Outline- and solid-angle orientation illusions have different determinants

Interactions between real and subjective contours in the Bourdon illusion

“…To the extent that our results can be compared to this perceptualleaming phenomenon, the direction of our interaction-real contours strongly influencing subjective contours, but not the reverse-appears opposite to the asymmetrical interaction that Vogels and Orban reported. Our finding of a small Bourdon illusion in Figure R-R is consistent with the earlier results of Rozvany and Day (1980), but contrary to the results of Wenderoth and O'Connor (1987b) and Wendoroth et al (1986), who found significant repulsion effects in real outline figures. In solid figures, Wenderoth and O'Connor (1987b) found significant Bourdon illlusions when the test contours were colinear, but there was no apparent displacement when the test contours were objectively bent.…”

“…Our finding of a small Bourdon illusion in Figure R-R is consistent with the earlier results of Rozvany and Day (1980), but contrary to the results of Wenderoth and O'Connor (1987b) and Wendoroth et al (1986), who found significant repulsion effects in real outline figures. In solid figures, Wenderoth and O'Connor (1987b) found significant Bourdon illlusions when the test contours were colinear, but there was no apparent displacement when the test contours were objectively bent. However, we found significant displacements-unbending-in our objectively bent induction contours in four of the seven illusions in the present experiment (see Figure 3).…”

“…Outline figures produce weaker Bourdon illusions, and sometimes produce apparent displacements in the opposite direction-that is, the test contours sometimes appear displaced away from the induction contours in outline figures (Luckiesh, 1922(Luckiesh, /1965Rozvany & Day, 1980;Wenderoth & O'Connor, 1987b;Wenderoth, O'Connor, & Johnson, 1986). Solid figures produce stronger Bourdon illusions, and subjective contours produce still greater effects (Walker & Shank, 1987).…”

Anomalous orientation effects in the Bourdon illusion

“…Besides the solid variety ofthe figure, outline versions also have been studied (Wenderoth & O'Connor, 1987). Day, Mitchell, and Stecher (1990) showed that the illusion in the solid variety occurs also with right angle and parallel edge figures.…”

The Bourdon illusion in haptic space

“…The data from the visual illusion renders this unlikely. Wenderoth and O'Connor (1987) have shown that a small gap in the visual figure actually enhances the illusion. In any case, it is now clear that the visual illusion occurs strongly with a right angle arrangement of the components arranged base to base with an even greater angular gap between them .…”