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The Relationship Between Visual Performance and Macular Pigment in Non-Diseased Eyes

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The Relationship Between Visual Performance and Macular Pigment in Non-Diseased Eyes ARTICLE The Relationship between Visual Performance and Macular Pigment in Non-diseased Eyes BY K MEAGHER, JM NOLAN, S BEATTY ision is one of the fundamental hand, refers to the ability of the visual the sensitivity of the brain to receive them. tools with which we interact system to discern the foreground from the Rod cells, of which there are with the world around us, and background, and is measured at a variety of approximately 90 million in the retina, good vision allows us to enjoy spatial frequencies (target sizes), and rests contain an opsin called rhodopsin, which is so many aspects of life to the on the ability of the visual system to discern activated under conditions of low levels of full. Ophthalmologists, by virtue of their objects of differing luminance within our incident light (as little as six photons), and Vprofession, are only too aware of the threat field of view. For example, we can easily these cells are therefore useful for low light to vision that ophthalmic pathology can distinguish a medium-sized object at low conditions or nightvision. Cones are represent. As a consequence, however, the contrast (i.e. a grey object in front of a white necessary for fine detailed vision, colour variability in visual performance and background), yet smaller-sized objects perception and visual resolution (acuity), experience in the normal eye has been require a greater degree of contrast with the and require greater amounts of incident light necessarily overlooked, as have measures to background if they are to be perceived. to be activated.The three types of cones, optimise vision in the absence of ocular Contrast sensitivity declines with known as S, M and L cones, contain pathology. increasing age [1,2] and poor contrast differing opsins, which alter the spectral The variability in visual performance in the sensitivity is associated with reduced absorption properties of the photopigments. non-diseased eye is largely attributable to function related to vision, and this is reflected S-cone cells are activated bythe shorter differences in visual perception which are in difficulty with everyday tasks such as ‘blue’ wavelengths (approximately <470nm) subject to the respective and interactive reading, distinguishing traffic lights and road and are thus blue light sensitive, while M- influences of environment, anatomy and age. signage. Of note, CS, as a measure of visual cone cells are activated by medium The optical (blue light filtering), anatomic performance, is more reflective of a subject’s wavelengths (<530nm) and are green (central retinal) and biochemical (antioxidant) visual functioning than is VA, whether in a sensitive. The final type, L-cone cells, are properties of macular pigment (MP) are ideal diseased or in a healthy eye [3]. activated by the longer wavelengths of the to optimise vision in the normal eye and to visual spectrum (<560nm), thereby maintain effective physiological functionality The neurophysiological process of explaining their constituent photopigments’ of the visual system into old age. vision sensitivity to red. MP is composed of three carotenoids, The visual experience is created through a Beyond each cone’s primary colour lutein (L), zeaxanthin (Z) and meso - wide array of neurobiological processes. sensitivity, or spectrum absorption range, all zeaxanthin (MZ), to the exclusion of all other Photoreceptors in the inner layers of the three cone types are partially sensitive to the carotenoids in the diet and serum. MZ is the retina, known as rod and cone cells, contain other two spectral ranges. This allows dominant carotenoid in the epicentre of the photopigments within their cellular ganglion cells to compare the stimulation of macula, while Z and L are the dominant membranes, which convert the incident light the cones, which results in the visual carotenoids in the mid-periphery and (visible electromagnetic radiation) into perception of a wide spectrum of colours periphery of the macula, respectively. ‘electrical’ signals. Rod and cone (and not solely red, blue and green). photopigments are similar, in that they Measures of visual function consist of retinene bound (with both ends of Photoreceptor cell degradation / Visual function, and the experience it results the molecule) to retinal proteins, or opsins. death in for the subject, is a composite of a wide In order to produce a signal, the retinene The ultra-structural cellular composition of array of optical and neurophysiological must be activated by incident light, which photoreceptors, required for the necessarily processes, which are the result of an converts the retinene from 11-cis-retinal to sensitive nature of both rod and cone cells, exquisitely specialised and evolved visual 11-trans-retinal, detaching one end of the confers a susceptibility to oxidatively- system, and which can be reflected in a retinene molecule from the opsin, thus induced cell death. Reactive oxygen species plethora of psychophysical measures of initiating a nerve pulse or signal. The (ROS) are the inevitable by-products of visual performance, including object retinene then reverts to 11-cis-retinal and oxygen metabolism, and are therefore recognition, colour discrimination and depth re-binds to the opsin, to begin over anew. generated in large quantities in the human perception (to name but a few). These signals are compared and processed retina, a tissue which has the highest Visual acuity (VA), for example, is the by retinal ganglion cells and transmitted to oxygen metabolism in the mammalian world most commonly employed tool to assess the occipital cortex via the optic nerve, [4]. However, ROS production in tissue is spatial vision by eye care professionals, and where they are interpreted and converted increased further when irradiated with light, is a measure of the resolving power of the into a visual experience. Visual resolution especially short wavelength (high energy) eye at 100% contrast. (acuity) is a measure that is a direct light such as visible blue light. Indeed, the Contrast sensitivity (CS), on the other correlate of the strength of these signals and threshold for retinal injury is one thousand ARTICLE times lower for blue light than for orange resulting in a bluish tinge or edge to an light (thereby limiting light-induced light under ambient conditions [4]. image, a phenomenon known as CA andone generation of ROS) and MP’s powerful ROS- Furthermore, the photoreceptor outer which adversely impacts CS. neutralising capacity, suggest that it is these segment membranes contain MP’s pre-receptoral filtration of blue light properties that limit age-related degradation polyunsaturated fatty acids (PUFAs) in very attenuates CA by reducing the amount of of photoreceptors, with a consequential and high concentrations, which, because of their defocused blue light incident upon the retina, parallel preservation of visual function into electron-dense molecular structures, are thereby optimising CS and enhancing visual old age [14]. susceptible to oxidation. As a result, a performance and experience [6,8]. Again, cytotoxic chain reaction ensues, thereby optimisation of visual function, in terms of Conclusion damaging the photoreceptors by depletion of CS under photopic and mesopic conditions, High levels of MP attenuate the adverse membrane PUFAs, in addition to is best achieved following supplementation impact of CA and light scatter on visual perpetuating the production of ROS. with a formulation containing all three of performance, because of the optical Reactive oxygen species, because of their MP’s constituent carotenoids (L, Z and MZ; properties of the pigment’s constituent inherent instability (free radicals, for Macushield™) [7]. carotenoids. Further, the biochemical example, contain one unpaired electron) and properties of L, Z and MZ are important for regardless of whether they are generated as Non-optical visual benefits of optimal physiological functionality of the a result of high oxygen metabolism or macular pigment neurobiological components of the visual irradiation with blue light, damage DNA, system, and appear to be important in the lipids, proteins and other important Neural efficiency maintenance of this system into old age. components essential for cellular functions. The functionality of the macular carotenoids is not limited to pre-receptoral light filtration Optical visual benefits of macular or anti-oxidant capacity, nor are these pigment compounds located exclusively in retinal Take home message tissue. Research has shown that L and Z – Augmentation of macular pigment Glare accumulate in the occipital and frontal (MP), and optimisation of its spatial Clinically, there are two types of glare, cortices, where they are the predominant profile, is best achieved following referred to as glare discomfort and glare xanthophylls [9], and where they are supplementation with a formulation that contains all three of MP’s disability. Discomfort glare is caused by thought to play an important role on gap constituent carotenoids (L, Z, and intense environmental light sources that junction communication [10] and in the MZ; Macushield ™). cause distraction and / or discomfort, such maintenance of neuronal membrane − L, Z and MZ are powerful as headlights of oncoming cars. integrity, thereby contributing to the antioxidants, and because they exert Glare disability is the result of light scatter, conditions required to optimise physiological this effect synergistically, the where the incident light is scattered by functionality of neural
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