Dissociating size representation for action and for conscious judgment: Grasping visual illusions without apparent obstacles

Visual illusions provide important evidence for the co-existence of unconscious and conscious representations. Objects surrounded by other figures (e.g., a disc surrounded by smaller or larger rings, Ebbinghaus/Titchener illusion) are consciously perceived as different in size, while the visuo-motor system supposedly uses an unconscious representation of the discs’ true size for grip size scaling. Recent evidence suggests other factors than represented size, e.g., surrounding rings conceived as obstacles, affect grip size. Use of the diagonal illusion avoids visual obstacles in the path of the reaching hand. Results support the dual representation theory. Grip size scaling follows actual size independent of illusory effects, which clearly bias conscious perception in direct comparisons of lengths (Experiment 1) and in finger-thumb span indications of perceived length (Experiment 2).