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Biomedical subjects

J van de Kraats

Publications and source records attributed to J van de Kraats.

11 recordsLinked to original sources
PubMed2001

Wavelength dependence of the Stiles-Crawford effect explained by perception of backscattered light from the choroid.

To explain the wavelength dependence of the directional sensitivity of human foveal cones (Stiles-Crawford I effect) we extended an existing fundus reflectance model for calculation of the total absorption by visual pigment. We took experimental data from literature for both the psychophysical and the optical Stiles-Crawford effect and optimized parameters to fit the experimental data. The wavelength dependence of the Stiles-Crawford effect could be well described with the geometrical optics model. Essential elements are self-screening and the inclusion of backscattered choroidal light for perception.

Choroid↗
PubMed2000

Slow optical changes in human photoreceptors induced by light.

PURPOSE: The basic assumption of fundus reflection densitometry is that changes in reflectance are solely determined by photolysis and regenerating visual pigments. This study was undertaken to investigate small but systematic deviations from this rule. METHODS: Spectral reflectance changes (450-740 nm) of the fovea were measured during light and dark adaptation over a period of 66 minutes in five healthy subjects. The directional properties of the fundus reflection were examined with a custom-built scanning laser ophthalmoscope (SLO) at 514, 633, and 790 nm. The same instrument was also used to find the spatial distribution of the reflectance changes. RESULTS: In addition to fast changes consistent with visual pigment, slower reflectance changes (lasting 10-20 minutes) were observed at all wavelengths including 740 nm. Because visual pigment does not absorb at 740 nm, a second mechanism must be involved. Factor analysis generated two factors (i.e., spectral curves) that explained more than 97% of the variations in the time course of the spectral reflectance. Total reflectance was modeled by means of an existing model for fundus reflection, and it was found that the first factor strongly resembled the rapid changes in absorption of the cone pigments. The second factor seems linked to slow changes in cone reflectance. Measurements with the SLO showed a clear increase in directionally dependent reflectance from 6 to 30 minutes in the dark. This was observed only in the central 6 degrees of the retina. CONCLUSIONS: The characteristics of the slow reflectance changes all point to the cone photoreceptors as the origin. Most likely, alterations in the index of refraction between the interphotoreceptor matrix and photoreceptors lie at the base of this hitherto unknown phenomenon.

Adult↗
PubMed2000

Influence of lutein supplementation on macular pigment, assessed with two objective techniques.

PURPOSE: Macular pigment (MP) may protect against age-related macular degeneration. This study was conducted to determine the extent of changes in the macular pigment density as a consequence of oral supplementation with lutein. A second purpose was to compare two objective measurement techniques. METHODS: In the first technique, reflectance maps were made with a scanning laser ophthalmoscope. Digital subtraction of log reflectance maps and comparison between the foveal area and a 14 degrees temporal site provided MP density estimates. In the second technique, spectral fundus reflectance of the fovea was measured with a fundus reflectometer and analyzed with a detailed optical model, to arrive at MP density values. Eight subjects participated in this study. They took 10 mg lutein per day for 12 weeks. Plasma lutein concentration was measured at 4-week intervals. RESULTS: After 4 weeks, mean blood level of lutein had increased from 0.18 to 0.90 microM. It stayed at this level throughout the intake period and declined to 0.28 microM 4 weeks after termination. Measurement of the density of MP showed a within-subject variation of 10% with MP maps and 17% with spectral reflectance analysis. MP density showed a mean linear 4-week increase of 5.3% (P: < 0.001) and 4.1% (P: = 0. 022), respectively. CONCLUSIONS: Supplementation with lutein significantly increased the density of the MP. Analyzing reflectance maps with a scanning laser ophthalmoscope provided very reliable estimates of MP.

Adolescent↗
PubMed1996

Foveal cone mosaic and visual pigment density in dichromats.

1. Optical reflectance spectra of the fovea were measured in ten subjects with normal colour vision, ten protanopes and seven deuteranopes. Four conditions were used: perpendicular and oblique angle of incident and reflected light on the retina, both in a dark-adapted and a fully bleached state. 2. The spectra were analysed to assess the effects of dichromacy on the cone mosaic. A replacement model, i.e. one where the total number of cones remains unchanged and all cones are filled with a single type of pigment, was found to fit our data best. 3. The analysis of the spectral fundus reflectance also provided estimates for densities of photo-labile and photo-stable retinal pigments and fraction of long wavelength-sensitive (LWS) cones. Visual pigment density was 0.39 for protanopes and 0.42 for deuteranopes, significantly lower than the 0.57 found for colour normals. Macular pigment density was 0.54 for colour normals, 0.46 for protanopes and 0.42 for deuteranopes. 4. For colour normals the LWS cone fraction was 0.56, in agreement with psychophysical literature. The LWS cone fraction for protanopes was -0.04, and for deuteranopes 0.96, consistent with their Rayleigh matches.

Color Vision Defects↗
PubMed1996

The pathways of light measured in fundus reflectometry.

We measured the spectral reflectance of the fovea of ten normal subjects in four conditions, i.e. under dark-adapted and bleached conditions and at two retinal angles of incidence. The objective was to study optical pathways through the photoreceptor layer, resulting in a model that simultaneously explains spectral, directional and bleaching properties of the fovea. On theoretical grounds, we propose that small reflections from the stack of discs in the cone outer segments are the origin of the directional component of foveal reflection. Non-directional reflection occurs at the inner limiting membrane and at all layers posterior to the outer segments. With four reflectance spectra as input, the model allows determination of the density of the photostable absorbers, the lens, macular pigment, melanin and blood. Because of the simplified modeling of the layers posterior to the photoreceptor layer, the values for the density of melanin and blood are not necessarily comparable to physiological data. The density of the visual pigment calculated with this model is consistent with psychophysical data, with estimates for the ten subjects ranging from 0.41 to 0.80. The long wavelength sensitive cone fraction is calculated as 0.56.

Blood↗
PubMed1993

Scanning laser densitometry in multiple evanescent white dot syndrome.

A 28-year-old man with multiple evanescent white dot syndrome (MEWDS) in the left eye was examined with a scanning laser densitometer. The first measurements were taken in the acute stage, and repeat examinations were performed during the process of recovery. Fundus pictures were obtained from a 20 degrees retinal field, in dark and light adapted stages. From these images visual pigment density maps were derived. In the acute stage of the disease, maps revealed small round areas of absent visual pigment, which did not always correspond with the visible white dots. The areas of absent visual pigment density were also larger than the white dots seen on funduscopic examination. Single spot densitometry at the fovea was also performed and showed no significant density difference of the foveal cones. Rod density difference measured at a locus 16 degrees in the temporal retina was much lower than normal with an increased time constant of rhodopsin regeneration. Ten weeks after the onset of the disease, no white dot lesions were visible on funduscopic examination. Rod density difference and regeneration time had become normal again, but with scanning laser densitometry the abnormal areas of no pigment were still faintly visible. It is concluded that these findings, completed with data of electroretinography, anomaloscopic testing, and perimetry, are in agreement with a metabolic disturbance at the level of the retinal pigment epithelium-photoreceptor complex.

Acute Disease↗
PubMed1991

Rod densitometry in the aging human eye.

Retinal densitometry is a noninvasive physiologic technique used to examine the visual pigments in living human eyes. To assess possible age-related disturbances of rod photopigment kinetics, retinal densitometry was done in 44 eyes of 44 healthy subjects (age range, 12-78 yr). With progressing age, a significant but small increase in photopigment density difference (bleached versus dark adapted eye) and an increase in the time constant of rhodopsin regeneration was found. The increased density difference in rods was consistent with morphologic findings of increased rod outer segment diameter and disc content in older subjects. To explain this change in terms of the decreased specular reflections at the level of the inner limiting membrane was inadequate because age effects were independent of wavelength in the region of 450-550 nm. To control for the effects of ocular stray light from the lens, subjects older than 40 yr with a clear crystalline lens were measured and compared with those with pseudophakia. No statistically significant difference was found between the two groups. Increased rod density difference contrasts sharply with an earlier reported decrease in this parameter for foveal cones. The slowing of the regeneration rate is a phenomenon common to rods and cones. It may be a result of a gradual metabolic dysfunction of the retinal pigment epithelium in older subjects.

Adolescent↗
PubMed1989

Retinal densitometer with the size of a fundus camera.

This paper describes a small, user-friendly fundus reflection densitometer. All optics and part of the electronics are contained in a box with the size of a fundus camera. A personal computer is used for control and on-line display of output. A single 30 W halogen lamp provides bleaching and measuring light. A chopper wheel generates 24 light pulses in 100 msec time frames: 16 pulses of measuring light at different wavelengths covering the spectrum, four pulses of bleaching light (optionally), and four dark pulses for assessing the dark current of the photomultiplier. The fundus can be viewed when the bleaching light is on. The measuring field has four widths ranging from 1.6 to 5.4 deg; the bleaching light is fixed at 25 deg. A fixation aid may be positioned anywhere in the bleaching field. A microprocessor sorts the quanta, detected by the photomultiplier after reflection from the fundus, in 16 channels labeled with wavelength information. Real-time changes in spectral reflection can be viewed on a monitor. Due to optimal design of entrance and exit pupils foveal density differences of up to 0.5 were recorded in human subjects. This is higher than ever reported before with retinal densitometry.

Adult↗
PubMed1989

Imaging retinal densitometry with a confocal Scanning Laser Ophthalmoscope.

We describe a novel use of the Scanning Laser Ophthalmoscope (SLO), viz. as an imaging retinal densitometer. In our SLO a helium-neon or an argon laser beam is moved in a raster pattern over the retina; the reflected light is descanned (confocal SLO) and collected by a photomultiplier. Images of the fundus subtending 22 by 18 deg are displayed on a TV monitor. Single frames taken with 514 nm light were stored in a computer in arrays of 256 by 256 pixels and density differences between dark adapted and bleached images were calculated. With a full bleach density differences of about 0.35 were found in the center of the fovea; at retinal eccentricities of 15-20 deg we found 0.15. After selective bleaching with 633 nm light substantial density differences were only seen in the foveal area. We conclude that the confocal SLO is a very suitable instrument for imaging fundus reflectometry.

Densitometry↗
PubMed1984

User friendly system for electrodiagnosis.

A laboratory built computer system for clinical electrophysiology of vision is described to illustrate how modern technology may ease the task of handling otherwise rather complicated electronic equipment and of elaborating the data obtained. An example is given of a procedure for clinical electroretinography in which the task of the operator is virtually confined to using a single command letter, which is specified in continuously updated instructions displayed on a monitor. After the patient has left, the operator checks and if necessary corrects the automatically defined maxima of a and b waves of the electroretinogram. The print-out consists of graphs of amplitude and latency versus stimulus intensity in which the normal range is indicated. The task of the operator during an EOG measurement is confined to checking the collaboration of the patient, since the complete procedure including the elaboration of the data is automated.

Computers↗