation. Both forms of measurement documented surprisingly-large perceptual exaggerations of haptic slant. For somewhat steep ramps, the haptic exaggeration of perceived slant was even greater than the visual exaggeration observed when the same participants judged the orientations of the ramps when looking at them afterward. For example, Hajnal et al. found that participants standing on a 16° ramp judged it to be about 35-40° (both verbally, and as measured by hand gesture) based on their haptic experience, whereas when looking down at a 16° ramp (while standing on a level surface at the base of the ramp) they judged it to be only about 22-24°. The same pattern (higher estimates based on haptic perception) was found for a 14.5° ramp by Durgin et al. (2009) who collected visual estimates before having people step onto the ramp.

Haptic evaluations of the surface under one's feet are more valuable for immediate motor planning than for distal route planning. Hajnal et al. (2011) therefore suggested that these distortions might be the perceptual consequence of dense coding of orientations near horizontal that led to functionally exaggerated perceived orientation for the more precise control of action.

Kinsella-Shaw et al. (1992) had previously reported that participants were good at matching haptic inclines underfoot to visual inclines. To rule out the possibility that the haptic exaggerations were learned from calibrating haptic experience to visual experience, Hajnal et al. (2011) also tested a population of four congenitally blind individuals using verbal report. The blind individuals' estimates were quite similar to those of the sighted participants, though they were slightly higher. This indicates that the haptic exaggeration of the apparent inclination of surfaces on which one stands exists even in the absence of visual experience.
