Orientation illusions vary in size and direction as a function of task-dependent attention

Adding an upright inner square frame to an outer tilted square frame causes a central rod's perceived orientation to be directionally opposite the usual rod-and-frame illusion (RFI). Zoccolotti et al. (1997) attributed this double rod-and-frame illusion (DRFI) to Rock's (1990) ¿hierarchical organisation principle¿. In experiment 1, this explanation predicted results for small (11°) but not larger outer frame orientations (22° and 33°). Two experiments using the DRFI varied bottom-up, goal-driven attention and compared direct and indirect measures of the framework's influence. In experiment 2 the RFI angular function was compared to two other DRFI conditions: a direct measure of perceived rod orientation and an indirect measure of the inner frame. These conditions induced directionally opposite effects. In experiment 3 direct and indirect measures of the inner frame's perceived tilt were compared. Angular functions differing in size and direction were obtained. Experiment 4 replicated previous results, using a different psychophysical procedure. All results were consistent with the hierarchical organisation mechanism, but suggested different processing strategies due to different attentional weights. They were also consistent with other recent findings based upon the Bayesian approach to accounts of illusory phenomena (e.g. Jazayeri & Movshon, 2006; 2007; Weiss et al, 2002).