Biomedical subjects

D Kersten

Publications and source records attributed to D Kersten.

5 recordsLinked to original sources

PubMed1992

Structure-from-motion based on information at surface boundaries.

Existing computational models of structure-from-motion--the appearance of three-dimensional motion generated by moving two-dimensional patterns--are all based on variations of optical flow or feature point correspondence within the interior of single objects. Three separate phenomena provide strong evidence that in human vision, structure-from-motion is significantly affected by surface boundary cues. In the first, a rotating cylinder is seen, though no variation in optical flow exists across the apparent cylinder. In the second, the shape of the bounding contour of a moving pattern dominates the actual differential motion within the pattern. In the third, the appearance of independently moving objects changes significantly when the boundary between them becomes indistinct. We describe a simple computational model sufficient to account for these effects. The model is based on qualitative constraints relating possible object motions to patterns of flow, together with an understanding of the patterns of flow that can be discriminated in practice.

Animals↗

PubMed1991

Apparent surface curvature affects lightness perception.

The human visual system has the remarkable capacity to perceive accurately the lightness, or relative reflectance, of surfaces, even though much of the variation in image luminance may be caused by other scene attributes, such as shape and illumination. Most physiological, and computational models of lightness perception invoke early sensory mechanisms that act independently of, or before, the estimation of other scene attributes. In contrast to the modularity of lightness perception assumed in these models are experiments that show that supposedly 'higher-order' percepts of planar surface attributes, such as orientation, depth and transparency, can influence perceived lightness. Here we show that perceived surface curvature can also affect perceived lightness. The results of the earlier experiments indicate that perceiving luminance edges as changes in surface attributes other than reflectance can influence lightness. These results suggest that the interpretation of smooth variations in luminance can also affect lightness percepts.

Computer Simulation↗

PubMed1991

Dutch general practice care for patients with epilepsy: results from the Dutch National Survey of Morbidity and Interventions.

This study aimed to assess the share which Dutch general practice has in the care of patients with epilepsy. During a 3-month period 400,000 patient contacts in 103 general practices with a total list of 335,000 patients were registered, 1536 of these, concerning 1059 patients, concerned epilepsy. Contacts with patients with known epilepsy in Dutch General Practice were handled mainly by the practice nurse and most involved repeat prescriptions. Patients suspected of having epilepsy had more attention from the General Practitioner. The involvement of General Practitioners in the care of epilepsy was found to be small, but not unimportant. GPs are in a crucial position as regards the detection of epilepsy. They can enlarge their role in respect of patients with known epilepsy and improve continuity of care.

Drug Prescriptions↗

PubMed1991

How to bridge the gap between hospital and home?

Many countries have separate organizations for delivering nursing care to patients in the hospital and to patients at home in the community. This separation creates extra difficulties in organizing continuous care for patients discharged from hospital who need nursing aftercare (in The Netherlands, at a rough estimate, 10% of all hospital patients). Special arrangements have to be made to prevent these patients missing out completely. By means of a questionnaire sent to all Dutch regional community nursing organizations, this study makes an inventory of the organizational measures that have been taken in this field in The Netherlands and analyses a model of the effects these measures (and also the effects on contextual factors) have on the number of problems reported by the community in respect of continuity of care. In The Netherlands, the initiative in arranging continuity of care is taken by the regional community nursing organizations or by their operational teams. In most cases, they have been able to make arrangements with the hospitals about the selection of aftercare patients and the transfer of information by transfer forms. Sometimes special continuity nurses are appointed by the community, among other things to organize regular consultation with the hospitals. An eight-item Likert scale was developed to measure the extent of continuity problems. Analysis of the effect of several factors in the model by a stepwise multiple regression analysis, revealed that the consistent use of transfer forms by hospitals was an important tool in reducing the problems. Furthermore, regional community nursing organizations themselves had fewer problems compared with the operational teams to whom arranging continuity is sometimes delegated. Increased personnel capacity within the community is also beneficial. The effect of these last two factors (organizational level and personnel capacity) is, however, overruled, by the negative effect of the size of the regional community nursing organization: the more inhabitants, the more problems there are.

Aftercare↗

PubMed1990

Human discrimination of fractal images.

In order to transmit information in images efficiently, the visual system should be tuned to the statistical structure of the ensemble of images that it sees. Several authors have suggested that the ensemble of natural images exhibits fractal behavior and, therefore, has a power spectrum that drops off proportionally to 1/f beta (2 less than beta less than 4). In this paper we investigate the question of which value of the exponent beta describes the power spectrum of the ensemble of images to which the visual system is optimally tuned. An experiment in which subjects were asked to discriminate randomly generated noise textures based on their spectral drop-off was used. Whereas the discrimination-threshold function of an ideal observer was flat for different spectral drop-offs, human observers showed a broad peak in sensitivity for 2.8 less than beta less than 3.6. The results are consistent with, but do not provide direct evidence for, the theory that the visual system is tuned to an ensemble of images with Markov statistics.

Computer Simulation↗