**7.4 Scale expansion theory**

For large scale (locomotor) space, Durgin and Li (2011a) have proposed that the special role of proprioception of gaze direction in estimating distance (e.g., Wallach & O'Leary, 1982) may encourage scale expansion near horizontal with a gain of 1.5. Because their model provides impressive quantitative predictions of perceptual matching tasks (Li, Phillips & Durgin, 2011), it seems to capture an important feature of locomotor space perception. Durgin and Li have proposed that the 1.5 gain in the scaling of perceived slant may be driven by the 1.5 gain in gaze proprioception. That is, for horizontal ground surfaces to look flat requires a 1.5 gain in the optical slant. Thus, it might be argued that the expanded scale of perceived gaze declination also creates pressure for an expanded scale of visual slant.
