[Sensus orthopedicus].

Li and Zaidi (Vision Research 40 (2000) 217; 41 (2001) 1519) have recently argued that there are two necessary conditions for the perception of 3D shape from texture: (1) the texture pattern must have a disproportionate amount of energy along directions of principal curvature; and (2) the surface must be viewed with a noticeable amount of perspective. In the present article we present evidence that these conclusions are only valid under a limited set of non-generic viewing conditions. Other relevant factors that need to be considered in this context include the distribution of curvature on an object's surface and the set of possible viewing directions from which it can be observed. For generic viewing directions and patterns of curvature, the perception of surface curvature from texture is only minimally affected by the orientation spectrum of the texture pattern or the amount perspective in its optical projection. Li and Zaidi (Vision Research 41 (2001) 1519) have also identified two characteristic patterns of image contours, which they claim to be the only possible source of information within textured images for determining the direction of surface slant or the sign of surface curvature. In the present article we attempt to show that these characteristic patterns can only arise in natural vision for a limited set of non-generic viewing directions. We also review several other factors that can influence the perceived direction of slant or the perceived sign of curvature, which have been identified previously by other investigators.

Form Perception↗

PubMed2002

Short-term temporal recruitment in structure from motion.

Temporal integration was investigated in the minimal conditions necessary to perform a structure-from-motion (SFM) task. Observers were asked to discriminate three-dimensional (3D) surface orientations in conditions in which the stimulus displays simulated velocity fields providing, in each frame transition, either sufficient (3 moving dots) or insufficient information (1 or 2 moving dots) to perform the task. When only two moving dots were shown in each frame transition of the stimulus displays (Experiment 1), we found that performance decreased as dot-lifetime increased. A facilitation effect of the overall display duration was also found. The negative effect of dot-lifetime on performance contrasts with what found in Experiment 2 with three dots in each frame transition, where performance improved with increasing dot-lifetime up to 170 ms, and then reached a plateau. Finally, for an optimal dot-lifetime of 150 ms, we found that performance was still above chance when each frame transition specified the motion of only one dot (Experiment 3). These results indicate that temporal recruitment alone can support the recovery of 3D information from sparse motion signals, thus providing a strong indication for the importance of temporal integration in the perceptual analysis of the optic flow. Our results reveal, moreover, that temporal integration in SFM has different characteristics, depending on whether, in each frame transition, the stimulus displays provide either sufficient (3 or more moving dots) or insufficient information (1 or 2 moving dots) to specify the higher-order properties of the optic flow necessary for 3D surface recovery.

Form Perception↗

PubMed2002

Binocular integration of partially occluded surfaces.

Normal binocular vision can provide a view of an object partially occluded so that no part of it is seen by both eyes but all of it is seen by one or other eye. We used two-dimensional filtered noise textures to explore the conditions under which the visual system can piece together the monocular fragments of such occluded surfaces. When the fragments seen by left and right eyes are drawn from a continuous texture with strong horizontal correlation, observers see coherent surfaces reliably located in depth. When textures are discontinuous or have weaker horizontal correlation, or the left and right eyes' views represent unnatural depth relationships, no coherent surface is perceived, and binocular rivalry ensues. The discovery of coherent surfaces under our conditions seems to reflect the operation of a high-level integration process, failures of which drive rivalry.

Form Perception↗