Biomedical subjects

Andrea Li

Publications and source records attributed to Andrea Li.

4 recordsLinked to original sources

PubMed2004

Three-dimensional shape from non-homogeneous textures: carved and stretched surfaces.

We examined the perception of 3D shape for surfaces folded, carved, or stretched out of textured materials. The textures were composed of sums of sinusoidal gratings or of circular dots, and were designed to differentiate between orientation and frequency information present in perspective images of the surfaces. Correct perception of concavities, convexities, saddles, and slants required the visibility of signature patterns of orientation modulations. These patterns were identical to those identified previously for developable surfaces (A. Li & Q. Zaidi, 2000; Q. Zaidi & L. Li, 2002), despite the fact that textures were statistically homogeneous on developable surfaces but not on carved or stretched surfaces. Frequency modulations in the image were interpreted as cues to distance from the observer, which led to weak but qualitatively correct percepts for some carved and stretched surfaces but to misperceptions for others, similar to the misperceptions for developable surfaces (A. Li & Q. Zaidi, 2003). Irrespective of whether texture on the surface is homogeneous or non-homogeneous, similar neural modules can be used to locate signature orientation modulations and thus extract shape from texture cues.

Cues↗

PubMed2003

Observer strategies in perception of 3-D shape from isotropic textures: developable surfaces.

We document the limitations of isotropic textures in conveying three-dimensional shape. We measured the perceived shape and pitch of upright and pitched corrugated surfaces overlaid with different classes of isotropic textures: patterns containing isotropic texture elements, isotropically filtered noise patterns, and patterns containing ellipses or lines of all orientations. Frequency modulations arising from surface slant were incorrectly interpreted as changes in surface distance, resulting in concavities being misclassified as convexities, and right and left slants as concavities. In addition, images of pitched surfaces exhibited oriented flows that confound surface shape and surface pitch. Observers related oriented flow patterns to particular surface shapes with a bias for perceiving convex surfaces. When concave and convex curvatures were concurrently visible, the number of correct shape classifications increased slightly. Isotropic textures thus convey correct 3-D shapes of developable surfaces only in some conditions, and the same perceptual strategies lead to non-veridical percepts in other conditions.

Computer Simulation↗

PubMed2002

Limitations on shape information provided by texture cues.

This paper uses visual, empirical and formal methods (Li & Zaidi, Vision Research, 40 (2000) 217; Li & Zaidi, Vision Research, 41 (22) (2001a) 2927) to examine the roles of oriented texture components in conveying veridical percepts of concave and convex surfaces that are pitched towards or away from the observer. The results show that pairs of components, oriented symmetrically around the axis of maximum curvature, combine to provide the geodesic orientation modulations that are critical for veridical shape perception. The degree of pitch determines the orientations of the critical pair of components. Perspective is crucial to the veridical perception of concavities and convexities, regardless of the degree of pitch. The results of this paper reconfirm that veridical shape perception depends on extracting critical patterns of oriented energy, but also show that the class of textures capable of conveying veridical percepts of developable shapes in general views is even more restricted than that identified by Li and Zaidi (Journal of Optical Society of America A, 18 (2001b), 2430).

Cues↗

PubMedDate not supplied

Three-dimensional shape perception from chromatic orientation flows.

The role of chromatic information in 3-D shape perception is controversial. We resolve this controversy by showing that chromatic orientation flows are sufficient for accurate perception of 3-D shape. Chromatic flows required less cone contrast to convey shape than did achromatic flows, thus ruling out luminance artifacts as a problem. Luminance artifacts were also ruled out by a protanope's inability to see 3-D shape from chromatic flows. Since chromatic orientation flows can only be extracted from retinal images by neurons that are responsive to color modulations and selective for orientation, the psychophysical results also resolve the controversy over the existence of such neurons. In addition, we show that identification of 3-D shapes from chromatic flows can be masked by luminance modulations, indicating that it is subserved by orientation-tuned neurons sensitive to both chromatic and luminance modulations.

Color↗