Biomedical subjects

P Werkhoven

Publications and source records attributed to P Werkhoven.

6 recordsLinked to original sources

PubMed1992

Visual processing of optic acceleration.

We present data on the human sensitivity to optic acceleration, i.e. temporal modulations of the speed and direction of moving objects. Modulation thresholds are measured as a function of modulation frequency and speed for different periodical velocity vector modulation functions using a localized target. Evidence is presented that human detection of velocity vector modulations is not directly based on the acceleration signal (the temporal derivative of the velocity vector modulation). Instead, modulation detection is accurately described by a two-stage model: a low-pass temporal filter transformation of the true velocity vector modulation followed by a variance detection stage. A functional description of the first stage is a second order low-pass temporal filter having a characteristic time constant of 40 msec. In effect, the temporal low-pass filter is an integration of the velocity vector modulation within a temporal window of 100-140 msec. A non-trivial link of this low-pass filter stage to the temporal characteristics of standard motion detection mechanisms will be discussed. Velocity vector modulations are detected in the second-stage, whenever the variance of the filtered velocity vector exceeds a certain threshold variance in either the speed or direction dimension. The threshold standard deviations for this variance detection stage are estimated to be 17% for speed modulations and 9% for motion direction modulations.

Fixation, Ocular↗

PubMed1991

Reversed rotary motion perception.

A stroboscopically presented revolving annulus composed of dots is used to elicit rotary motion perception. Observers judge the direction of rotary motion. We find sharp and gradual transitions in the probability for reversed motion perception as a function of the angle of rotation between successive frames. These transitions reveal that matches between nonsuccessive frames can dominate motion perception. The transitions are scale invariant. The strength of a match is discussed in terms of a motion strength function, which is a separable function of the angle of rotation between successive frames and the frame repetition rate. The dependence of motion strength on the frame repetition rate (time function) is computed from the transitions. The similarity of this time function for rotary motion with the time function for linear motion [Psychol. Rev. 88, 171 (1981)] suggests that mechanisms for the discrimination of rotary motion address local detectors of linear motion.

Humans↗

PubMed1991

Visual processing of rotary motion.

Local descriptions of velocity fields (e.g., rotation, divergence, and deformation) contain a wealth of information for form perception and ego motion. In spite of this, human psychophysical performance in estimating these entities has not yet been thoroughly examined. In this paper, we report on the visual discrimination of rotary motion. A sequence of image frames is used to elicit an apparent rotation of an annulus, composed of dots in the frontoparallel plane, around a fixation spot at the center of the annulus. Differential angular velocity thresholds are measured as a function of the angular velocity, the diameter of the annulus, the number of dots, the display time per frame, and the number of frames. The results show a U-shaped dependence of angular velocity discrimination on spatial scale, with minimal Weber fractions of 7%. Experiments with a scatter in the distance of the individual dots to the center of rotation demonstrate that angular velocity cannot be assessed directly; perceived angular velocity depends strongly on the distance of the dots relative to the center of rotation. We suggest that the estimation of rotary motion is mediated by local estimations of linear velocity.

Adult↗

PubMed1990

Extraction of motion parallax structure in the visual system. I.

We present a paradigm to estimate local affine motion parallax structure from a varying image irradiance pattern. The method is based on a matching of jets of irradiance rather than the local image irradiance pattern or features of it. It does not put any constraints on the structure of the image irradiance pattern. Moreover, the aperture problem does not arise and additional ad hoc constraints on the velocity field like the "smoothness constraint" are superfluous. An implementation is designed such that the affine structure of the local motion parallax field is represented through simple combinations of the outputs of physiologically plausible receptive fields and their temporal derivatives.

Cybernetics↗

PubMed1990

Extraction of motion parallax structure in the visual system. II.

We have developed an algorithm to extract local affine motion parallax structure of a varying image irradiance pattern in Part I. In Part II, we present computational results of this algorithm for situations in which the information about the affine structure is contained in only two successive images. This applies to a large class of problems (e.g. two-image motion sequences and stereoscopic vision). The results show the effects of spatio-temporal discretisation and internal noise on the performance of this algorithm.

Algorithms↗

PubMed1990

Effects of element orientation on apparent motion perception.

We present an ambiguous motion paradigm that allows us to quantify the influence of aspects of form relevant to the perception of apparent motion. We report on the role of bar element orientation in motion paths. The effect of orientation differences between bar elements in a motion path is small with respect to the crucial role of the orientation of bar elements relative to motion direction. Motion perception between elements oriented along the motion direction dominates motion perception between elements oriented perpendicularly to motion direction. The perception of apparent motion is affected by bar length and width and is anisotropic.

Attention↗