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Biomedical subjects

J T Todd

Publications and source records attributed to J T Todd.

At least 55 records · Page 3Linked to original sources
PubMed1990

Visual perception of smoothly curved surfaces from double-projected contour patterns.

This research was designed to examine how human observers are able to perceive the 3-dimensional structure of smoothly curved surfaces from projected patterns of surface contours. Displays were generated by using a method of double projection that made it possible to cover a surface with a wide range of contour patterns of varying geometric structure and to eliminate systematic variations of image shading. The compelling impression of 3-dimensional form from these patterns provides strong evidence that the ability of observers to perceptually interpret surface contours is considerably less restrictive than would be reasonable to expect on the basis of existing computational models. Results suggest that the perceptual analysis of surface contours is able to exploit statistical regularities of contour structure over appropriately large regions of visual space.

Adult↗
PubMed1989

Ordinal structure in the visual perception and cognition of smoothly curved surfaces.

In theoretical analyses of visual form perception, it is often assumed that the 3-dimensional structures of smoothly curved surfaces are perceptually represented as point-by-point mappings of metric depth and/or orientation relative to the observer. This article describes an alternative theory in which it is argued that our visual knowledge of smoothly curved surfaces can also be defined in terms of local, nonmetric order relations. A fundamental prediction of this analysis is that relative depth judgments between any two surface regions should be dramatically influenced by monotonicity of depth change (or lack of it) along the intervening portions of the surface through which they are separated. This prediction is confirmed in a series of experiments using surfaces depicted with either shading or texture. Additional experiments are reported, moreover, that demonstrate that smooth occlusion contours are a primary source of information about the ordinal structure of a surface and that the depth extrema in between contours can be optically specified by differences in luminance at the points of occlusion.

Cognition↗
PubMed1987

Perception of three-dimensional form from patterns of optical texture.

The research described in the present article was designed to investigate how patterns of optical texture provide information about the three-dimensional structure of objects in space. Four experiments were performed in which observers were asked to judge the perceived depth of simulated ellipsoid surfaces under a variety of experimental conditions. The results revealed that judged depth increases linearly with simulated depth although the slope of this relation varies significantly among different types of texture patterns. Random variations in the sizes and shapes of individual surface elements have no detectable effect on observers' judgments. The perception of three-dimensional form is quite strong for surfaces displayed under parallel projection, but the amount of apparent depth is slightly less than for identical surfaces displayed under polar projection. Finally, the perceived depth of a surface is eliminated if the optical elements in a display are not sufficiently elongated or if they are not approximately aligned with one another. A theoretical explanation of these findings is proposed based on the neural network analysis of Grossberg and Mingolla.

Attention↗
PubMed1986

Perception of solid shape from shading.

Observers judged the slants and tilts of numerous regions within shaded images of ellipsoid surfaces that varied in shape, orientation, surface reflectance, and direction of illumination. The perceived three-dimensional structure of each surface was calculated from these judgments. Much of the error in observers' responses resulted from a tendency to perceive surfaces whose axes were aligned with the display screen. The presence of specular highlights or cast shadows, in contrast, had no effect on performance. The results of the experiment indicate that several assumptions of certain formal models for perception of shape from shading are not psychologically valid. The most notable of these assumptions are that the visual system initially assumes that all surfaces have Lambertian reflectance and that illuminant direction must be known before shape detection can proceed. These assumptions are often accompanied by a third assumption that surface orientation is detected locally, and global shape determined by smoothing over local surface orientation estimates. The present experiment indicates that an alternative approach offered by Koenderink and van Doorn may be more psychologically accurate, as it avoids all three assumptions.

Humans↗
PubMed1985

Perception of structure from motion: is projective correspondence of moving elements a necessary condition?

A fundamental assumption of almost all existing computational analyses of the perception of structure from motion is that moving elements on the retina projectively correspond to identifiable moving points in three-dimensional space. The present investigation was designed to determine the psychological validity of this assumption in several different contexts. The results demonstrate that the ability of human observers to perceive structure from motion is much more general than would be reasonable to expect on the basis of existing theory. Observers can experience a compelling kinetic depth effect even when the pattern of optical motion is contaminated by large amounts of visual noise (e.g., where the signal to noise ratio is less than 0.15). Moreover, the optical deformations of shading, texture, or self-occluding contours, which would be treated as noise by existing computational models, are analyzed by human observers as perceptually salient sources of information about an object's three-dimensional form. These results suggest that the modular analyses of visual information that currently dominate the literature will have to be modified if they are to account for the high level of generality exhibited by human observers.

Attention↗
PubMed1984

Simulation of curved surfaces from patterns of optical texture.

Previous research on the perceptual analysis of optical texture has been severely restricted by the lack of an appropriate technique for distributing texture on curved surfaces in a uniform manner. In an effort to overcome this problem, the present article presents a new algorithm for generating stochastically regular distributions of texture on any smooth surface regardless of its curvature. We also present a new technique for representing the global organization of a textured image based on the formal concept of a projected area field.

Depth Perception↗
PubMed1983

Perception of gait.

Our ability of perceive the identity and naturalness of a human gait is examined in a series of four experiments involving computer-animated stick figures. The results indicate that the perceived naturalness of a walking or running gait can be influenced by the motion of any limb segment, but the perceived identity of these gaits is primarily determined by the movements of the lower leg (i.e., the tibia). The results also demonstrate that a perceptually salient walking gait can be transformed into running (or vice versa) by adding or subtracting a constant value to the angle of the lower leg over the entire step cycle. The size of this constant value is affected by the shape of the lower leg angle function and the motion of other limb segments.

Gait↗
PubMed1983

Perception of surface curvature and direction of illumination from patterns of shading.

Three experiments examine the perceptual salience of shading information for the visual specification of three-dimensional form. The observers in these experiments were required to estimate the surface curvature and direction of illumination depicted in computer-synthesized images of cylindrical surfaces, both with and without texture. The results indicate that the shininess of a surface enhances the perception of curvature, but has no effect on perceived direction of illumination; and that shading is generally less effective than texture for depicting surfaces in three dimensions. These and other findings are used to evaluate the psychological validity of several mathematical analyses of shading information that have recently been proposed in the literature.

Humans↗
PubMed1983

Perception of growth from changes in body proportions.

Three experiments examine the perception of age level from computer-generated line drawings of a human body, a Martian, a robot, and a flower that were mathematically transformed using a model of human growth developed by P.B. Medawar. The results suggest that observers' age level judgments are related by a power function to the size of the head relative to the rest of the body but that the exponents of these functions vary systematically across the different body forms. These results are discussed in relation to other research on biological event perception.

Growth↗
PubMed1982

Visual information about rigid and nonrigid motion: a geometric analysis.

A mathematical analysis is presented that attempts to describe the available visual information about rigid and nonrigid motion and the three-dimensional structure of rigidly moving objects. Unlike other approaches, the analysis is based on the geometric relations among a set of trajectories defined over an extended region of space-time. Two experiments are reported in which observers viewed computer simulations of moving objects and were required to judge whether the observed motion appeared to be rigid or nonrigid. The results suggest that the mathematical limitations of a trajectory-based analysis of visual information are consistent with the perceptual limitations of actual human observers.

Discrimination Learning↗
PubMed1982

Visual perception of relative mass in dynamic events.

Two experiments are reported which examine how the dynamic property of relative mass in collision events is specified by kinematic properties of a visual motion display. In most cases observers are accurate at detecting the 'heavier' of two objects, although they do not take advantage of the completely general optic information that is available. Instead, they rely on limited information that breaks down at extreme values of elasticity and relative initial velocity. In addition, observers appear to utilize different information for relative mass with different types of collisions. It is suggested that reliance on such limited information may be appropriate for perceivers operating in the restricted context of a terrestrial environment.

Humans↗
PubMed1981

Issues related to the prediction of craniofacial growth.

The prediction of craniofacial growth involves four central issues: (1) What frame of reference should be adopted for measuring change? (2) What type of coordinate system should be used? (3) How should the change be described? (4) How can the change be explained biologically? In an effort to address these issues within a common framework, we are presenting a mathematical model for predicting the course of craniofacial growth in any given individual. The model is derived from a few basic assumptions about the long-range effects gravitational pressure on the remodeling of bone and is expressed formally as a single geometric transformation. The validity of the model is examined empirically, using data for twenty individuals from the Denver Child Research Council's longitudinal growth study. The predictions of the model are found to be in close correspondence with the actual morphologic changes in each individual over periods ranging from 8 to 17 years. These findings suggest that a transformational approach to the study of human growth may provide clinicians with a valuable tool for long-range treatment planning.

Adolescent↗