Orientation dependence of human line-length judgements matches statistical structure in real-world scenes

A set of 42 photographs of real-world scenes were filtered by convolution with a Laphacian-of-Gaussian kernel. The linear density of zero-crossings along notional lines in the filtered images was measured as a function of the orientation of those lines. The zero-crossing density was found to be greatest along near-vertical lines and least along horizontal lines. The peaks in this function were much broader than the troughs, and there was weak evidence of a small decline in density along lines at and very close to vertical. Psychophysical measurements of the apparent length of a line as a function of its orientation (by the current author and others) show very similar variation, subject to an overall scaling factor. This close correspondence is taken as strong evidence for some sort of environmental determination of the orientation dependence of human length judgements.