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Zhi Li, Frank Durgin; Hills look less steep from the edge: Proprioceptive error and frontal tendency affect the perception of downhill slopes. Journal of Vision 2009;9(8):81. doi: 10.1167/9.8.81.
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A steep incline looks very steep from the top, as others have documented. However, we observed that such an incline appeared less steep when standing at its edge. Why should slopes appear more exaggerated when standing back from the edge where gaze can be nearly parallel to the slope (making head orientation a perfect cue)? And why should slopes appear less steep as one approaches the edge? To address the first question, we asked people wearing a head-tracker and blindfold to tilt their heads down by amounts ranging from 10 to 60 deg. The values of actual head pitch observed were always about half that requested, suggesting that proprioceptive error could play a substantial role in downhill slope overestimation. Before answering the second question, we sought to document the perceptual phenomenon using immersive VR. Our software corrected optical distortions of HMD optics. We simulated a sloped surface with 3D blades of grass to provide a rich stereo environment in a scene that also contained a visible horizon. Participants judged 20 slopes (4–42 deg, by steps of 2) while standing at the edge of the hill or from a meter back. A golf ball 2 m down the hill served as a reference point, but participants began each trial looking toward the horizon. Between and within subjects, all slopes were judged much shallower when standing near the edge. Between-subject effects were by 20 deg for the steepest slopes analyzed, consistent with our observations of real hills. Whereas frontal tendency can help explain slope overestimation when facing a hill from the bottom, proprioceptive error may be fundamental to explaining overestimation from the top (see Hajnal & Durgin, this VSS). Nonetheless, effects of frontal tendency can account for why slopes appear less steep when standing near the edge.
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