Biomedical subjects

James T Todd

Publications and source records attributed to James T Todd.

16 recordsLinked to original sources

PubMed2006

The effects of phase on the perception of 3D shape from texture: psychophysics and modeling.

Two experiments are reported in which observers judged the apparent shapes of elliptical cylinders with eight different textures that were presented with scrambled and unscrambled phase spectra. The results revealed that the apparent depths of these surfaces varied linearly with the ground truth in all conditions, and that the overall magnitude of surface relief was systematically underestimated. In general, the apparent depth of a surface is significantly attenuated when the phase spectrum of its texture is randomly scrambled, though the magnitude of this effect varies for different types of texture. A new computational model of 3D shape from texture is proposed in which apparent depth is estimated from the relative density of edges in different local regions of an image, and the predictions of this model are highly correlated with the observers' judgments.

Depth Perception↗

PubMed2005

Mapping the parietal cortex of human and non-human primates.

The present essay reviews a series of functional magnetic resonance imaging (fMRI) studies conducted in parallel in humans and awake monkeys, concentrating on the intraparietal sulcus (IPS). MR responses to a range of visual stimuli indicate that the human IPS contains more functional regions along its anterior-posterior extent than are known in the monkey. Human IPS includes four motion sensitive regions, ventral IPS (VIPS), parieto-occipital IPS (POIPS), dorsal IPS medial (DIPSM) and dorsal IPS anterior (DIPSA), which are also sensitive to three-dimensional structure from motion (3D SFM). On the other hand, the monkey IPS contains only one motion sensitive area (VIP), which is not particularly sensitive to 3D SFM. The human IPS includes four regions sensitive to two-dimensional shape and three representations of central vision, while monkey IPS appears to contain only two shape sensitive regions and one central representation. These data support the hypothesis that monkey LIP corresponds to the region of human IPS between DIPSM and POIPS and that a portion of the anterior part of human IPS is evolutionarily new. This additional cortical tissue may provide the capacity for an enhanced visual analysis of moving images necessary for sophisticated control of manipulation and tool handling.

Animals↗

PubMed2005

Visual discrimination of local surface structure: slant, tilt, and curvedness.

In four experiments, observers were required to discriminate interval or ordinal differences in slant, tilt, or curvedness between designated probe points on randomly shaped curved surfaces defined by shading, texture, and binocular disparity. The results reveal that discrimination thresholds for judgments of slant or tilt typically range between 4 degrees and 10 degrees; that judgments of one component are unaffected by simultaneous variations in the other; and that the individual thresholds for either the slant or tilt components of orientation are approximately equal to those obtained for judgments of the total orientation difference between two probed regions. Performance was much worse, however, for judgments of curvedness, and these judgments were significantly impaired when there were simultaneous variations in the shape index parameter of curvature.

Depth Perception↗

PubMed2005

The effects of field of view on the perception of 3D slant from texture.

Observers judged the apparent signs and magnitudes of surface slant from monocular textured images of convex or concave dihedral angles with varying fields of view between 5 degrees C and 60 degrees C. The results revealed that increasing the field of view or the regularity of the surface texture produced large increases in the magnitude of the perceptual gain (i.e., the judged slant divided by the ground truth). Additional regression analyses also revealed that observers slant judgments were highly correlated with the range of texture densities (or spatial frequencies) in each display, which accounted for 96% of the variance among the different possible dihedral angles and fields of view.

Depth Perception↗

PubMed2004

The visual perception of 3D shape.

A fundamental problem for the visual perception of 3D shape is that patterns of optical stimulation are inherently ambiguous. Recent mathematical analyses have shown, however, that these ambiguities can be highly constrained, so that many aspects of 3D structure are uniquely specified even though others might be underdetermined. Empirical results with human observers reveal a similar pattern of performance. Judgments about 3D shape are often systematically distorted relative to the actual structure of an observed scene, but these distortions are typically constrained to a limited class of transformations. These findings suggest that the perceptual representation of 3D shape involves a relatively abstract data structure that is based primarily on qualitative properties that can be reliably determined from visual information.

Depth Perception↗

PubMed2004

Perceptual biases in the interpretation of 3D shape from shading.

Two experiments are reported in which observers judged the sign and magnitude of surface curvature from shaded images of an indoor scene. The depicted surfaces were illuminated by an area light in the ceiling or floor, and the illumination was attenuated with distance in a physically correct manner. The displays were presented both with and without cast shadows, specular highlights and surface inter-reflections in all possible combinations. The overall pattern of results revealed a strong perceptual bias to interpret the images as convex rather than concave, and a weaker bias to prefer illumination from above rather than from below, though there were large individual differences in the relative strengths of these biases. For displays that did not contain cast shadows or specular highlights, the accuracy of observers' judgments about the sign of surface curvature was no greater than chance, but performance was significantly improved when those aspects of normal shading were included in the rendering model. An analysis of the apparent depth magnitudes revealed that convex surfaces produce much greater perceived depth than concave surfaces with comparable relief.

Depth Perception↗

PubMed2004

Pointing out of the picture.

The eyes of portrayed people are often noticed to 'follow you' when you move with respect to a flat painting or photograph. We investigated this well-known effect through extensive measurements of pictorial relief and apparent orientation of the picture surface for a number of viewing conditions, including frontal and oblique views. We conclude that cases of both oblique and frontal viewing are very similar in that perception simply follows what is indicated by the proximal stimulus, even though this may imply that the (perceived) physical and pictorial spaces segregate. The effect of foreshortening then causes an apparent narrowing of pictorial objects. We find no evidence for any 'correction' mechanisms that might be specifically active in oblique viewing conditions.

Depth Perception↗

PubMed2004

Perception of three-dimensional shape from specular highlights, deformations of shading, and other types of visual information.

There have been numerous computational models developed in an effort to explain how the human visual system analyzes three-dimensional (3D) surface shape from patterns of image shading, but they all share some important limitations. Models that are applicable to individual static images cannot correctly interpret regions that contain specular highlights, and those that are applicable to moving images have difficulties when a surface moves relative to its sources of illumination. Here we describe a psychophysical experiment that measured the sensitivity of human observers to small differences of 3D shape over a wide variety of conditions. The results provide clear evidence that the presence of specular highlights or the motions of a surface relative to its light source do not pose an impediment to perception, but rather, provide powerful sources of information for the perceptual analysis of 3D shape.

Depth Perception↗

PubMed2004

Lightness constancy in the presence of specular highlights.

Visible surfaces in a natural environment often have multiple components of reflectance, including a diffuse component, by which light is scattered isotropically in all possible directions, and a specular component, by which light is reflected anisotropically within a limited range of directions. The research described in the present article was designed to investigate how these different components of reflectance influence the perception of lightness. Human observers were presented with shaded images of smoothly curved surfaces and asked to compare the relative lightness of different surface regions whose diffuse and specular components of luminance were independently manipulated. The results revealed that observers are able to discount the presence of specular highlights so that the relative lightness among different regions is determined almost entirely by the diffuse component of surface reflectance.

Environment↗

PubMed2004

The perception of doubly curved surfaces from anisotropic textures.

Most existing computational models of the visual perception of three-dimensional shape from texture are based on assumed constraints about how texture is distributed on visible surfaces. The research described in the present article was designed to investigate how violations of these assumptions influence human perception. Observers were presented with images of smoothly curved surfaces depicted with different types of texture, whose distribution of surface markings could be both anisotropic and inhomogeneous. Observers judged the pattern of ordinal depth on each object by marking local maxima and minima along designated scan lines. They also judged the apparent magnitudes of relative depth between designated probe points on the surface. The results revealed a high degree of accuracy and reliability in all conditions, except for a systematic underestimation of the overall magnitude of surface relief. These findings suggest that human perception of three-dimensional shape from texture is much more robust than would be reasonable to expect based on current computational models of this phenomenon.

Anisotropy↗

PubMed2004

Attention to 3-D shape, 3-D motion, and texture in 3-D structure from motion displays.

We used fMRI to directly compare the neural substrates of three-dimensional (3-D) shape and motion processing for realistic textured objects rotating in depth. Subjects made judgments about several different attributes of these objects, including 3-D shape, the 3-D motion, and the scale of surface texture. For all of these tasks, we equated visual input, motor output, and task difficulty, and we controlled for differences in spatial attention. Judgments about 3-D shape from motion involve both parietal and occipito-temporal regions. The processing of 3-D shape is associated with the analysis of 3-D motion in parietal regions and the analysis of surface texture in occipito-temporal regions, which is consistent with the different behavioral roles that are typically attributed to the dorsal and ventral processing streams.

Adult↗

PubMed2004

Distortions of distance and shape are not produced by a single continuous transformation of reach space.

We investigated whether distortions of perceived distance and shape could be captured by a single continuous one-to-one transformation of the underlying space. In Experiment 1, the participants reached to touch points around the perimeter of spherical targets viewed at five different distances, to yield simultaneous measures of perceived distance and shape. Different participants reached while using dynamic monocular, static binocular, or dynamic binocular vision. Thin plate spline (TPS) analysis was applied so as to transform a Cartesian grid in such a way as to carry the original target points to the mean reach locations. In all cases, discontinuities appeared in the transformed grid from folding of the space. In Experiment 2, the participants reached to points that lay at the same locus in reach space, but on different portions of the visible target spheres (e.g., front vs. side). The participants reached to different locations when the points were different with respect to shape (e.g., front vs. side) but reached to the same locations when the points were the same with respect to shape (left vs. right side). TPS analysis revealed discontinuities from holes torn in the underlying space. The results show that perceived distance and perceived shape entail different distortions and cannot be captured by a single continuous transformation of reach space.

Adult↗

PubMed2003

The visual perception of 3-D shape from multiple cues: are observers capable of perceiving metric structure?

Three experiments are reported in which observers judged the three-dimensional (3-D) structures of virtual or real objects defined by various combinations of texture, motion, and binocular disparity under a wide variety of conditions. The tasks employed in these studies involved adjusting the depth of an object to match its width, adjusting the planes of a dihedral angle so that they appeared orthogonal, and adjusting the shape of an object so that it appeared to match another at a different viewing distance. The results obtained on all of these tasks revealed large constant errors and large individual differences among observers. There were also systematic failures of constancy over changes in viewing distance, orientation, or response task. When considered in conjunction with other, similar reports in the literature, these findings provide strong evidence that human observers do not have accurate perceptions of 3-D metric structure.

Aptitude↗

PubMed2003

Perceptual representation of visible surfaces.

Two experiments are reported in which we examined the ability of observers to identify landmarks on surfaces from different vantage points. In Experiment 1, observers were asked to mark the local maxima and minima of surface depth, whereas in Experiment 2, they were asked to mark the ridges and valleys on a surface. In both experiments, the marked locations were consistent across different observers and remained reliably stable over different viewing directions. These findings indicate that randomly generated smooth surface patches contain perceptually salient landmarks that have a high degree of viewpoint invariance. Implications of these findings are considered for the recognition of smooth surface patches and for the depiction of such surfaces in line drawings.

Depth Perception↗

PubMed2002

Generic and non-generic conditions for the perception of surface shape from texture.

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

Pappus in optical space.

Optical space differs from physical space. The structure of optical space has generally been assumed to be metrical. In contradistinction, we do not assume any metric, but only incidence relations (i.e., we assume that optical points and lines exist and that two points define a unique line, and two lines a unique point). (The incidence relations have generally been assumed implicitly by earlier authors.) The condition that makes such an incidence structure into a projective space is the Pappus condition. The Pappus condition describes a projective relation between three collinear triples of points, whose validity can--in principle--be verified empirically. The Pappus condition is a necessary condition for optical space to be a homogeneous space (Lobatchevski hyperbolic or Riemann elliptic space) as assumed by, for example, the well-known Luneburg theory. We test the Pappus condition in a full-cue situation (open field, broad daylight, distances of up to 20 m, visual fields of up to 160 degrees diameter). We found that although optical space is definitely not veridical, even under full-cue conditions, violations of the Pappus condition are the exception. Apparently optical space is not totally different from a homogeneous space, although it is in no way close to Euclidean.

Adult↗