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Perceiving distance accurately by a directional process of integrating ground information


By itself, the absolute distance of an object cannot be accurately judged beyond 2–3 m (refs 1–3). Yet, when it is viewed with reference to a flat terrain, humans accurately judge the absolute distance of the object up to 20 m, an ability that is important for various actions4,5,6,7,8. Here we provide evidence that this is accomplished by integrating local patches of ground information into a global surface reference frame. We first show that restricting an observer's visual field of view to the local ground area around the target leads to distance underestimation, indicating that a relatively wide expanse of the ground surface is required for accurate distance judgement. Second, as proof of surface integration, we show that even with the restricted view, the observer can accurately judge absolute distance by scanning local patches of the ground surface, bit by bit, from near to far, but not in the reverse direction. This finding also reveals that the surface integration process uses the near-ground-surface information as a foundation for surface representation, and extrapolation to the far ground surface around the target for accurate absolute distance computation.

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Figure 1: The effect of visual field size on judged distance.
Figure 2: Profile illustrations of the physical and perceived ground surface through a limited field of view.
Figure 3: The critical ground surface for accurate distance judgement.
Figure 4: Scanning direction affects judged absolute distance.


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This research was supported in part by a grant from NIH to Z.J.H. and T.L.O. and by a RIG grant from the University of Louisville to Z.J.H.

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Correspondence to Teng Leng Ooi or Zijiang J. He.

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Wu, B., Ooi, T. & He, Z. Perceiving distance accurately by a directional process of integrating ground information. Nature 428, 73–77 (2004).

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