Biomedical subjects

J M du Buf

Publications and source records attributed to J M du Buf.

5 recordsLinked to original sources

PubMed1992

Modelling spatial vision at the threshold level.

Some available single- and multiple-channel models are reviewed. Multichannel models are generalized and tested against threshold data on various stimulus sets. Without using the explicit assumption of spatial probability summation, simple multichannel models are shown to provide good simultaneous predictions of threshold curves of sinewave gratings and other gratings. They fail in predicting threshold curves of disk-shaped stimuli. If global or local spatial probability summation within channels is incorporated into the models, correctly shaped threshold curves of disks can be predicted. However, the predicted curves appear still too low if compared to measured curves. The same holds for noise gratings. Possible extensions of the models, based on local summation between channel responses and/or models consisting of initial isotropic channels (retina) followed by anisotropic channels (cortex), are discussed.

Humans↗

PubMed1992

Brightness versus apparent contrast. 1: Incremental and decremental disks with varying diameter.

This study describes the matching of the brightness and of the apparent contrast of foveal disks, presented as an increment or decrement with varying diameter against a 300 cd.m-2 background. If the brightness in the centre of the disks is matched with a constant reference brightness, the well-known spatial Broca-Sulzer phenomenon is obtained. This effect is not found if the apparent contrast of the disks is matched instead. All matching results and detection thresholds indicate that luminance increments and decrements are processed asymmetrically by the visual system: for decrements the threshold curve and curves of isobrightness as well as apparent iso-contrast are all shifted towards larger disk radii relative to those for increments. Two nonlinear single-channel models are tested against the data.

Contrast Sensitivity↗

PubMed1992

Abstract processes in texture discrimination.

In this study some experiments on texture segmentation are reported using the local Gabor power spectrum. The techniques applied are: (1) supervised pixel classification; (2) boundary detection by spectral dissimilarity estimation; (3) region-based segmentation based on Gaussian spectral estimation; and (4) the same as (3) but based on central moments of the local spectrum. It is shown that very-acceptable-to-excellent results can be obtained. It is argued, however, that the shortcomings of region-based and boundary-based approaches require that both processes should act in parallel, not only in digital image processing but also in the modelling of visual perception.

Form Perception↗

PubMed1992

Brightness versus apparent contrast. 2: Large-field asymmetry.

Experiments were performed on the quasi-static perception of brightness and of apparent contrast of a foveal 1-deg disk, presented either as a luminance increment or decrement against a 300 cd.m-2 background. Results suggest that the perceptual attributes of brightness and apparent (or subjective) contrast should be distinguished. For an equal brightness difference with respect to the background, luminance increments are more effective than decrements. For an equal apparent contrast it is found that increments and decrements, up to 100 cd.m-2, are about equally effective; for higher values luminance decrements are more effective. Brightness increments and decrements can both be described by a Stevens power function of the respective luminance increments and decrements. Apparent contrast can, apart from applying a usual luminance contrast formula, also be described as a power function of the luminance difference with the background.

Contrast Sensitivity↗

PubMed1992

Brightness versus apparent contrast. 3: Blurred disks and concentric cosine gratings.

Matching either the centre-brightness or the apparent contrast of incremental 1-deg disks, with varying edge-sharpness, yields quite different results. These suggest that the maximum brightness gradient at the edge determines apparent-contrast perception. However, no significant differences are found in matching the brightness maxima, the brightness minima, or the apparent contrast of concentric cosine gratings with varying spatial frequency. This means that spatial frequency influences both brightness extremes in the same way, and that apparent contrast is a function of the difference between the brightness extremes. The grating results therefore deviate from the results obtained with sharp as well as blurred disks. These contradictions complicate the construction of a model which unifies suprathreshold disk and grating perception.

Contrast Sensitivity↗