Biomedical subjects

D Kersten

Publications and source records attributed to D Kersten.

At least 19 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↗

PubMed1987

Statistical efficiency for the detection of visual noise.

Visual detection thresholds were measured for one-dimensional static noise patterns centered at 2 c/deg on a logarithmic scale, for several widths and spatial frequency bandwidths in the presence and absence of a one-dimensional dynamic visual noise mask. Human performance was compared with the ideal observer's performance for the same task. The ideal's contrast thresholds increase with the fourth root of bandwidth. For thresholds measured in the presence and absence of noise, the fourth-root-law for bandwidth fitted the human observers' data from 1 to 6 octaves for a space constant of 2 degrees and from 2 to 6 octaves for a space constant of 0.25 degree. Detection efficiencies were quite high (30-60%) for patterns of 1- and 6-octave bandwidths. These results can be interpreted either in terms of a channel with an adjustable spatial frequency bandwidth or as very efficient combination of information between spatial frequency channels.

Form Perception↗

PubMed1987

Contrast discrimination in noise.

Even the highest contrast sensitivities that humans can achieve for the detection of targets on uniform fields fall far short of ideal values. Recent theoretical formulations have attributed departures from ideal performance to two factors--the existence of internal noise within the observer and suboptimal stimulus information sampling by the observer. It has been postulated that the contributions of these two factors can be evaluated separately by measuring contrast-detection thresholds as a function of the level of externally added visual noise. We wished to determine whether a similar analysis could be applied to contrast discrimination and whether variation of the increment threshold with pedestal contrast is due to changes in internal noise or sampling efficiency. We measured contrast-increment thresholds as a function of noise spectral density for near-threshold and suprathreshold pedestal contrasts. The experiments were conducted separately for static and dynamic noise. Our findings indicate that the same formulation can be applied to contrast discrimination and that changes in the estimated values of internal noise, rather than changes in sampling efficiency, play the major role in determining properties of contrast discrimination. Implications for models of contrast coding in vision are discussed.

Humans↗

PubMed1987

Contrast discrimination in peripheral vision.

Contrast discrimination provides a psychophysical method for studying contrast coding in vision. Our purpose was to compare properties of contrast discrimination in central and peripheral vision. We used forced-choice procedures to measure contrast-increment thresholds as a function of pedestal contrast. Our stimuli were 2-cycle/deg Gaussian-windowed sine-wave grating patches. They were centered at retinal loci ranging from 10 degrees nasal to 20 degrees temporal on the horizontal meridian. At each eccentricity, curves relating increment threshold to pedestal contrast had the same shape. When increment thresholds and pedestal contrasts were both normalized by the contrast thresholds at the retinal eccentricity in question, the curves became superimposed and fell along the same dipper-shaped contrast-discrimination function. We conclude that, after scaling by the local contrast sensitivity, properties of contrast discrimination are qualitatively and quantitatively similar from 0 degree to 20 degrees on the retina. These findings suggest that mechanisms of contrast coding are similar in central and peripheral vision.

Discrimination, Psychological↗

PubMed1987

Predictability and redundancy of natural images.

One aspect of human image understanding is the ability to estimate missing parts of a natural image. This ability depends on the redundancy of the representation used to describe the class of images. In 1951, Shannon [Bell. Syst. Tech. J. 30, 50 (1951)] showed how to estimate bounds on the entropy and redundancy of an information source from predictability data. The entropy, in turn, gives a measure of the limits to error-free information compaction. An experiment was devised in which human observers interactively restored missing gray levels from 128 X 128 pixel pictures with 16 gray levels. For eight images, the redundancy ranged from 46%, for a complicated picture of foliage, to 74%, for a picture of a face. For almost-complete pictures, but not for noisy pictures, this performance can be matched by a nearest-neighbor predictor.

Forecasting↗

PubMed1987

Complications of subconjunctival 5-fluorouracil following glaucoma filtering surgery.

The antimetabolite 5-fluorouracil (5-FU) has been used to inhibit fibroblastic growth to increase the success rate in glaucoma filtration surgery. Subconjunctival 5-FU can have adverse side effects. This paper describes two cases in detail of the ocular side effects of 5-FU. The first case is representative of the ocular surface problems. The second case deals with bleb leaks. Relative contraindications to 5-FU treatment are pre-existing ocular surface problems, early postoperative bleb leaks and fornix-based conjunctival flaps in filtering procedures. New drug delivery methods are presently being investigated to help decrease the adverse side effects and increase the efficacy of 5-FU.

Aged↗

PubMed1986

A study of histochemical changes in mucus from patients with ulcerative colitis, Crohn's disease, and diverticular disease of the colon.

The colonic mucosa produces a protective and lubricating layer of mucus. In certain conditions, the quantity and quality of this mucus is impaired. This study assessed the histochemical changes in mucus in inflammatory bowel disease compared with the severity and extent of the condition. Biopsy specimens were taken from 62 patients (32 with ulcerative colitis; ten with colonic Crohn's disease; ten with diverticular disease; ten with normal controls) and sections stained with high iron diamine-alcian blue to distinguish sulphated mucins from sialomucins. Normal subjects showed a predominance of sulphated mucins. The patients with Crohn's and diverticular disease also demonstrated this normal pattern. Of the 20 patients with ulcerative colitis, and without demonstrable dysplastic changes, only one showed a moderate increase in sialomucins. However, of the 12 patients with extensive colitis and dysplastic changes, ten had an increase in sialomucins. Thus, the predominant sialomucin pattern was seen mainly in patients with dysplasia. It may, therefore, indicate patients at high risk of malignancy.

Colitis, Ulcerative↗

PubMed1986

The use of glaucoma shell tamponade in leaking filtration blebs.

Our experience with the use of glaucoma shell tamponade for bleb leaks in six patients is described. All patients had posterior lip sclerectomies, with leaking blebs diagnosed intraoperatively (2 patients), early (2 patients) and late (2 patients) following surgery. The glaucoma shell works by applying a high resistance to excessive aqueous humor run-off through the leaking wound, and promotes conjunctival epithelial closure by approximating the wound edges. In all six cases the use of the glaucoma shell tamponade was fully effective. The leaks were sealed off and diffuse functioning filtering blebs were maintained, with very low intraocular pressure. We recommend the use of the glaucoma shell tamponade as the treatment of choice for bleb leaks prior to considering surgical repair.

Aged↗

PubMed1985

Reduction in the stearic to oleic acid ratio in the circulating red blood cells: a possible tumour marker in solid human neoplasms.

Gas liquid chromatography study of the 18 carbon chain length fatty acids (C18FA) of the human red blood cells (RBCs) was performed on 65 patients with various clinical disorders. It was found that the stearic to oleic acid ratio (SI) of the RBCs was significantly lower (P less than 0.001) in patients with malignant conditions (n = 20, SI = 0.62 +/- 0.16) compared with pathological non-malignant diseases (n = 10, SI = 1.19 +/- 0.2) and the normal control group (n = 35, SI = 1.57 +/- 0.5). Our early results suggest that the increased unsaturation (oleic acid) in the circulating RBCs could be used as a chemical marker in various solid neoplasms.

Chromatography, Gas↗

PubMed1984

Spatial summation in visual noise.

Contrast thresholds were measured for sinusoidal gratings, with Gaussian spatial and temporal envelopes, as a function of spatial extent in the presence and absence of dynamic white noise. Spatial frequencies ranged from 0.5 to 32 c/deg. Efficiency is defined as the ratio of the ideal's contrast-energy threshold to that of the observer under study, each at the same performance level. For spatial frequencies from 0.5 to 8 c/deg, efficiencies for gratings in noise ranged from 8 to 30% for widths less than one cycle, but dropped rapidly as the width was increased beyond one cycle. Spatial summation of gratings in noise resembles the performance of a cross-correlator whose template is matched to a signal about 1 cycle wide (between 1/e points). The psychometric function slope is consistent with this idea.

Form Perception↗

PubMed1984

Symmetry and constancy in the perception of negative and positive luminance contrast.

The perception of suprathreshold luminance contrast was investigated by forced-choice psychophysical procedures that were designed to define contrast equivalence relations. Observers compared the perceived contrast of rectangular bars that were presented for 500 msec at 3.9 deg on opposite sides of the fovea. The results show a nearly symmetrical relation between the perception of negative and positive contrast that is largely invariant over four decades of background luminance. Thus, for any fixed background luminance, equal absolute contrasts evoke approximately equal perceived contrasts. Symmetry also held with variations in the width, the eccentricity, and the focus of the bars. Symmetry was investigated further by determining equivalent contrast relations for negative contrasts as a function of background luminance and by contrast scaling. These results show evidence for nearly perfect contrast constancy for targets of low to moderate contrast and departures form constancy for high-contrast targets. These new findings on negative contrast, symmetry, and contrast constancy are discussed in relation to underlying mechanisms for contrast perception and classic experiments on brightness and lightness constancy.

Dark Adaptation↗

PubMed1984

[The evaluation of non-isothermal short-term tests of drug solutions].

Non-isothermal short-term tests to examine the stability of drug solutions, are critically discussed with regard to their application to simple and complex reaction mechanisms. The influence of additional temperature dependences (ionic product of water, activity coefficient) is assayed. By example of the decomposition of tetracaine hydrochloride and acetylsalicylic acid, the evaluation of experimental results is discussed considering these factors.

Aspirin↗

PubMed1983

Light and dark bars; contrast discrimination.

Contrast increment thresholds were measured for light and dark bars as a function of the base contrast of the bars. The bars were superimposed on a uniform field of 340 cd/m2. They had either rectangular or Gaussian luminance profiles, varied in width from 0.1 degree to 10 degrees, and in duration from 10 to 200 msec. For the 200-msec presentations, the resulting contrast discrimination functions all had approximately the same shape when contrast was defined as (Lmax - Lmin)/(Lmax + Lmin), and closely resembled corresponding results for sine-wave gratings. The similarity in shape of contrast discrimination functions for light and dark bars is attributed to a retinal nonlinear intensity transformation. The 10 msec contrast discrimination functions differed from the 200-msec functions in ways that can be explained by differences in temporal integration.

Darkness↗

PubMed1983

Convergence accommodation.

Steady-state accommodation responses were measured in both eyes as a function of vergence angle and direction of lateral gaze. The measurements were made with a binocular laser optometer. Small speckle patterns were used as fusional stimuli in an otherwise dark field. These patterns have the advantage of providing no blur stimulus to accommodation. Convergence accommodation for vergence angles ranging from 0 to 25 deg was measured for lateral-gaze angles of +32, -32, and 0 deg. The average accommodation of the two eyes was linearly related to vergence angle over the observer's accommodation range but was independent of the angle of lateral gaze. The mean convergence accommodation/convergence ratio for three subjects, in diopters per meter-angle, was 0.9. Our measurements of convergence accommodation using laser-speckle targets are in good agreement with previous studies that used small pupils. Accommodation responses for binocular viewing of letters of a Snellen chart were also measured. When luminance was reduced, night myopia was observed. No similar effect was found for convergence accommodation. Accommodation to a dim target corresponded closely to the convergence accommodation.

Accommodation, Ocular↗