The bourdon illusion occurs with straight-, right-angle-, and parallel-edge figures

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. Our investigation will be restricted to the traditional, straight edge solid figures.…”

The Bourdon illusion in haptic space

“…One is an identical trend in the illusion when a particular stimulus is varied. Day et al (1989) showed that the visual illusion is reversed when'the triangular components of the figure are arranged base to base instead of apex to apex. They also showed that the illusion in this arrangement is reduced by about half.…”

“…Day et al, (1989) showed that when the two triangular components of the conventional Bourdon figure ( Figure I) are each rotated through 180°so that they are base to base instead of apex to apex, the illusion is reversed in direction and reduced by about half. That is to say, when the triangular components are base to base, the outer ends of the figure, now the apexes, appear to be displaced upward to the left rather than downward to the right as in Figure I.…”

“…The perfectly straight edge AC appears to be slightly bent about its center B in the same direction as the opposite obtuse angle edge DBE. Day, Mitchell, and Stecher (1989) have shown that the illusion is reversed in direction and reduced in size when the two elongated components of the figure are arranged base to base rather than apex to apex as in Figure 1. The purpose of the two experiments reported here was to establish whether or not the Bourdon illusion occurs in the active haptic mode, and, if so, whether it too is reversed and reduced when the two triangular components of the stimulus object are arranged with their bases contiguous (see Figure 2).…”

The basis of the Bourdon illusion

“…and Day, Mitchell;and Stecher (1990) Wenderoth, Criss, and van der Zwan (1990), Wenderoth and O'Connor (1987a, 1987b), and Wenderoth, O'Connor, and Johnson (1986 Day (1990) and Day et aL (i990).…”