Abstract/Summary

The horizontal–vertical illusion, in which the vertical dimension is overestimated relative to the horizon- tal direction, has been explained in terms of the statistical relationship between the lengths of lines in the world, and the lengths of their projections onto the retina (Howe & Purves, 2002). The current study shows that this illusion affects the apparent aspect ratio of shapes, and investigates how it interacts with binocular cues to surface slant. One way in which statistical information could give rise to the horizontal– vertical illusion would be through prior assumptions about the distribution of slant. This prior would then be expected to interact with retinal cues to slant. We determined the aspect ratio of stereoscopically viewed ellipses that appeared circular. We show that observers’ judgements of aspect ratio were affected by surface slant, but that the largest image vertical:horizontal aspect ratio that was considered to be a surface with a circular profile was always found for surfaces close to fronto-parallel. This is not consistent with a Bayesian model in which the horizontal–vertical illusion arises from a non-uniform prior proba- bility distribution for slant. Rather, we suggest that assumptions about the slant of surfaces affect appar- ent aspect ratio in a manner that is more heuristic, and partially dissociated from apparent slant.