What is second-order vision for? Discriminating illumination versus material changes

Abstract

The human visual system is sensitive to second-order modulations of the local contrast (CM) or amplitude (AM) of a carrier signal. Second-order cues are detected independently of first-order luminance signals; however, it is not clear why vision should benefit from second-order sensitivity. Analysis of the first- and second-order contents of natural images suggests that these cues tend to occur together, but their phase relationship varies. We have shown that in-phase combinations of LM and AM are perceived as a shaded corrugated surface whereas the anti-phase combination can be seen as corrugated when presented alone or as a flat material change when presented in a plaid containing the in-phase cue. We now extend these findings using new stimulus types and a novel haptic matching task. We also introduce a computational model based on initially separate first- and second-order channels that are combined within orientation and subsequently across orientation to produce a shading signal. Contrast gain control allows the LM + AM cue to suppress responses to the LM - AM when presented in a plaid. Thus, the model sees LM - AM as flat in these circumstances. We conclude that second-order vision plays a key role in disambiguating the origin of luminance changes within an image.

Reduced sensitivity for visual textures affects judgments of shape-from-shading and step-climbing behaviour in older adults

“…The right leaning component is paired with a positively correlated change in texture amplitude (regions of high amplitude texture align with regions of high luminance) and thus simulates, and is perceived as, a shaded, corrugated surface. The left leaning component has been paired with a negatively correlated change in texture amplitude and appears as a flat reflectance change: like strips of material laid across the undulations (Schofield et al, 2006;2010;Schofield, Rock & Georgeson, 2011). This apparent distinction between undulations and material changes is especially strong in the plaid configuration depicted in Figure 1b.…”

“…In the first (Figure 3c) the contrast of the texture was unchanged such that any shading due to illumination would produce the normal, yoked (in-phase), combination of luminance and texture amplitude (but not contrast) changes that results from multiplicative (real) shading of a matte surface: we call this the Uniform condition. Due to the natural properties of shaded textures (see Schofield et al, 2006Schofield et al, , 2010 this condition resulted in the peak of the luminance highlight caused by the spotlight to co-inside with a peak in texture amplitude (LM+AM; cf Experiment 1). In the second treatment ( Figure 3d) the contrast of the texture was reduced on the riser such that once illuminated the luminance and texture amplitude varied in anti-phase across the riser.…”

“…This apparent distinction between undulations and material changes is especially strong in the plaid configuration depicted in Figure 1b. Such correlated parings of luminance and texture amplitude variations are common in the environment where shadows fall across textures and where undulations produce shading on visually textured surfaces (Schofield, 2000;Schofield et al 2010). Computational modelling of human performance with a 2 stage filtering mechanism as described above has shown that successful discrimination between shading and reflectance indeed depends on the high-frequency, fine detail, elements in the texture (Schofield et al, 2010;Sun & Schofield, 2011).…”

“…Schofield and colleagues (Schofield, Heese, Rock & Georgeson, 2006;Schofield, Rock, Sun, Jiang & Georgeson, 2010;Sun & Schofield, 2011) have recently shown that modulations in the luminance amplitude of visual textures (texture amplitude) can be used to discriminate between changes in illumination -such as shading and shadows -and changes in the reflectance properties of a surface.…”

“…Such correlated parings of luminance and texture amplitude variations are common in the environment where shadows fall across textures and where undulations produce shading on visually textured surfaces (Schofield, 2000;Schofield et al 2010). Computational modelling of human performance with a 2 stage filtering mechanism as described above has shown that successful discrimination between shading and reflectance indeed depends on the high-frequency, fine detail, elements in the texture (Schofield et al, 2010;Sun & Schofield, 2011). However, the work described above had not established whether the high-frequency losses associated with ageing are sufficiently large and of the correct nature to influence the discrimination or shading and material changes.…”