Dirk van Norren

Spectral transmission of the human ocular media.

Abstract The existing literature on transmission of the human ocular media in the visible region is reviewed. Added is an ocular density curve derived from the CIE scotopic sensitivity function and the absorption curve of human rhodopsin. This ocular density curve fits the literature data very well and can serve as the transmission curve of a standard observer. The extent of individual differences in ocular density is calculated to be ±25 per cent of the average density values.

Light adaptation of primate cones: An analysis based on extracellular data

Extracellular photo-cone responses were isolated in the intact rhesus monkey eye by fractional recording across the outersegment layer in the fovea with a bipolar microelectrode. The cone response vs intensity function was determined in the presence of adapting backgrounds up to 10(6) td. Increment and decrement responses, as well as the response to the steady backgrounds were recorded. All steady state and transient response data could be described with the equation V/Vm = In/(In + sigma n) with n = 0.74 and V and I representing total response and total incident light intensity. This invariant response function is shifted both along the intensity and the response axis with increasing background intensity. The decrease in sensitivity, corresponding to these shifts, could be attributed to cellular adaptation (sigma-adaptation), pigment bleaching and response compression. An analysis of the threshold vs intensity function shows how each of these mechanisms contributes to produce Weber behaviour.

Origin of the oscillatory potentials in the primate retina.

The current source and sink profiles of the oscillatory potentials (OPs) of the primate (macaque) electroretinogram (ERG) were studied. The intraretinal local ERG (LERG) and the local resistivity were recorded with a bipolar microelectrode in the intact macaque eye in response to a 10 deg and full field stimulus. A statistical method, a principle component analysis, was performed on high pass digitally filtered LERGs to extract and more closely study the oscillatory components of the LERGs at a number of electrode depths in the retina. The OPs were found to represent radial current flow and to have a current distribution different from those of the receptor and b-wave components. They are generated more proximal than the a- and b-waves, most probably by the bipolar cells. However, the interplexiform cell cannot be excluded as a generator. If so, the OPs should represent activity in a feed-back neural pathway in the inner part of the retina, also in the primate. The OPs can be used to monitor the inner neuronal layer but further differentiation in depth regarding consecutive peaks is not possible.

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.

Optical density of the aging human ocular media in the visible and the UV

We analyzed the literature on the absorption in the young and aging human eye media. Five templates were derived to provide an adequate description of the spectra from 300 to 700 nm for the lens, cornea, aqueous, and vitreous. Two templates were found in all media. They stand for Rayleigh scatter and the absorbance of tryptophan. Three additional templates for the lens represent absorbance in kynurenine derivatives, such as 3-hydroxykynurenine glucoside (3HKG), and absorbance in two substances found at older age. Except for Rayleigh scatter, all templates have a Gaussian shape. Aging-trend functions were derived that show a linear slope on an age-squared scale. The result can be used to correct for media losses in visual perception tasks, in fundus reflectometry, and in studies on light damage.

Fundus reflectance: historical and present ideas

In 1851 Helmholtz introduced the ophthalmoscope. The instrument allowed the observation of light reflected at the fundus. The development of this device was one of the major advancements in ophthalmology. Yet ophthalmoscopy allows only qualitative observation of the eye. Since 1950 attempts were made to address the challenging, quantitative assessment of the amount of light reflected by the fundus. At first, only comparative measurements were possible, applied in the study of macular and visual pigments. With improvements in light detecting techniques, and with the advent of microprocessors, the measurement of spectral and spatial distribution of the reflectance became feasible. This led to the development of models that explained the observed wavelength dependence and the directional behavior of light reflected from the fovea. The models allowed a quantitative assessment of many parameters on absorption and reflection by structures in the human eye. This paper provides a review of both the experimental and theoretical progress, and summarizes the results of fundamental and clinical research using fundus reflectometry.

Origin of the electroretinogram in the intact macaque eye—II: Current source-density analysis

Local electroretinograms (LERGs), evoked by 400 msec flashes of white light, were recorded as a function of retinal depth with a microelectrode in the intact eye of a macaque monkey. Retinal resistivity was measured as the potential difference between the tips of a bipolar microelectrode, resulting from a 300 msec current pulse which was passed through the retina. From the light evoked potentials and the resistivity the local currents in the retina were calculated. Current sources and sinks were localised for three LERG components. The receptor component had a current source in the outer segments (90% depth) and a sink in the ONL (70% depth). Both the b-wave and the d.c.-component had a source around 40% depth and a sink around 70% depth. The distribution of b-wave sources and sinks in primate seems to deviate from that in lower species, since no large current source is found in the most proximal layers.

Human and macaque blue cones studied with electroretinography.

Abstract The spectral sensitivity of the human and macaque blue cone system was measured using an electroretinographic criterion response. It was necessary to take very low criteria (about 1 μV) since the blue system was proved to show a low maximum response. The human and macaque blue cone curve have distinctly different peak wavelengths of 432 and 440 nm respectively. The waveform of the blue system response to 10 msec flashes appeared very similar to that of the long wavelength mechanisms, although a 5–10 msec longer latency was observed. At higher adaptive states the blue and red-green responses showed distinct differences.

A continuously recording retinal densitometer

Abstract A densitometer is described which enables continuous monitoring of the radiances of three different light beams reflected from the fundus of the eye. The optical system is designed such that within a 30 msec period a deep red reference light, a measuring light, a bleaching light and a dark period are sequentially presented to the eye. A microprocessor based electronic system counts, sorts and averages the output pulses of the photomultiplier that receives the reflected light. A theoretical relationship is derived which describes the fluctuations in the output of the densitometer as a function of the radiance of the measuring light. The uncertainty of measurements with an artificial eye is about 0.002 density units. The maximum change in measured density between a fully bleached and a fully dark adapted retina is for most subjects between 0.25 and 0.40. Twenty measurements on one subject show a value of 0.295 ± 0.021. These density changes are nearly twice as high as those hitherto reported in the literature and are attributable to optimization of the optical system. We discuss the relation between observed density changes and changes in the visual photopigments and argue that either the amount of intra ocular stray light in our measurements must be low or the time constant of cone regeneration must be much longer than 120 sec.

Origin of the electroretinogram in the intact macaque eye—I: Principal component analysis

Responses to 400 msec flashes of white light were recorded at various depths in the retina of the intact macaque eye. A statistical technique, principal component analysis (PCA), was used to isolate independent components from the LERG, using the changing contribution of a cells response to the LERG with retinal depth. Two approaches were evaluated: first, PCA was performed on the complete LERG waveforms and, second, on small sections of the LERG. The first method yielded a component strongly resembling the receptor response, apart from some inconsistencies in the proximal layers, and a component in which the b-wave was the most prominent feature, but which still contained some other responses. Hence, the components were not suitable to describe the LERG in terms of responses of specific cell types. The second method uses PCA to determine whether or not the potential change within a small time window is accounted for by a single component. This method, which essentially uses the knowledge that different cell types respond with different delay times, yielded three components. These were identified as the receptor component, the b-wave and the d.c.-component. The voltage-depth profiles of these components were assessed.