On the role of external reference frames on visual judgements of parallelity

In a previous study we found large systematic errors (up to 40 degrees) whe n subjects adjusted the orientation of a horizontal test bar until it appea red parallel to a horizontal reference bar, both bars rotating about their vertical axes. The deviations increased linearly with the separation angle but vanished when the orientation of the reference bar was either parallel or perpendicular to the median line. In order to test the assumption that e xternal references caused these deviations to vanish, the same task was rep eated in four different conditions: in the normal condition the horizontal aperture, formed by a cabin, and the facing wall of the room were frontopar allel to the subject; in the other conditions either the room, the cabin or both were oriented 30 degrees to the right with respect to the subject. It was found that, depending on the subject, the occurrence of the vanishing deviations covaried with the orientation of the cabin or the room. Evidentl y, subjects are influenced by the external references provided by the walls of the room and the sides of the cabin. The results indicate that a descri ption of visual space by a Riemannian metric of constant curvature is not v alid in a visual environment containing external references. (C) 2001 Elsev ier Science B.V. All rights reserved.