**3. Bias in the perceived orientation of locomotor surfaces: Hills and ramps**

While there are clear and consistent visual and spatial biases in the perception of surfaces within reach of the hands, visual biases measured in the perception of locomotor surfaces have tended to be seem even more exaggerated. These exaggerations were initially documented by Kammann (1967) who proposed that they might reflect the heightened energetics of the gravitational vertical, and by Ross (1974), but were most extensively documented by Proffitt et al. (1995). Although many of the theoretical conclusions of Proffitt et al. have been called into question by more recent work, their basic documentation of the judged orientation of 8 distinct hills on their campus remains a useful basic source regarding the overestimation of hills. In particular, Proffitt et al. reported that paths with physical slants of 4°, 5° or 6° were typically perceived as being about 20° in orientation when viewed with gaze forward, as shown in Figure 3.

Fig. 3. Verbal estimates of the (visual) slants of eight hills from Proffitt et al. (1995), ± 1 SEM. Hills were viewed from the base with gaze forward.

Because the range of orientations in most locomotor settings is no more than ± 10° from horizontal, and physical/biomechanical limitations restrict bipedal locomotion to surfaces of less than about 35°, Proffitt et al. (1995) proposed that the exaggerated perception of hill orientation was an adaptive strategy to represent the range of human behavioural potential

These observations are particularly relevant to the present discussion because they emphasize that the form of the perceptual bias may not be a "vertical tendency." Rather, the linear gain of about 1.5 found in various measurements of perceived pitch between 0° and 45° from horizontal suggests that deviations from horizontal are being exaggerated and therefore that it is the horizontal that is special. Durgin and Li (2011a) have hypothesized that the expanded scaling (near horizontal) of the perceived pitch of gaze direction serves to expand the most highly utilized portion of the angular range for purpose of maintaining greater cognitive precision. Across a variety of contexts, they proposed that a gain of about 1.5 may provide efficient scaling for retaining greater precision in this lower half of the

range when sending neural pitch signals upstream to cognitive and motor areas.

with gaze forward, as shown in Figure 3.

Hills were viewed from the base with gaze forward.

**3. Bias in the perceived orientation of locomotor surfaces: Hills and ramps**  While there are clear and consistent visual and spatial biases in the perception of surfaces within reach of the hands, visual biases measured in the perception of locomotor surfaces have tended to be seem even more exaggerated. These exaggerations were initially documented by Kammann (1967) who proposed that they might reflect the heightened energetics of the gravitational vertical, and by Ross (1974), but were most extensively documented by Proffitt et al. (1995). Although many of the theoretical conclusions of Proffitt et al. have been called into question by more recent work, their basic documentation of the judged orientation of 8 distinct hills on their campus remains a useful basic source regarding the overestimation of hills. In particular, Proffitt et al. reported that paths with physical slants of 4°, 5° or 6° were typically perceived as being about 20° in orientation when viewed

Fig. 3. Verbal estimates of the (visual) slants of eight hills from Proffitt et al. (1995), ± 1 SEM.

Because the range of orientations in most locomotor settings is no more than ± 10° from horizontal, and physical/biomechanical limitations restrict bipedal locomotion to surfaces of less than about 35°, Proffitt et al. (1995) proposed that the exaggerated perception of hill orientation was an adaptive strategy to represent the range of human behavioural potential and energetic costs and thus to inform decisions about route selection during locomotion in the wild. A difficulty with this view is that perceived hill orientation decreases as one approaches a hill (Li & Durgin, 2010), and nearer portions of hills appear shallower than farther portions (Bridgeman & Hoover, 2008). In fact, as we will discuss below, there is continuity between orientation biases we have measured for small, near surfaces and those measured by Proffitt et al. for hills (Li & Durgin, 2010).

Proffitt further proposed that physical actions, such as stepping, were controlled by an *unbiased* perceptual representation (vision for action) that was contrasted with the exaggerated representation available for long-range cognitive planning. This view has since been challenged by studies of the haptic perception of locomotor inclines.
