FRAME-OF-REFERENCE AND HIERARCHICAL-ORGANIZATION EFFECTS IN THE ROD-AND-FRAME ILLUSION

Two hypotheses proposed as alternatives by Rock-frame of reference and hierarchical organisation of perception-were tested in a series of ex periments with the use of the rod-and-frame illusion. This illusion pr oduces errors in the apparent vertical due to the presence of a tilted frame surrounding the test rod. The apparent vertical is shifted in t he direction of the frame tilt. When an upright square was added insid e the tilted frame, rod-setting errors varied according to the visual characteristics of the display. In the case of a large display present ed in the dark (experiment 1), there continued to be large errors in t he direction of the outer-square tilt. This finding supports the frame -of-reference hypothesis, which proposes that the orientation of all o bjects in the visual field is dominated by the most peripheral referen ce. In the case of a small display presented in a lit environment (exp eriments 2 and 3), the direction of errors was the opposite. This latt er finding was taken to indicate that the rod was set with reference t o the perceived tilt of the inner upright square. Thus, according to a hierarchical-organisation hypothesis, the orientation of an object in the visual field is influenced by objects in the immediate surroundin gs not by outermost reference. Overall, the results confirm the presen ce of two qualitatively different classes of orientational phenomena: one is concerned with the definition of egocentric coordinates and one with an object-centred visual representation.