Curvature biases in stereoscopic vision: A nasotemporal asymmetry

The reliability of curvature judgments for linear elements was studied, wit h stereograms that contained a binocular are with curvature in depth, and e ither a binocular frontoparallel are or a monocular one, on a background re presenting a hemiellipsoid. The subjects made about 15% errors on binocular arcs with curvature in depth, and 60% - 80% of these occurred when both th e hemiellipsoid and the are were convex, the are being perceived as concave , by transparency through the hemiellipsoid. There were also about 15% - 30 % errors on frontoparallel arcs, but spread among all situations, with a sm all prevalence of concave judgments. Curvature in depth was assigned to the monocular stimuli in more than 60% of the cases. There was a curvature bia s when the monocular arcs were on the nasal side, and were viewed against a concave background. Assuming parallel viewing, nasal ingoing area were usu ally perceived as concave, and nasal outgoing arcs usually perceived as con vex, in agreement with geometrical likelihood. Nasal-side elements captured by one eye are, in general, those with the highest likelihood of having ma tching elements in the other eye. Then the observed nasal bias effect sugge sts that the matching process in stereopsis could be driven from the nasal sides of the projections in the two cerebral hemispheres.