It is known that perception of surface slant is more sensitive about the horizontal axis than it is about the vertical axis. This study investigates the anisotropy of slant perception by applying the approach of natural scene statistics, which measures various physical properties to find probability regularities in the environment. We sampled 17 environments (2 indoor and 15 outdoor ones) from those routinely experienced by the first author. In each environment, a 3D laser scanner measured the spatial layout by collecting the radial distances to the surrounding surfaces. On the basis of the measurements, we analyzed the probability distribution of the magnitude of surface slant about both the horizontal and vertical axes. Consequently, the probability of the slant about the horizontal axis, compared to that about the vertical axis, shows a remarkably high and narrow peak around 0° (i.e., the vertical plane). This indicates that the environment contains nearly vertical surfaces with a relatively high probability. The result suggests that our visual system often needs to perceive a small deviation from the vertical plane and accordingly becomes anisotropically sensitive to the slant about the horizontal axis.
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