Biomedical subjects

A M Kappers

Publications and source records attributed to A M Kappers.

15 recordsLinked to original sources

PubMed1996

Detection of first-order structure in optic flow fields.

We measured psychophysical thresholds for the detection of four different optic flow components in the presence of a translational velocity. We also measured thresholds for detection of rotation in the presence of expansion and for expansion in the presence of rotation. Our stimuli consisted of sparse random dot patterns. Detection thresholds are similar for all four optic flow components. Thus, our experiments indicate that our subjects use a factor that is similar in all first-order flow components, namely the relative orientation of the velocity vectors. First-order components can be extracted independently of both each other and translational velocity.

Humans↗

PubMed1996

Effects of changing viewing conditions on the perceived structure of smoothly curved surfaces.

Observers' perceptions of a male and a female torso were investigated using monocular and stereoscopic images under varying conditions of illumination. Observers judged the shapes of these torsos by adjusting a gauge figure to estimate the local slant and tilt at numerous probe points arranged in a lattice over the torso's surface. The results revealed that the judged surfaces in the monocular and stereoscopic conditions were related by an affine stretching transformation in depth that accounted for approximately 95% of the between-condition variance. There was also a strong affine component between the judgments obtained for the different illumination directions, although a further analysis of the residuals indicated that changing the direction of illumination influenced perceived structure in a piecewise manner.

Contrast Sensitivity↗

PubMed1995

Depth relief.

A study is reported of the depth relief in a simple three-dimensional scene consisting of a white, rough sphere on a planar support, illuminated in a natural manner. Viewing conditions included monocular and binocular as well as 'synoptical' viewing. In the synoptical condition the eyes are optically superimposed. The local surface attitude was probed via a gauge figure that had to appear as a circle painted upon the surface of the object. The measurements allow a reconstruction of the depth relief, which can be compared with the (known) range map. Idiosyncratic differences between three subjects were found that are the exact reverse of what was found for these same subjects for the case of pictorial relief reported in an earlier paper. Apparently the subjects put different weights on sources of sometimes conflicting, sometimes corroborative evidence. The relief is deepest for binocular vision, flatter for monocular vision, and flatter still for synoptical vision. Deviations from veridicality that cannot be explained by mere depth scaling exist and may be due to the nature of the monocular cues.

Cues↗

PubMed1994

Influence of surface attitude and curvature scaling on discrimination of binocularly presented curved surfaces.

We report on the ability of human observers to discriminate local second-order shape of quadratic stereo-defined surfaces. Local second-order shape can be specified by two parameters: the curvedness (a scale-dependent quantity describing overall curvature of a shape) and the shape index (a scale-independent quantity describing the shape's appearance). We measured shape index discrimination thresholds of shapes subject to attitude and curvedness transformations. Results show that neither slant nor curvedness manipulations affect or bias observers' judgements of surface shape. Furthermore, ideal detector simulations show that observers do not perform optimally: they do not exploit all available information.

Depth Perception↗

PubMed1994

On so-called paradoxical monocular stereoscopy.

Human observers are apparently well able to judge properties of 'three-dimensional objects' on the basis of flat pictures such as photographs of physical objects. They obtain this 'pictorial relief' without much conscious effort and with little interference from the (flat) picture surface. Methods for 'magnifying' pictorial relief from single pictures include viewing instructions as well as a variety of monocular and binocular 'viewboxes'. Such devices are reputed to yield highly increased pictorial depth, though no methodologies for the objective verification of such claims exist. A binocular viewbox has been reconstructed and pictorial relief under monocular, 'synoptic', and natural binocular viewing is described. The results corroborate and go beyond early introspective reports and turn out to pose intriguing problems for modern research.

Attention↗

PubMed1994

Haptic discrimination of doubly curved surfaces.

Active haptic discrimination of mathematically well-defined surfaces was investigated. The quadric surfaces were defined in terms of 'shape index'--a quantity describing shape--and 'curvedness'--a quantity describing overall curvature. In these experiments shape index was varied and curvedness kept constant. The influence of laterality (unilateral versus bilateral discrimination) was tested in separate sessions. No influence of shape on the discrimination of solid objects was found. From the results an estimate can be made of the just-noticeable difference in terms of shape-index units. There was a significant effect of laterality on discrimination: performance with unilateral (successive) examination (both with left and with right hand) was better than with bilateral (simultaneous) examination.

Form Perception↗

PubMed1994

Detection of vorticity in optical flow fields.

Psychophysical thresholds for the detection of vorticity in the presence of a translational component are determined for human observers. Stimuli consist of sparse random-dot flow patterns. The detection of vorticity depends critically on the translational component. The curvature of the flow lines, however, cannot be the only factor limiting human performance. On the basis of a comparison with an ideal detector, it is found that for small vorticities (< 0.5 rad/s) humans typically use the stimulus information in an optimal manner. For higher vorticities performance gets worse, possibly owing to matching problems. Lifetime and number of dots have hardly any influence on performance. Although this indicates that the human observer can already perform the task by using only local information, it does not imply that global information is disregarded. If the stimulus is disturbed locally, global information is used to full extent.

Humans↗

PubMed1994

Haptic identification of curved surfaces.

In two experiments, the active haptic identification of three-dimensional mathematically well-defined objects is investigated. The objects, quadric surfaces, are defined in terms of the shape index, a quantity describing the shape, and curvedness, a quantity describing overall curvature. Both shape index and curvedness are found to have a significant influence on haptic shape identification. Concave surfaces lead to a larger spread in responses than convex ones. Hyperbolic surfaces show a slight tendency to be identified with more difficulty than elliptic ones. Surfaces with a high curvedness are identified more easily than those with a low curvedness. Results from experiments with constant and with random curvedness are indistinguishable. It is concluded that shape index and curvedness are psychophysically not confounded.

Adult↗

PubMed1993

Estimating the gradient direction of a luminance ramp.

We report on the extent to which human observers are able to indicate the gradient direction of a luminance ramp. In our experiment modulation depth ranged from 1 to 64% and field sizes subtended 0.5 to 47.5 degrees of visual angle. Observers are not able to indicate the gradient direction for modulation depths below 6%. For values above this threshold, the angular standard deviation in the responses decreases proportionally with the logarithm of the modulation depth and is about 11 degrees for a modulation depth of 64%. The angular standard deviations for supra-threshold contrast are slightly increased for the field sizes of 0.5 and 47.5 degrees but are constant over a range of field sizes of 1 to 25 degrees. Thus, size invariance holds well for this range of field sizes.

Contrast Sensitivity↗

PubMed1993

Visuomotor control of the relative movement of random-dot patterns.

A series of experiments were made on human performance in controlling optical relative movement. The aim was to test the influence of different kinds of relative movement on visually controlled steering tasks. Within adjacent displays on a computer screen random dot patterns moved in a fixed direction at continually changing speeds (Exp. 1) or at constant speed and in continually changing directions (Exp. 2). The subject was required to compensate for the unpredictable modulations of the pattern movement by means of an isometric joystick. The task was to adjust relative movements involving pure translation, symmetric convergence, divergence, or shear. Analysis indicated that the task performance was not dependent on the special kind of relative movement. However, performance was significantly higher in tasks where directionally disturbed relative movement had to be controlled compared to those situations in which relative movement varied with respect to speed.

Adult↗

PubMed1993

Estimating local shape from shading in the presence of global shading.

In theory, global shading may help with the estimation of local surface structure from shading (e.g., in specifying the illuminant direction). Empirically, we do not know whether human observers combine the information given by the local and global shading to estimate local shape. Observers had to indicate the orientation of a local elongated perturbation with or without global shading information provided by a background surface. Our psychophysical results show the following: 1. Observers do not estimate the orientation of the local perturbation more accurately with global shading information than they do in the absence of such information. 2. Responses depend dramatically on the inclination between the illuminant direction and the viewing direction. For an inclination of 20 degrees, observers indicate more or less the orientation of the local ridge; however, for an inclination of 40 degrees, they indicate either the direction of the illuminant or an orientation close to the shadow edge of the perturbation. Most subjects show some combination of these behaviors. This behavior is not altered by global shading information. We conclude that in our paradigm, global shading information does not aid the estimation of local shape.

Female↗

PubMed1993

Shape from stereo: a systematic approach using quadratic surfaces.

We used quadratic shapes in several psychophysical shape-from-stereo tasks. The shapes were elegantly represented in a 2-D parameter space by the scale-independent shape index and the scale-dependent curvedness. Using random-dot stereograms to depict the surfaces, we found that the shape of hyperbolic surfaces is slightly more difficult to recognize than the shape of elliptic surfaces. We found that curvedness (and indirectly, scale) has little or no influence on shape recognition.

Adult↗

PubMed1993

Perception of local shape from shading.

Theoretically, metric solid shape is not determined uniquely by shading. Consequently, human vision has difficulty in categorizing shape when shading is the only cue. In the present research, subjects were required to categorize shaded quadric surfaces. We found that they were rather poor at this task; they confused hyperbolic and elliptic (both convex and concave) shapes easily. When a cast shadow visually indicated the direction of the illuminant, they were able to notice the concavity or convexity of elliptic shapes. However, they still confused elliptic and hyperbolic ones. Finally, when an animated sequence of eight intensity patterns belonging to one quadric shape had been displayed, the subjects were able to categorize the quadrics. However, the results are still quite moderate. Our experiments indicate that local shading structure is only a weak shape cue when presented in the absence of other visual cues.

Algorithms↗

PubMed1992

Surface perception in pictures.

Subjects adjusted a local gauge figure such as to perceptually "fit" the apparent surfaces of objects depicted in photographs. We obtained a few hundred data points per session, covering the picture according to a uniform lattice. Settings were repeated 3 times for each of 3 subjects. Almost all of the variability resided in the slant; the relative spread in the slant was about 25% (Weber fraction). The tilt was reproduced with a typical spread of about 10 degrees. The rank correlation of the slant settings of different observers was high, thus the slant settings of different subjects were monotonically related. The variability could be predicted from the scatter in repeated settings by the individual observers. Although repeated settings by a single observer agreed within 5%, observers did not agree on the value of the slant, even on the average. Scaling factors of a doubling in the depth dimension were encountered between different subjects. The data conformed quite well to some hypothetical fiducial global surface, the orientation of which was "probed" by the subject's local settings. The variability was completely accounted for by single-observer scatter. These conclusions are based upon an analysis of the internal structure of the local settings. We did not address the problem of veridicality, that is, conformity to some "real object."

Attention↗

PubMed1992

Visually controlled matching of pattern movement.

Subjects were asked to match the speeds of two moving random-dot patterns seen through circular apertures. The speed of one pattern that moved horizontally toward the right of a computer screen changed continuously. The speed of this pattern represented the target. It was to be matched with the speed of the second pattern, which moved in the opposite direction. The subject controlled the speed of the second pattern by means of an isometric joystick. The distance between the apertures on the screen as well as the subject's distance from the screen served as experimental parameters. In this way, the effects of both spatial and temporal transients of pattern speed on human tracking performance were studied. To avoid anticipation by the subject, the amplitude and the frequency of the target pattern speed changed pseudorandomly. The accuracy with which the subject performed the matching task was influenced by the mean pattern speed and the parameters of the visual field. Within lower velocity ranges, the subject's sensitivity to the instantaneous speed differences varied according to Weber's law. The cross-correlation of the velocity time courses decreased when the mean speed of the target pattern was increased. Two stimulus parameters had a strong influence on the modulation of the correlation value: (1) the angular size of the stimulus on the retina and (2) the retinal eccentricity of the stimulus.

Adult↗