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

The Macular Assessment Profile test - a new VDU-based technique for measuring the spatial distribution of the macular pigment, lens density and rapid flicker sensitivity

The measurement of macular pigment optical density (MPOD) in the eye is often carried out using optical techniques based on heterochromatic flicker photometry (HFP). These require the use of two spectrally-narrow beams, one at the wavelength of maximum absorption of the macular pigment (MP) and the other in the long wavelength region of the visible spectrum where MP absorption is negligible. A new technique for the measurement of MPOD spatial profiles has been developed by overcoming the current shortcomings associated with the use of visual displays. The new Macular Assessment Profile (MAP) test makes use of a 'notch' filter and a photometric model to measure and compute the peak MPOD value. Two other useful parameters are also computed from the same measurements. These describe the subject's sensitivity to rapid flicker and the absorption of blue light by the lens. MPOD profiles, lens density, rapid flicker sensitivity, and red/green (RG) and yellow/blue (YB) colour thresholds were measured in 54 normal subjects aged 18-61 years. The results confirm previous findings on ageing effects and demonstrate the complete absence of correlation between MPOD and the subject's YB chromatic thresholds. In contrast, RG chromatic sensitivity improves with higher levels of MPOD.

Directional model analysis of the spectral reflection from the fovea and para-fovea

Directional and nondirectional spectral reflection data from 0, 1, 2, 4, and 8 deg eccentricity, and the optic disk, were analyzed from 400 to 950 nm with an existing optical reflection model. The optical model, developed for the fovea, appeared to be also suitable for more eccentric locations. The optical densities of melanin and of the macular pigments zeaxanthin and lutein peaked in the fovea, in correspondence with literature data. The amplitude of the directional component, originating in the cone photoreceptors, had its maximum at 1 deg. The maximum of the directionality (peakedness) occurred at a slightly higher eccentricity.

In vivo quantification of the retinal reflectance spectral composition in elderly subjects before and after cataract surgery: Implications for the non-visual effects of light

Light is the signal that entrains the biological clock in humans to the 24-hour external time. Recently, it has been shown that short wavelengths play a key role in this process. In the present study, we describe a procedure to measure, objectively and in a quick way, the spectral composition of the light reaching the retina in vivo. The instruments involved are the foveal reflection analyzer (FRA) and the macular pigment reflectometer (MPR). By making use of these reflectometers, we show quantitatively that in subjects with cataracts, the light input is especially reduced in the short wavelength range. After cataract surgery during which the crystalline lens is replaced by a transparent artificial lens, the transmittance of the short wavelengths (between 420-500 nm) improved on average by a factor of 4. We conclude that this technique holds great promises for the chronobiological field because it allows for quantification of the spectral composition and light levels reaching the retina in vivo.

Essentials of photometry for clinical electrophysiology of vision

Electrophysiological testing of the visual system requires familiarity with photometry. This technical note outlines the concepts of photometry with a focus on information relevant to clinical ERG and VEP testing. Topics include photometric quantities, consideration of pupil size, specification of brief extended flash stimuli, and the influence of the spectral composition of visual stimuli. Standard units and terms are explained in the context of the ISCEV standards and guidelines for clinical electrophysiology of vision.

Design of a trichromatic cone array

Cones with peak sensitivity to light at long (L), medium (M) and short (S) wavelengths are unequal in number on the human retina: S cones are rare (<10%) while increasing in fraction from center to periphery, and the L/M cone proportions are highly variable between individuals. What optical properties of the eye, and statistical properties of natural scenes, might drive this organization? We found that the spatial-chromatic structure of natural scenes was largely symmetric between the L, M and S sensitivity bands. Given this symmetry, short wavelength attenuation by ocular media gave L/M cones a modest signal-to-noise advantage, which was amplified, especially in the denser central retina, by long-wavelength accommodation of the lens. Meanwhile, total information represented by the cone mosaic remained relatively insensitive to L/M proportions. Thus, the observed cone array design along with a long-wavelength accommodated lens provides a selective advantage: it is maximally informative.

Human color perception arises by comparing the signals from cones with peak sensitivities, at long (L), medium (M) and short (S) wavelengths. In dichromats, a characteristic distribution of S and M cones supports blue-yellow color vision: a few S and mostly M. When L cones are added, allowing red-green color vision, the S proportion remains low, increasing slowly with increasing retinal eccentricity, but the L/M proportion can vary 5-fold without affecting red-green color perception. We offer a unified explanation of these striking facts. First, we find that the spatial-chromatic statistics of natural scenes are largely symmetric between the L, M and S sensitivity bands. Thus, attenuation of blue light in the optical media, and chromatic aberration after long-wavelength accommodation of the lens, can give L/M cones an advantage. Quantitatively, information transmission by the cone array is maximized when the S proportion is low but increasing slowly with retinal eccentricity, accompanied by a lens accommodated to red light. After including blur by the lens, the optimum depends weakly on the red/green ratio, allowing large variations without loss of function. This explains the basic layout of the cone mosaic: for the resources invested, the organization maximizes information.

Design of a light stimulator for fetal and neonatal magnetoencephalography

The design, safety analysis and performance of a fetal visual stimulation system suitable for fetal and neonatal magnetoencephalography studies are presented. The issue of fetal, neonatal and maternal safety is considered and the maximum permissible exposure is computed for the maternal skin and the adult eye. The risk for neonatal eye exposure is examined. It is demonstrated that the fetus, neonate and mother are not at risk.

A study of unusual Rayleigh matches in deutan deficiency

Rayleigh match data were modeled with the aim of explaining the locations of match midpoints and matching ranges, both in normal trichromats and in subjects with congenital color deficiency. Model parameters included the wavelength of peak sensitivity of cone photopigments, the effective photopigment optical density, and the noise amplitude in the red-green color channel. In order to avoid the suprathreshold, perceptual effects of extreme L:M cone ratios on color vision, selective post-receptoral amplification of cone signals is needed. The associated noise is also amplified and this causes corresponding changes in red-green threshold sensitivity. We propose that the noise amplitude and hence the size of the matching range in normal trichromats relates to the known inter-subject variation in the relative numbers of L and M cones. If this hypothesis can be shown to account for the extremes of the red-green matching range measured in normal trichromats, it is of interest to establish the extent to which it also predicts the unexpected, small matching ranges that are observed in some subjects with red-green color deficiency. A subset of subjects with deutan deficiency that exhibited less common Nagel matches were selected for genetic analysis of their cone pigment genes in order to confirm the type of deficiency, and to predict the corresponding peak wavelength separation (delta lambda(max)) of their two, long-wavelength cone pigments. The Rayleigh match model predicted accurately the midpoint and the range for the spectral differences specified by the genes. The prediction also required plausible selection of effective optical density of the cone pigments and noise. The noise needed varied, but the estimates were confined to lie within the limits established from the matching ranges measured in normal trichromats. The model predicts correctly the small matching ranges measured in some deuteranomalous subjects, principally accounted for by a low estimate of noise level in the red-green channel. The model also predicts the "normal" matches made by some subjects that rely on two hybrid genes and therefore exhibit red-green thresholds outside the normal range, typical of mild deuteranomaly.

The effect of senescence on orientation discrimination and mechanism tuning

Accurately processing orientation information is a fundamental component of visual performance. Single-unit recordings have shown that the orientation tuning of individual neurons in macaque cortical areas V1 and V2 is reduced dramatically with age (M. T. Schmolesky, Y. Wang, M. Pu, & A. G. Leventhal, 2000; S. Yu, Y. Wang, X. Li, Y. Zhou, & A. G. Leventhal, 2006). These researchers suggest that losses in single-unit orientation selectivity result in declines in orientation discrimination and object recognition in older humans. Three experiments were conducted to determine whether human performance is affected by putative age-related changes in tuning of cortical neurons. Ten younger and ten older observers participated in this study. Experiment 1 demonstrated significant differences in the contrast sensitivity of the two age groups. Experiment 2 showed significant differences in orientation discrimination thresholds. However, when thresholds were plotted in terms of multiples of detection threshold, age-related differences were not observed. In Experiment 3, perceptual orientation tuning curves did not significantly differ in shape for younger and older subjects. As in Experiment 2, at any given contrast, there is a large difference in sensitivity between younger and older adults. This implies a model of orientation processing that allows the adult visual system to maintain consistent and reliable orientation information at the network and ultimately the perceptual level.

Directional and nondirectional spectral reflection from the human fovea

A model of the directional and nondirectional reflection spectrum of the human fovea is developed, incorporating reflectors, absorbers, and a wavelength-dependent optical Stiles-Crawford effect (OSCE). Data from 102 healthy subjects between 18 and 75 years obtained with the fundus reflection analyzer (FRA), an imaging spectrograph that measures the directional reflection profile of the human fovea in the pupil plane from 400 to 950 nm, were analyzed. Subgroups of young (<40 years) and old (>50 years) observers were defined. Mean results of the young group defined a template for directionality versus wavelength. For the whole group, mean reflection at 550 nm from the cones was 2.12%, from the retinal pigment epithelium 0.56%, and from the choroid 7.92%. Lens density, cone disc reflection, and blood layer thickness showed significant trends versus age. The model for the first time simultaneously describes the spectra of the directional and nondirectional reflection of the human fovea. Rayleigh scatter losses of the media and in preretinal layers were assumed zero in the nondirectional pathway. Mean density of the macular pigment of a subgroup (53 subjects, 19 to 75 years) correlated significantly with independent data from reflectance and autoflourescence images obtained by scanning laser ophthalmoscope (SLO) and data from flicker photometry.

Spectral transmission of intraocular lenses expressed as a virtual age

AIM

Currently available intraocular lenses (IOL) have widely different spectral filters. This study aimed to calculate the virtual age of IOL with regard to photoprotection and photoreception, i.e. the age of the natural lens that has similar effects on these aspects.

METHODS

With diffuse solar radiation as a light source blue light damage was calculated for natural lenses at all ages, commercially available IOL and Schott steep cut-off filters in the wavelength range 300-600 nm. Similarly, the input to the short wavelength sensitive cone system was calculated for the range 380-600 nm.

RESULTS

The virtual age for photoprotection of IOL and steep cut-off filters varied from under 0 to 66 years. Most IOL had similar ages for photoreception, and thus show a reasonable resemblance to the spectral properties of the natural lens. Two IOL and all steep cut-off filters had a lower age for photoreception than for photoprotection, and thus outperformed the natural lens.

CONCLUSION

The virtual age of an IOL relates its spectral filtering properties to what happens in the healthy crystalline lens. Many older IOL types offer less protection than the lens of a newborn. Middle age seems a reasonable choice for an IOL.