The role of relative disparity gradients in perceiving slant has been empirically supported by experiments showing that placing a frontal plane surface above and/or below a stereoscopically slanted surface significantly increases its perceived slant (Gillam et al.,
1984; van Ee & Erkelens,
1996; Gillam & Blackburn,
1998; Pierce, Howard, & Feresin,
1998; Gillam & Pianta,
2005; Gillam, Blackburn, & Brooks,
2007; Serrano-Pedraza, Phillipson & Read,
2010; Gillam, Sedgwick, & Marlow,
2011) and reduces its latency (Gillam et. al.,
1984; Gillam et al.,
1988; van Ee & Erkelens,
1996). This arrangement of surfaces is known as the “twist” configuration (Gillam et al.,
1988; Gillam & Pianta,
2005; Gillam et al.,
2007; Howard & Rogers,
2012). The two surfaces in a twist configuration form relative disparity gradients along the discontinuities at the top or bottom edges of the slanted surface (see
Figure 1a). In contrast, if equally proximal frontal plane surfaces are placed next to the slanted surface but in a configuration that does not create gradients of relative disparity, as in the “hinge” configuration (see
Figure 1b), slant perception is not enhanced and perceptual latency is not decreased (Gillam et al.,
1988; Pierce et al.,
1998; Gillam & Pianta,
2005; Gillam et al.,
2007). Thus, the increase in perceived slant for a surface in the twist configuration cannot be attributed to the presence of a reference surface, to slant contrast, or to the introduction of a derivative of absolute disparity (all of which apply to the hinge configuration).