Modulation of the rod-and-frame illusion by additional external stimuli.

The presence of an additional external upright frame was studied in three experiments to separate the role of visuovestibular, global, and local mechanisms in the rod-and-frame illusion (RFI). In the first experiment, carried out in a dark room, the external frame surrounded a large tilted frame. Rod-setting errors to the vertical were abolished with the additional-frame condition (at 22 degrees inner-frame tilt) confirming earlier findings. However, small, residual direct (at 11 degrees inner-frame tilt) and indirect effects (at 33 degrees inner-frame tilt) were still present, indicating the persistence of global visual processing. In the second experiment, the RFI in the dark was compared with the RFI with the lights on. Turning the light on abolished the effect at 22 degrees and 33 degrees frame tilt; however, a small direct effect was maintained at 11 degrees frame tilt. These two studies indicate that the addition of veridical vertical information abolishes the effect owing to visuovestibular mechanisms. In the third experiment, a small rod and frame was used with the lights on (a condition abolishing visual-vestibular interaction). In the case of a small gap between the rod and the inducing frame (a condition which maximises local processing), the effect of the outer upright frame was negligible; this indicates that the additional frame had no effect on local processing. In the case of a large gap (a condition which minimises local processing), the external square reduced the illusion, indicating its modulating effect on visual global processing. Overall, an upright external frame exerts a differential influence depending on which mechanisms contribute most to the RFI in a given experimental condition.