The color/shading effect and oriented double opponent neurons: a noise analysis

Shading and material effects are coupled in causing brightness variations in natural images, which is problematic for shape-from-shading inferences. Given the importance of orientation in visual cortex, we have previously conjectured that, in regions where changes in shading are parallel (in orientation) to changes in hue, a material effect is likely the source of brightness variations, whereas, in areas where shading changes are transverse -- non-parallel -- to those in hue, a shading effect is likely. This geometric model has been supported psychophysically by a color/shading effect on naturalistic images: when shading and hue flows are designed to be parallel, the depth percept is destroyed. We here extend this geometric model to its putative physiological realization involving oriented double-opponent (DO) cells in primary visual cortex (V1). A random noise paradigm is introduced onto the color shading effect, under the assumption that this noise can be, in effect, averaged away by the DO cells' receptive fields. Our goal is to determine whether experimental values for the spatial frequency responses of these cells match the noise-smoothing properties of putative receptive fields. Participants performed both quantitative ("which point appears closer?'') and qualitative ("which image appears more 3D?'') tasks on noisy images, and noise thresholds for perceiving depth were calculated and compared to physiological receptive field measurements. A strong correlation was found between participants’ performance on these tasks and the amount of noise added to each image. Furthermore, estimates of the spatial frequency response needed to cancel the noise were also calculated, providing further evidence in support of oriented DO cells’ role in the color-shading effect.