DISCRIMINATION OF 3-D SHAPE AND 3-D CURVATURE FROM MOTION IN ACTIVE VISION

We examined the ability of human observers to discriminate between dif ferent 3-D quadratic surfaces defined by motion, and with head positio n fed back to the stimulus to provide an up-to-date dynamical perspect ive view. We tested whether 3-D shape or 3-D curvature would affect di scrimination performance. It appeared that discrimination of 3-D quadr atic shape clearly depended on shape but not on the amount of curvatur e. Even when the amount of curvature was randomized, subjects' perform ance was not altered. On the other hand, the discrimination of 3-D cur vature clearly depended linearly on curvature with Weber fractions of 20% on the average and, to a small degree, on 3-D shape. The experimen t shows that observers can easily separate 3-D shape and 3-D curvature , and that Koenderink's shape index and curvedness provide a convenien t way to specify shape. These results warn us against using just any a rbitrary 3-D shape in 3-D shape perception tasks and indicate, for exa mple, that emphasizing 3-D shape in computer displays by exaggerating curvature does not have any effect.