Biomedical subjects

Tjeerd M H Dijkstra

Publications and source records attributed to Tjeerd M H Dijkstra.

5 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

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↗

PubMed2002

Object pose: perceiving 3-d shape as sticks and slabs.

Estimating the pose (three-dimensional orientation) of objects is an important aspect of 3-D shape perception. We studied the ability of observers to match the pose of the principal axes of an object with the pose of a cross consisting of three perpendicular axes. For objects, we used a long and a flat spheroid and eight symmetric objects with aspect ratios of dimensions of approximately 4:2:1. Stimulus cues were the contour and stereo for the spheroids, and contour, stereo, and shading for the symmetric objects. In addition, the spheroids were shown with or without surface texture and with or without active motion. Results show that observers can perform the task with standard deviations of a few degrees, though biases could be as large as 30 degrees. The results can be naturally decomposed in viewer-centered coordinates, and it turns out that the estimation of orientation in the frontoparallel plane (tilt) is more precise than estimation of orientation in depth (slant, roll). A comparison of long and flat spheroids shows that sticks lead to better performance than do slabs. This can even be the case within the same object; the pose of the stick-like aspect is seen with more precision than is the pose of the slab-like aspect. The largest biases occurred when the spheroids were displayed with the binocular contour as the only cue. We can explain these biases by assuming that subjects' settings are influenced by the orientation of the rim.

Depth Perception↗

PubMed2002

The beholder's share in the perception of orientation of 2-D shapes.

A considerable amount of research demonstrates that people perceive cardinal orientations (horizontal and vertical) more accurately than other orientations; this is termed the oblique effect. We investigated the interaction of this effect with the degree of elongation of the stimulus. Our stimuli were ellipses with a wide range of aspect ratios, varying from a circle (aspect ratio = 1) to a line (aspect ratio = 123.5). The task was to set a probe line in the same orientation as the long axis of the ellipse. In our first experiment, we determined that performance is degraded as the aspect ratio decreases; furthermore, the bias and response variability are linearly related to a transformation of aspect ratio (roundness). We found significant individual differences; the results show high within-subjects correlations and low between-subjects correlations. In our second experiment, we had observers judge the orientation of circles randomly mixed in with ellipses of low aspect ratio. The observers demonstrated intrinsic preferences and generated reproducible distributions of orientation settings with idiosyncratic profiles. These distributions predict the influence on the response to ellipses with an aspect ratio higher than one and can be considered as the beholder's share in the perception of shape orientation.

Adult↗

PubMedDate not supplied

Dynamical stability in the acquisition and performance of rhythmic ball manipulation: theoretical insights with a clinical slant.

Three experiments illustrate how a task-based approach and dynamical modeling of a perceptual-motor task can provide a useful framework for understanding functional and dysfunctional behavior. In the chosen task, subjects held a racket and bounced a ball rhythmically in the air with invariant ball amplitude. As such, the task could be cast into a mechanical model that encompassed the movements of the actor (racket) and the manipulandum (ball). In this form, the movement task is a dynamical system that displays dynamical stability, i.e., performance where perturbations die out by themselves. The hypothesis is that skilled actors seek to perform with this "passive" stability as it alleviates the control demands because perturbations do not require explicit corrections. In the experimental data, this strategy could be characterized by a single parameter, the acceleration of the racket at impact, which provided quantitative predictions. Experiment 1 established that subjects with normal sensorimotor functions indeed performed with racket acceleration values that were predicted to provide passive stability. Experiment 2 showed that subjects improved their skill over a course of 40 practice trials, as evidenced in decreased variability and accompanied by a change in racket acceleration toward values that provided optimal stability. In experiment 3, perturbations were applied and the subjects' adaptability was tested. When perturbations were large enough, subjects altered their racket timing to resume contact that provided stability. The results are discussed with their relevance to clinical contexts: How can such a task-based approach provide insights into the control of functional and dysfunctional movements? Can such behavioral results serve as a diagnostic tool? How can this approach to sensorimotor behavior stimulate physicians and therapists to progress therapeutic measures?

Biomechanical Phenomena↗