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Biomedical subjects

D D Hoffman

Publications and source records attributed to D D Hoffman.

11 recordsLinked to original sources
PubMed1992

Interpolation in structure from motion.

We investigated surface interpolation in displays of structure from motion (SFM). To do so, we introduced a new method for measuring surface perception in dynamic displays--the SFM probe. An SFM probe is a dot that moves rigidly with the dots on a simulated surface, and whose distance from that surface can be adjusted with a joystick or similar control. The displays we studied were random-dot cylinders containing a vertical strip devoid of feature points (the gap). Subjects adjusted an SFM probe, presented in the gap, until the probe dot appeared to be on the surface. Variability in probe-dot placement decreased with increasing texture density on the cylinder and increased with increasing gap width. Subjects showed a consistent bias to place the probe dot outside the cylinder. This bias increased with increasing texture density for the SFM displays. (The opposite bias was found in a static two-dimensional interpolation task with an arc whose curvature matched that of the cylinder: Subjects placed the probe dot inside the arc.) This outside bias is inconsistent with several theoretical approaches to surface interpolation.

Adult↗
PubMed1991

Unity of perception.

Perceptual scientists have recently enjoyed success in constructing mathematical theories for specific perceptual capacities, capacities such as stereovision, auditory localization, and color perception. Analysis of these theories suggests that they all share a common mathematical structure. If this is true, the elucidation of this structure, the study of its properties, the derivation of its consequences, and the empirical testing of its predictions are promising directions for perceptual research. We consider a candidate for the common structure, a candidate called an "observer". Observers, in essence, perform inferences; each observer has a characteristic class of perceptual premises, a characteristic class of perceptual conclusions, and its own functional relationship between these premises and conclusions. If observers indeed capture the structure common to perceptual capacities, then each capacity, regardless of its modality or manner of instantiation, can be described as some observer. In this paper we develop the definition of an observer. We first consider two examples of perceptual capacities: the measurement of visual motion, and the perception of depth from visual motion. In each case, we review a formal theory of the capacity and abstract its structural essence. From this essence we construct the definition of observer. We then exercise the definition in discussions of transduction, perceptual illusions, perceptual uncertainty, regularization theory, the cognitive penetrability of perception, and the theory neutrality of observation.

Attention↗
PubMed1990

Discriminating rigid from nonrigid motion: minimum points and views.

Theoretical investigations of structure from motion have demonstrated that an ideal observer can discriminate rigid from nonrigid motion from two views of as few as four points. We report three experiments that demonstrate similar abilities in human observers: In one experiment, 4 of 6 subjects made this discrimination from two views of four points; the remaining subjects required five points. Accuracy in discriminating rigid from nonrigid motion depended on the amount of nonrigidity (variance of the interpoint distances over views) in the nonrigid structure. The ability to detect a rigid group dropped sharply as noise points (points not part of the rigid group) were added to the display. We conclude that human observers do extremely well in discriminating between nonrigid and fully rigid motion, but that they do quite poorly at segregating points in a display on the basis of rigidity.

Adult↗
PubMed1989

Parts of visual objects: an experimental test of the minima rule.

Three experiments were conducted to test Hoffman and Richards's (1984) hypothesis that, for purposes of visual recognition, the human visual system divides three-dimensional shapes into parts at negative minima of curvature. In the first two experiments, subjects observed a simulated object (surface of revolution) rotating about a vertical axis, followed by a display of four alternative parts. They were asked to select a part that was from the object. Two of the four parts were divided at negative minima of curvature and two at positive maxima. When both a minima part and a maxima part from the object were presented on each trial (experiment 1), most of the correct responses were minima parts (101 versus 55). When only one part from the object--either a minima part or a maxima part--was shown on each trial (experiment 2), accuracy on trials with correct minima parts and correct maxima parts did not differ significantly. However, some subjects indicated that they reversed figure and ground, thereby changing maxima parts into minima parts. In experiment 3, subjects marked apparent part boundaries. 81% of these marks indicated minima parts, 10% of the marks indicated maxima parts, and 9% of the marks were at other positions. These results provide converging evidence, from two different methods, which supports Hoffman and Richard's minima rule.

Attention↗
PubMed1989

Structure from two orthographic views of rigid motion.

We study the inference of rigid three-dimensional interpretations for the structure and motion of four or more moving points from but two orthographic views of the points. We develop an algorithm to determine whether image data are compatible with a rigid interpretation. As a corollary of this result we find that the measure of false targets (roughly, nonrigid objects that appear rigid) is zero. We find that if the two views have at least one rigid interpretation, then in fact there is a canonical one-parameter family of rigid interpretations; we show how to compute this family, and we describe precisely how the rigid interpretations vary within it. Since only two views are used, this analysis is relevant also to stereo vision.

Algorithms↗
PubMed1987

Minimum points and views for the recovery of three-dimensional structure.

Mathematical analyses of motion perception have established minimum combinations of points and distinct views that are sufficient to recover three-dimensional (3D) structure from two-dimensional (2D) images, using such regularities as rigid motion, fixed axis of rotation, and constant angular velocity. To determine whether human subjects could recover 3D information at these theoretical levels, we presented subjects with pairs of displays and asked them to determine whether they represented the same or different 3D structures. Number of points was varied between two and five; number of views was varied between two and six; and the motion was fixed axis with constant angular velocity, fixed axis with variable velocity, or variable axis with variable velocity. Accuracy increased with views, decreased with points, and was greater with fixed-axis motion. Subjects performed above chance levels even when motion was eliminated, indicating that they exploited regularities in addition to those in the theoretical analyses.

Depth Perception↗
PubMed1985

Inferring the relative three-dimensional positions of two moving points.

We show that four orthographic projections of two rigidly linked points are compatible with at most four interpretations of the relative three-dimensional positions of the points if the points rotate about a fixed axis--even when the points as a system undergo arbitrary rigid translations. A fifth view (projection) yields a unique interpretation and makes zero the probability that randomly chosen image points will receive a three-dimensional interpretation. Assuming that the points rotate at a constant angular velocity, instead of adding a fifth view, also yields a unique interpretation and makes zero the probability that randomly chosen image points will receive a three-dimensional interpretation.

Form Perception↗
PubMed1982

The interpretation of biological motion.

The term biological motion has been coined by Johansson (1973) to refer to the ambulatory patterns of terrestrial bipeds and quadripeds. In this paper a computational theory of the visual perception of biological motion is proposed. The specific problem addressed is how the three-dimensional structure and motions of animal limbs may be computed from the two dimensional motions of their projected images. It is noted that the limbs of animals typically do not move arbitrarily during ambulation. Rather, for anatomical reasons, they typically move in single planes for extended periods of time. This simple anatomical constraint is exploited as the basis for utilizing a "planarity assumption" in the interpretation of biological motion. The analysis proposed is: (1) divide the image into groups of two or three elements each; (2) test each group for pairwise-rigid planar motion; (3) combine the results from (2). Fundamental to the analysis are two "structure from planar motion" propositions. The first states that the structure and motion of two points rigidly linked and rotating in a plane is recoverable from three orthographic projections. The second states that the structure and motion of three points forming two hinged rods constrained to move in a plane is recoverable from two orthographic projections. The psychological relevance of the analysis and possible interactions with top down recognition processes are discussed.

Gait↗
PubMed1982

Equation counting and the interpretation of sensory data.

Many problems in biological information processing require the solution to a complex system of equations in many unknown variables. An equation-counting procedure is described for determining whether such a system of equations will indeed have a unique solution, and under what conditions the solution should be interpreted as 'correct'. Three examples of the procedure are given for illustration, one from auditory signal processing and two from vision.

Animals↗
PubMed1982

Inferring local surface orientation from motion fields.

The problem of inferring local surface orientation from changing images is studied computationally by deriving conditions under which the motion information is sufficient for an information processing system, biological or otherwise, to infer unique descriptions of the local surface orientation. The analysis is based on shape-from-motion proposition, which states, that given the first spatial derivatives of the orthographically projected velocity and acceleration fields of a rigidly rotating regular surface, then the angular velocity and the surface normal at each visible point on that surface are uniquely determined up to a reflection. The proof proceeds in two steps. First it is shown that surface tilt and one component of the angular velocity are uniquely determined by the first spatial derivatives of the velocity field. Then it is shown that surface slant and the remaining two components of the angular velocity are uniquely determined if the first spatial derivatives of the acceleration field are also available.

Humans↗