Surface reflectance and motion characteristics affect perceived bumpiness of 3D-rotating objects

Dynamic visual information projected onto the retina (optic flow) facilitates 3D shape recognition. While optic flow generated by a moving diffusely-reflecting (matte) object is directly linked to its first order shape properties, the flow generated by a specular object is tightly related to its second-order shape characteristics (Koenderink and van Doorn, 1980). Dovencioglu et al. (2015) demonstrated that reflectance-dependent optic flow yields differences in perceived local curvature of rotating matte and specular objects. Here we investigated these perceptual differences in a global shape task. Stimuli were bumpy spheres with the object boundary masked by a Gaussian aperture. The bumpiness level was varied by adjusting the amplitudes of randomly applied sinusoids. We measured ‘percent judged bumpier’ in a 2IFC task, where the reference object was always specular and of intermediate bumpiness. Seven observers completed 5(bumpiness) × 3(material: specular, matte, intermediate) × 3(rotation axes) × 30(repetition) trials. Results indicate that matte objects were judged as less bumpy than specular ones. Moreover – unlike for matte objects -the perceived bumpiness of specular objects was not affected by the object’s rotation axis, suggesting that specular flow characteristics remain largely robust across different types of object motion.