2. Pathological conditions

#### 2.1. Corneal diseases

The major function of the cornea is to direct and refract light to the retina as well as provide structural support to the eyeball. Thus, preserving transparency and corneal shape is highly important in visual function [4, 5]. In various corneal diseases, the cornea is damaged through inflammation, swelling, and dystrophy [6]. The transparency of the cornea is the function of tight controls on water content, diameter of the collagen fibrils, and the spacing between the fibrils. The collagen fibrils have a diameter of 27–35 nm and the distances between fibrils are 41.4–60 nm [7]. The precise pattern of the collagen fibrils enables efficient light transmittance with minimal scattering or absorbance in a healthy eye. Any increase or decrease in the distance between the fibrils will compromise the transmittance of light [7].

Corneal edema is one example of a condition that disrupts the uniformity of these fibrils. The increased water content that results in edema changes the distance between fibrils, and thus can affect the overall transparency of the cornea. Reduced transparency, as we know, induces scattering when light enters the eye. Furthermore, scarring of cornea or deposits in the cornea can lead to the scattering of light as well. Post-surgical scarring is known to decrease vision and increase glare [8]. Additionally, certain medications like amiodarone causes cornea verticillata or deposits in the cornea that leads to the scattering of light rays [9].

Moreover, the type of light scatter that occurs can either be backwards or forward light scatter, depending on the angle of deviation light enters the eye. In backward light scatter, the scattering of light causes less light to reach the retina. While in forward light scatter, the scattering of light causes a luminance over the retinal image.

Reduced transparency that leads to increased reflection and scatter of light can potentially cause disability glare. The disability glare along with diffraction and high-order aberration attribute to distorted retinal image, and thus impaired visual function. Components of vision such as contrast sensitivity can be hampered and if scattering is severe can lead to a deficit in visual acuity [4]. Therefore, those with corneal aberrations and abnormalities experience intensified forms of disability glare as well as reduced contrast sensitivity and visual acuity.

Keratoconus is a corneal dystrophy that leads to the progressive thinning of the center of cornea. Corneal thinning causes the center to protrude outward resulting in a cone shape cornea. Those with keratoconus can experience blurred vision as well as sensitivity to light [6]. Being reactive to light can make individuals with this corneal disease vulnerable to disability glare. Jinabhi and colleagues surveyed forward light scatter and visual function in subjects with mild to moderate keratoconus with no corneal scarring or history of ocular surgeries [10]. In the study, keratoconic and normal ocular healthy subjects underwent contrast sensitivity testing and glare testing to evaluate their visual function. The subjects with keratoconus exhibited lower contrast sensitivity than normal ocular subjects in testing. These results agreed with previous studies done and suggested contrast sensitivity was commonly compromised in keratoconus. Furthermore, keratoconic subjects also presented with intraocular scatter that resembled the increased scattering found in older populations or to those with early cataracts. Greater light scatter makes an individual with keratoconus more susceptible to disability glare [10]. More evidence of glare sensitivity in keratoconus could be found in a study done by Mäntyjärvi and Latinen. These researchers measured contrast sensitivity under glare conditions in keratoconic and ocular healthy subjects. The Pelli-Robson chart was used to measure contrast sensitivity. The chart contained letters of decreasing contrast that provided a quick and accessible way to measure contrast sensitivity [11]. The subjects were asked to read the Pelli-Robson chart under glare illuminance provided by the BAT. Then contrast sensitivity performance with and without glare was compared. The results of the comparison demonstrated that subjects with keratoconus experienced greater contrast sensitivity loss when tested under glare conditions than normal subjects [10]. Visual impairments being significantly greater in keratoconic subjects advocates the need for disability glare testing in measuring visual function. Disability glare performance can distinguish between normal individuals and those with ocular pathologies. Thus, in the case of corneal disease, disability glare can be a helpful diagnostic tool and could be a potential method of monitoring the disease progression. NEI VFQ (REF) or similar survey techniques can be used in conjunction to assist in evaluation of quality of vision and may be used in assessing glare related problems (Figure 1).

#### 2.2. Glaucoma

Light is focused to the retina to receive visual information of the world around us. Thus, the transmittance of light is integral to how we visually function. To this accord, the human visual system is finely tuned to allow the maximum amount of light transmission to the retina with least scatter. The retinal anatomy is also tuned to decreased sensitivity to shorter wavelength light and the retinal pigment epithelium and macular pigment allows the absorbance of stray light. However, disability glare interrupts the direction of light to the eye thereby interfering with the way we see [2]. This is especially debilitating, and the effects of glare are worsened in those who suffer from ocular pathologies. The many layers and components of the eye is involved in directing and processing light and cues to interpret our surrounding. Thus, a disease that impacts any part of the eye can exasperate disability glare decreasing the ability

The impact of disability glare makes it an important visual function to measure. However, currently there is no standardized way to measure glare [3]. There are both commercial and self-made device that hope to address this problem. However, more evaluation will be necessary to solidify their validity for research and clinical use. As a result, much of disability glare

The major function of the cornea is to direct and refract light to the retina as well as provide structural support to the eyeball. Thus, preserving transparency and corneal shape is highly important in visual function [4, 5]. In various corneal diseases, the cornea is damaged through inflammation, swelling, and dystrophy [6]. The transparency of the cornea is the function of tight controls on water content, diameter of the collagen fibrils, and the spacing between the fibrils. The collagen fibrils have a diameter of 27–35 nm and the distances between fibrils are 41.4–60 nm [7]. The precise pattern of the collagen fibrils enables efficient light transmittance with minimal scattering or absorbance in a healthy eye. Any increase or decrease in the

Corneal edema is one example of a condition that disrupts the uniformity of these fibrils. The increased water content that results in edema changes the distance between fibrils, and thus can affect the overall transparency of the cornea. Reduced transparency, as we know, induces scattering when light enters the eye. Furthermore, scarring of cornea or deposits in the cornea can lead to the scattering of light as well. Post-surgical scarring is known to decrease vision and increase glare [8]. Additionally, certain medications like amiodarone causes cornea

Moreover, the type of light scatter that occurs can either be backwards or forward light scatter, depending on the angle of deviation light enters the eye. In backward light scatter, the scattering of light causes less light to reach the retina. While in forward light scatter, the scattering of

distance between the fibrils will compromise the transmittance of light [7].

verticillata or deposits in the cornea that leads to the scattering of light rays [9].

light causes a luminance over the retinal image.

to see and perform daily activities such as driving.

58 Causes and Coping with Visual Impairment and Blindness

in visual function and pathology is still under research.

2. Pathological conditions

2.1. Corneal diseases

Glaucoma is globally the second most common cause of blindness and it affects over – millions worldwide and is a very large socio-economic burden to the health care system [12]. The risk of glaucoma increases with increase age and elevated intraocular pressure is a major risk factor in glaucoma. Lowering intraocular pressure remains the only proven alterable risk factor that has shown to slow down the disease progression. Although, the exact pathogenesis in glaucoma remains to be identified, glaucoma leads to progressive damage to the to the optic nerve fiber

that disability glare had one of the strongest relationship with the severity of visual field loss [14]. This relationship suggest that progression of glaucoma will be likely accompanied by increasing disability glare. Furthermore, the outcomes of this study affirm disability glare as a concerning visual impairment of glaucoma. In addition, the observed correlation between disability glare and visual field loss can potentially explain the components of the visual system that is involved in glare tolerance. This can in turn further the understanding of overall

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As it is apparent that glaucoma patients suffer from disability glare, they found this impairment most concerning when driving. In surveying the value of various activities, glaucoma patients rate driving as highly important to maintaining their independence [15]. And so, understanding the impairments glaucoma patients face when driving is essential to addressing the concerns and preserving the quality of life for these individuals. Janz and colleagues surveyed open-angle glaucoma drivers and non-drivers about the types of visual problems they encounter during driving at a 6-month and a 54-month period. These surveys were also accompanied by ophthalmologic examinations. From the surveys and examinations, increasing visual field loss accounted for the differences between subjects who stayed drivers and subjects who became nondrivers because of their declining vision [15]. Thus, it can be inferred that those who are still drivers only had mild to moderate visual field loss. Despite little visual field loss, those drivers still reported many visual complications. One of the highest complaints from the drivers were tasks involving glare, which was said to be more troubling than visual search, peripheral vision, or visual processing speed which showed a lot of variation. Glare was a consistent issue among glaucoma drivers. Furthermore, glare was noted as one of the first issues subjects recognized when they first began to struggle with driving [15]. The study presents the driving challenges faced by glaucoma patients due to their sensitivity to disability glare. As mentioned earlier, driving is deemed as an important task to glaucoma patients to sustain autonomy. Therefore, assessing and managing disability glare is imperative to treating the visual impairments experienced by these individuals. Furthermore, since glare is one of the first detectable visual problems, disability glare test can potentially be utilized as a tool to identify the progression or worsening of a glaucoma in a patient. Though it is important to note that in the current state, it may be able to identify progression of the disease but may not give idea of the localization of the retinal damage in this disease. It will be interesting to evaluate the glare tolerance in various quadrants to see if the quantification of glare in specific

locations is more sensitive than the non-specific glare tolerance testing.

The lens is a specialized structure that relies on its transparency, high refractive index, and curved surface to project clear images to the retina. Most of the lens comprises of concentric elongated fibers covered with an epithelium on its anterior surface. The epithelium along with the superficial fiber cells secrete an elastic extracellular matrix that encases the lens in what is known as the capsule [1]. Below the capsule, at the equator of the epithelium is where new fiber cells arise and differentiate [2]. The newer fiber cells constitute the periphery of the lens, named the cortex [1]. While the center of the lens is comprised of older fiber cells, some

visual function.

2.3. Cataracts

Figure 1. An optical coherence tomography image from a patient with early age-related macular degeneration. The drusen bodies are visible in the retinal pigment epithelium.

layer and changes in visual field that is in part associated to the level of intraocular pressure. If left unmanaged, glaucoma leads to progressive vision loss and blindness [12, 13].

Glaucoma affects several aspects of an individual's daily activities and task. Nelson and colleagues had articulated five major areas of difficulties in individuals with glaucoma. These difficulties include: (1) near vision issues, (2) peripheral vision issues, (3) dark adaptation and glare, (4) personal care and (5) household tasks, and outdoor mobility [14]. Their study measured both visual function and self-reported visual impairments. Subjects underwent multiple functional vision tests to assess the full spectrum of their visual capacity. The tests carried out included: Humphrey Visual Field Analyzer for visual field, Critical Flicker Frequency, Brightness Acuity Test (BAT) for disability glare, Goldmann-Weekers Dark Adaptometer for dark adaptation, Frisby Stereotest for stereopsis, and Farnsworth desaturated D-15 color test for color discrimination [14]. When comparing the results of the functional vision test to the self-reported impairments of the subject, there was a strong correlation between those two measures. Among the functional vision tests, disability glare testing done by the BAT best accounted for the difficulties the subjects reported. Nelson et al. also showed that disability glare had one of the strongest relationship with the severity of visual field loss [14]. This relationship suggest that progression of glaucoma will be likely accompanied by increasing disability glare. Furthermore, the outcomes of this study affirm disability glare as a concerning visual impairment of glaucoma. In addition, the observed correlation between disability glare and visual field loss can potentially explain the components of the visual system that is involved in glare tolerance. This can in turn further the understanding of overall visual function.

As it is apparent that glaucoma patients suffer from disability glare, they found this impairment most concerning when driving. In surveying the value of various activities, glaucoma patients rate driving as highly important to maintaining their independence [15]. And so, understanding the impairments glaucoma patients face when driving is essential to addressing the concerns and preserving the quality of life for these individuals. Janz and colleagues surveyed open-angle glaucoma drivers and non-drivers about the types of visual problems they encounter during driving at a 6-month and a 54-month period. These surveys were also accompanied by ophthalmologic examinations. From the surveys and examinations, increasing visual field loss accounted for the differences between subjects who stayed drivers and subjects who became nondrivers because of their declining vision [15]. Thus, it can be inferred that those who are still drivers only had mild to moderate visual field loss. Despite little visual field loss, those drivers still reported many visual complications. One of the highest complaints from the drivers were tasks involving glare, which was said to be more troubling than visual search, peripheral vision, or visual processing speed which showed a lot of variation. Glare was a consistent issue among glaucoma drivers. Furthermore, glare was noted as one of the first issues subjects recognized when they first began to struggle with driving [15]. The study presents the driving challenges faced by glaucoma patients due to their sensitivity to disability glare. As mentioned earlier, driving is deemed as an important task to glaucoma patients to sustain autonomy. Therefore, assessing and managing disability glare is imperative to treating the visual impairments experienced by these individuals. Furthermore, since glare is one of the first detectable visual problems, disability glare test can potentially be utilized as a tool to identify the progression or worsening of a glaucoma in a patient. Though it is important to note that in the current state, it may be able to identify progression of the disease but may not give idea of the localization of the retinal damage in this disease. It will be interesting to evaluate the glare tolerance in various quadrants to see if the quantification of glare in specific locations is more sensitive than the non-specific glare tolerance testing.

#### 2.3. Cataracts

layer and changes in visual field that is in part associated to the level of intraocular pressure. If

Figure 1. An optical coherence tomography image from a patient with early age-related macular degeneration. The

Glaucoma affects several aspects of an individual's daily activities and task. Nelson and colleagues had articulated five major areas of difficulties in individuals with glaucoma. These difficulties include: (1) near vision issues, (2) peripheral vision issues, (3) dark adaptation and glare, (4) personal care and (5) household tasks, and outdoor mobility [14]. Their study measured both visual function and self-reported visual impairments. Subjects underwent multiple functional vision tests to assess the full spectrum of their visual capacity. The tests carried out included: Humphrey Visual Field Analyzer for visual field, Critical Flicker Frequency, Brightness Acuity Test (BAT) for disability glare, Goldmann-Weekers Dark Adaptometer for dark adaptation, Frisby Stereotest for stereopsis, and Farnsworth desaturated D-15 color test for color discrimination [14]. When comparing the results of the functional vision test to the self-reported impairments of the subject, there was a strong correlation between those two measures. Among the functional vision tests, disability glare testing done by the BAT best accounted for the difficulties the subjects reported. Nelson et al. also showed

left unmanaged, glaucoma leads to progressive vision loss and blindness [12, 13].

drusen bodies are visible in the retinal pigment epithelium.

60 Causes and Coping with Visual Impairment and Blindness

The lens is a specialized structure that relies on its transparency, high refractive index, and curved surface to project clear images to the retina. Most of the lens comprises of concentric elongated fibers covered with an epithelium on its anterior surface. The epithelium along with the superficial fiber cells secrete an elastic extracellular matrix that encases the lens in what is known as the capsule [1]. Below the capsule, at the equator of the epithelium is where new fiber cells arise and differentiate [2]. The newer fiber cells constitute the periphery of the lens, named the cortex [1]. While the center of the lens is comprised of older fiber cells, some originating from embryonic and fetal development, known as the nucleus [1]. Maintaining the transparency of the lens depends on the integrity of the arrangement of these fiber cells. However, as we age, oxidative damage and protein instability can accumulate, forming opacity in the lens and disrupting vision.

impairments while driving, difficulties in dimly lit environments and especially with disability glare [19]. Thus, the purpose of disability glare devices and testing methods is to provide

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There is evidence that supports that those with cataracts often experience a decrease in contrast sensitivity when compared to the age-match ocular healthy groups without cataract [20]. The contrast sensitivity loss in patients with cataract is even more pronounced under glare luminance [21]. Furthermore, cataract patients also have lower contrast sensitivity in mesopic conditions [1]. This becomes an issue when driving at night because that activity integrates mesopic light levels, contrast sensitivity, and the presence of glare. Thus, patients with cataract frequently complain of debilitating problems related to driving at night, under foggy, or rainy conditions, particularly with the addition of glare from incoming headlights [22]. Thus, as an importance of safety and the quality of life issue for those with cataracts, disability glare testing that accurately measures the challenges of night time driving is necessary. Disability glare in the daytime can also present visual impairments. Glare during the day predominantly originates from incoming rays of the sun. Unlike nighttime glare, daylight glare can be more

There are numerous devices available that intend to simulate glare under the various conditions such as night, foggy, or rainy conditions, however, glare devices are not yet standardized [2]. Thus, the foundation on how to measure disability on those with cataracts have not been set. Though, the present literature already provided some insight to the impairments of cataracts. Research continues to find a valid, repeatable, and reproducible method for testing

Overall it is shown that glare induces a significant loss in visual function and individuals with cataract have further decline in visual acuity and contrast sensitivity in a range conditions with

Centered at the retina is the macula which is highly packed with cone photoreceptors, and xanthophyll pigments that give it a darken appearance [24]. The macula is responsible for the majority of our photopic visual acuity, despite only comprising of less than 4% of the retinal space [24]. A disease known as age-related macular degeneration causes a gradual breakdown of these photoreceptors in the macula as well as changes in the retinal pigment. These damages lead to a decline in central vision [24]. Age-related macular degeneration (AMD) is divided into non-exudative (dry AMD) and exudative type (wet AMD). Early stages of dry AMD symptoms may go unnoticed, but patients slowly experience vision loss and can ultimately be converted to the wet AMD [25]. Some of the symptoms of AMD includes: decrease vision,

As mentioned previously, the macula is comprised of xanthophyll pigments, specifically lutein and zeaxanthin. The role of these pigments is thought to have protective effects on the macula, as this is an area vital to visual function. Lutein and zeaxanthin are believed to filter some of

additional information and insight that cannot be given with visual acuity testing.

accurately measured under photopic conditions [23].

blurry vision, metamorphopsia, and central scotomas [25].

the disability glare.

2.4. Macular degeneration

glare.

Cataracts is a disease cause by an opacification or cloudiness of the lens in the eye. The disease affects certain components of the lens, thus understanding the anatomy of the lens is important to pathophysiology of cataracts. There are various types of cataracts, but age-related cataract can be mainly divided into one of three types cortical, nuclear, and posterior capsular. Although, mixed type with features of three cataract types cortical, nuclear and posterior sub capsular are not uncommon. Each type has its own pathophysiology, anatomical differences and prevalence in the population [16]. Nuclear cataracts affect the oldest fiber cells of the lens which are the those formed in embryonic and fetal life. Evidence supports that nuclear cataracts arise due to the accumulation of reactive oxidative species that disrupt the normal protein and lipid components of fiber cells in the nucleus. The resulting cataracts causes patient to experience increase light scatter [17, 18]. However, cortical cataracts occur in matured fiber cells that arise later in life which lie closer to the surface of the lens. The progression of the cortical cataract encircles the outer circumference of the lens. The damages due to cortical cataract is much greater than that of nuclear cataract, the effects [17, 18]. On the other hand, posterior subscapular cataracts take place at the posterior surface of the lens where the cells just below the capsule are swollen. Since, the pathology of posterior subscapular cataracts is at the optical axis, visual function particularly reading tasks are greatly compromised. Furthermore, swelling of the posterior fiber cells impairs visual function even more by increasing the scattering of light [17, 18]. Clinically the cataract that causes the most glare related disability is the posterior subcapsular cataract. This is due two reasons (1) the entrance angle of the peripheral light rays is more oblique than central light rays and (2) the area that the posterior capsule cataract covers is also greater compared to nuclear cataract. Clinically in age related cataract we see mixed type of cataracts that has features that combine the nuclear, cortical and to some extent posterior subcapsular cataract.

The light is refracted through the lens before reaching the retina to be processed, and any sort of opacity that disrupts light transmittance can increase light scatter particularly if the opacity is large and spread throughout the lens. Being prone to disability glare, makes glare one of the biggest visual complaints and impairments experienced by those suffering from cataracts. Glare devices have an integral part in the research behind cataracts and currently, a large basis of literature is focused on the effects of disability glare on cataracts and how to accurately assess these visual challenges. Most glare devices available are geared toward cataract testing with the purpose of mimicking visual problems in real life in a clinical setting with the additional purpose of evaluating, monitoring and treatment of the disease state [2].

Clinically, cataracts are commonly evaluated by visual acuity charts which poses some problems. Visual acuity testing optotypes are at 100% contrast with black letters on white background and do not simulate real life scenario. In many cases, patients with cataract will have good visual acuities meeting legal standards of driving but still report experiencing visual impairments while driving, difficulties in dimly lit environments and especially with disability glare [19]. Thus, the purpose of disability glare devices and testing methods is to provide additional information and insight that cannot be given with visual acuity testing.

There is evidence that supports that those with cataracts often experience a decrease in contrast sensitivity when compared to the age-match ocular healthy groups without cataract [20]. The contrast sensitivity loss in patients with cataract is even more pronounced under glare luminance [21]. Furthermore, cataract patients also have lower contrast sensitivity in mesopic conditions [1]. This becomes an issue when driving at night because that activity integrates mesopic light levels, contrast sensitivity, and the presence of glare. Thus, patients with cataract frequently complain of debilitating problems related to driving at night, under foggy, or rainy conditions, particularly with the addition of glare from incoming headlights [22]. Thus, as an importance of safety and the quality of life issue for those with cataracts, disability glare testing that accurately measures the challenges of night time driving is necessary. Disability glare in the daytime can also present visual impairments. Glare during the day predominantly originates from incoming rays of the sun. Unlike nighttime glare, daylight glare can be more accurately measured under photopic conditions [23].

There are numerous devices available that intend to simulate glare under the various conditions such as night, foggy, or rainy conditions, however, glare devices are not yet standardized [2]. Thus, the foundation on how to measure disability on those with cataracts have not been set. Though, the present literature already provided some insight to the impairments of cataracts. Research continues to find a valid, repeatable, and reproducible method for testing the disability glare.

Overall it is shown that glare induces a significant loss in visual function and individuals with cataract have further decline in visual acuity and contrast sensitivity in a range conditions with glare.

#### 2.4. Macular degeneration

originating from embryonic and fetal development, known as the nucleus [1]. Maintaining the transparency of the lens depends on the integrity of the arrangement of these fiber cells. However, as we age, oxidative damage and protein instability can accumulate, forming opac-

Cataracts is a disease cause by an opacification or cloudiness of the lens in the eye. The disease affects certain components of the lens, thus understanding the anatomy of the lens is important to pathophysiology of cataracts. There are various types of cataracts, but age-related cataract can be mainly divided into one of three types cortical, nuclear, and posterior capsular. Although, mixed type with features of three cataract types cortical, nuclear and posterior sub capsular are not uncommon. Each type has its own pathophysiology, anatomical differences and prevalence in the population [16]. Nuclear cataracts affect the oldest fiber cells of the lens which are the those formed in embryonic and fetal life. Evidence supports that nuclear cataracts arise due to the accumulation of reactive oxidative species that disrupt the normal protein and lipid components of fiber cells in the nucleus. The resulting cataracts causes patient to experience increase light scatter [17, 18]. However, cortical cataracts occur in matured fiber cells that arise later in life which lie closer to the surface of the lens. The progression of the cortical cataract encircles the outer circumference of the lens. The damages due to cortical cataract is much greater than that of nuclear cataract, the effects [17, 18]. On the other hand, posterior subscapular cataracts take place at the posterior surface of the lens where the cells just below the capsule are swollen. Since, the pathology of posterior subscapular cataracts is at the optical axis, visual function particularly reading tasks are greatly compromised. Furthermore, swelling of the posterior fiber cells impairs visual function even more by increasing the scattering of light [17, 18]. Clinically the cataract that causes the most glare related disability is the posterior subcapsular cataract. This is due two reasons (1) the entrance angle of the peripheral light rays is more oblique than central light rays and (2) the area that the posterior capsule cataract covers is also greater compared to nuclear cataract. Clinically in age related cataract we see mixed type of cataracts that has features that combine the nuclear, cortical and

The light is refracted through the lens before reaching the retina to be processed, and any sort of opacity that disrupts light transmittance can increase light scatter particularly if the opacity is large and spread throughout the lens. Being prone to disability glare, makes glare one of the biggest visual complaints and impairments experienced by those suffering from cataracts. Glare devices have an integral part in the research behind cataracts and currently, a large basis of literature is focused on the effects of disability glare on cataracts and how to accurately assess these visual challenges. Most glare devices available are geared toward cataract testing with the purpose of mimicking visual problems in real life in a clinical setting with the

Clinically, cataracts are commonly evaluated by visual acuity charts which poses some problems. Visual acuity testing optotypes are at 100% contrast with black letters on white background and do not simulate real life scenario. In many cases, patients with cataract will have good visual acuities meeting legal standards of driving but still report experiencing visual

additional purpose of evaluating, monitoring and treatment of the disease state [2].

ity in the lens and disrupting vision.

62 Causes and Coping with Visual Impairment and Blindness

to some extent posterior subcapsular cataract.

Centered at the retina is the macula which is highly packed with cone photoreceptors, and xanthophyll pigments that give it a darken appearance [24]. The macula is responsible for the majority of our photopic visual acuity, despite only comprising of less than 4% of the retinal space [24]. A disease known as age-related macular degeneration causes a gradual breakdown of these photoreceptors in the macula as well as changes in the retinal pigment. These damages lead to a decline in central vision [24]. Age-related macular degeneration (AMD) is divided into non-exudative (dry AMD) and exudative type (wet AMD). Early stages of dry AMD symptoms may go unnoticed, but patients slowly experience vision loss and can ultimately be converted to the wet AMD [25]. Some of the symptoms of AMD includes: decrease vision, blurry vision, metamorphopsia, and central scotomas [25].

As mentioned previously, the macula is comprised of xanthophyll pigments, specifically lutein and zeaxanthin. The role of these pigments is thought to have protective effects on the macula, as this is an area vital to visual function. Lutein and zeaxanthin are believed to filter some of the harmful short-wave length blue light [24, 25]. Additionally, these pigments can also act as antioxidants to tackle free radicals and eradicate reactive oxygen species that damage the photoreceptors of the macula. Furthermore, lutein and zeaxanthin has shown to absorb straylight which can decrease the amount of harmful light entering the retina and possibly lower glare. The protective properties of these pigments led researchers to believe that increasing these pigmentations could potentially improve visual function.

3. Allied visual functions

well.

3.1.1. Stereopsis

tion [30].

3.1. Issues involved in glare testing

Disability glare plays an impairing role in many ocular pathologies such as the ones previously mentioned [2]. Thus, glare testing is not only valuable to understanding visual function, but it can also serve as a tool to evaluate the efficacy of treatments and surgeries of ocular diseases as

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Though obvious that disability glare affects visual function, it still under study of what component of vision is most impaired by glare. Vision involves visual acuity, contrast sensitivity, stereopsis and many other components that can potentially be impaired by glare. Disability glare is commonly evaluated by either visual acuity or contrast sensitivity [28] (Figure 2). However, disability glare has shown to influence those aspects of vision differently, and so are important factors to consider when testing glare. Furthermore, glare is also tested under various light conditions such as photopic and mesopic. This is to mimic the changing luminance from day to night. Disability glare effects also varies from different light conditions; thus, presenting its own specific challenges in each light level [29]. Since glare testing is highly specific, appropriate variables must be incorporated for reliable and interpretable results.

Knowing the role of glare in visual function, proper glare testing methodology and devices are important. There are many components involved in glare testing some of which are the type of stimuli, glare source, and conditions. These factors play a role in the effectiveness of measuring disability glare and creating a real-world simulation. The capability of a glare testing method or device depends mainly on three criteria: discriminative ability, reliability, and validity [28]. Since glare methods and devices vary on the components they incorporate, so do their performance on the criteria mentioned. However, most current devices do fail to meet all three criteria, and thus there is still no standard way to measure glare. While there is a lack of standardization, there are a number commercial machines that are utilized in clinics and research [28]. Some of these devices are potentially valuable assessment tools but further research is necessary to evaluate their validity. However, there are many self-made devices created by researchers to address the glare test problem. Those have also shown good discriminative and repeatability. Despite positive findings, these devices and methods are still new

and require much more additional research to assess their accuracy and validity.

Stereopsis is the visual function of depth perception in a 3D world. The visual system integrates binocular disparity to interpret the placement of objects in space. Primarily a binocular visual function, good and balanced acuity of both eyes are necessary for proper depth percep-

As with some visual functions, stereopsis has shown to decrease with age even when visual acuity is still good. It is speculated that the decline in stereopsis is due to changes the eye undergoes with aging. The refractive and ocular motor system that can change with age can

One of the visual functions believed to be improved is disability glare and glare recovery. Stringham and Hammond looked at the relationship between disability glare and macular pigments. They measured macular pigment optical density (MPOD) of their subjects and compared that to their disability glare scores. The disability glare score was attained by measuring the level of illuminance from Maxwellian-View optical system that is high enough to induce disability glare when viewing sinusoidal gratings at 100% contrast [26]. From this test, the disability glare scores calculated showed a strong correlation to the macular pigment density. The researchers attributed the lower disability glare when there is a greater pigment density to the filtering effect of macular pigments. This was supported by the lack of correlation they showed between disability glare scores and macular pigment density when the glare source excluded the wavelengths of light that macular pigments are believed to filter [26]. These results provide compelling evidence for the involvement of the macula in disability glare. Disability glare is most associated with issues involving the optical media of the eye like the cornea and lens. However, as research has shown, the effects of disability glare can also be mediated by macular pigment. This provides more insight to visual function as well as the visual impairments that result from ocular diseases.

In additional studies, Stringham and Hammond recruited normal subjects who were given daily a 500-mg tablet that contained 10 mg of lutein and 2 mg of zeaxanthin over a 6 months period [27]. The research recruited 40 participants consisting of 23 women and 17 men. The subjects were assessed at 1,2,4 and 6-month period where their disability glare, photostress recovery, and macular pigment optical density (MPOD) were measured. As the researchers had done previously, disability glare was tested by utilizing the by the Maxwellian-view optical system to determine the illuminance level sufficient to cause visual impairment. All the subjects except for two had shown increase MPOD at the end of 6 months. The study subject also displayed reduced disability glare compared to baseline, tolerating greater veiling lights before any effects to their vision. On average, the participants tolerated 58% more glare (p < 0.0001) [27]. These results proposed a correlation between MPOD and tolerance to disability glare. This was further supported by the two subjects who did not experience any changes. These subjects that did not show an improvement in the MPOD also did not show an increased tolerance to glare. The researchers inferred that the macular pigment reduce glare disability by acting similarly to a yellow filter that cuts out short wavelength light and decreases veiling luminance [27].

From the relationship between macular pigment and disability glare, we speculate that the disability glare experienced by those suffering with macular degeneration can be partially due to the reducing level of MPOD. Moreover, knowing that MPOD can be supplemented and increased leaves possibility to improve the visual function of those with AMD, especially in the visual impairment of disability glare.
