Hierarchical organisation in perception of orientation

According to Rock [1990, in The Legacy of Solomon Asch (Hillsdale, NJ: Lawr ence Erlbaum Associates)], hierarchical organisation of perception describe s cases in which the orientation of an object is affected by the immediatel y surrounding elements in the visual field. Various experiments were perfor med to study the hierarchical organisation of orientation perception. In mo st of them the rod-and-frame-illusion (RFI: change of the apparent vertical measured on a central rod surrounded by a tilted frame) was measured in th e presence/absence of a second inner frame. The first three experiments sho wed that, when the inner frame is vertical, the direction and size of the i llusion are consistent with expectancies based on the hierarchical organisa tion hypothesis. An analysis of published and unpublished data collected on a large number of subjects showed that orientational hierarchical effects are independent from the absolute size of the RFI. In experiments 4 to 7 we examined the perceptual conditions of the inner st imulus (enclosure, orientation, and presence of luminance borders) critical for obtaining a hierarchical organisation effect. Although an inner vertic al square was effective in reducing the illusion (experiment 3), an inner c ircle enclosing the rod was ineffective (experiment 4). This indicates that definite orientation is necessary to modulate the illusion. However, orien tational information provided by a vertical or horizontal rectangle present ed near the rod, but not enclosing it, did not modulate the RFI (experiment 5). This suggests that the presence of a figure with oriented contours enc losing the rod is critical. In experiments 6 and 7 we studied whether the p resence of luminance borders is important or whether the inner upright squa re might be effective also if made of subjective contours. When the subject ive contour figure was salient and the observers perceived it clearly, its effectiveness in modulating the RFI was comparable to that observed with lu minance borders.