Biomedical subjects

A M Lelkens

Publications and source records attributed to A M Lelkens.

3 recordsLinked to original sources

PubMed1984

Illusory motion in visual displays.

The apparent motion of a change in the structure of a random check pattern is studied by spatially masking it with another noise pattern and it is compared with phi motion. A fundamental difference with phi motion is the insensitivity of second order correlators (Reichardt mechanisms) to this apparent motion. The following experimental characteristics distinguish this motion from phi motion: it induces no motion after-effect, it is not transparent to another simultaneous motion, it is strongly influenced by spatial masking and it does not evoke optokinetic nystagmus. A fourth order detector is introduced which is sensitive to this illusory motion as well as to phi motion. Simulation experiments with this detector together with the subjective reports of the observers lead us to the conclusion that human subjects inadvertently treat the coarsest spatial structures as signal and the finest as the disturbing noise.

Adult↗

PubMed1983

The frequency of seeing square wave density modulations in random dot patterns.

A theoretical and experimental study on the detection of a square wave pattern of random dots is presented. Our theory applies to two-alternative forced choice experiments as well as to "seen"-"not seen" discrimination experiments. In our experiments we used the forced choice method. The theory provides a good description of the psychometric function of the percentage of correct decisions versus contrast. From these frequency of seeing (FOS) curves the apparent signal-to-noise ratio of the detection mechanism of human subjects can be derived. It is found that this ratio can have values of up to 100% and that it decreases with the number of dots in the stimulus and with the number of periods of the square wave. It is shown that a more precise description of the human performance can be obtained from the mid-points of the FOS-curves (i.e. the thresholds) rather than from the slopes.

Bionics↗

PubMed1983

Increment thresholds with various low background intensities at different locations in the peripheral retina.

A set of increment threshold data as a function of test-flash diameter, background luminance, and retinal eccentricity is presented. It is shown that for low background intensities the results can readily be described by simple transformations of flash diameter and background luminance: The threshold is independent of eccentricity if the quotient of diameter and eccentricity is constant and if the flash is presented on a background for which the product of background luminance and the square of eccentricity is constant. At an eccentricity of 50 deg, Ricco's law is violated: A small stimulus has a threshold 10 times as high as a large stimulus. On the basis of results found by other investigators for smaller eccentricities, it is concluded that the receptive field size at 50 deg of eccentricity is more than 10 deg (for low background luminances). For eccentricities smaller than 50 deg, a data analysis is given in order to derive an appropriate measure of the size of the sample units. This analysis shows that with increasing background luminance the decrease in the size of the sample unit is steplike rather than gradual.

Adult↗