Supplementation with macular carotenoids reduces psychological stress, serum cortisol, and sub-optimal symptoms of physical and emotional health in young adults.

PURPOSE: Oxidative stress and systemic inflammation are the root cause of several deleterious effects of chronic psychological stress. We hypothesize that the antioxidant and anti-inflammatory capabilities of the macular carotenoids (MCs) lutein, zeaxanthin, and meso-zeaxanthin could, via daily supplementation, provide a dietary means of benefit. METHODS: A total of 59 young healthy subjects participated in a 12-month, double-blind, placebo-controlled trial to evaluate the effects of MC supplementation on blood cortisol, psychological stress ratings, behavioural measures of mood, and symptoms of sub-optimal health. Subjects were randomly assigned to one of three groups: placebo, 13 mg, or 27 mg / day total MCs. All parameters were assessed at baseline, 6 months, and 12 months. Serum MCs were determined via HPLC, serum cortisol via ELISA, and macular pigment optical density (MPOD) via customized heterochromatic flicker photometry. Behavioural data were obtained via questionnaire. RESULTS: Significant baseline correlations were found between MPOD and Beck anxiety scores (r = -0.28; P = 0.032), MPOD and Brief Symptom Inventory scores (r = 0.27; P = 0.037), and serum cortisol and psychological stress scores (r = 0.46; P < 0.001). Supplementation for 6 months improved psychological stress, serum cortisol, and measures of emotional and physical health (P < 0.05 for all), versus placebo. These outcomes were either maintained or improved further at 12 months. CONCLUSIONS: Supplementation with the MCs significantly reduces stress, cortisol, and symptoms of sub-optimal emotional and physical health. Determining the basis for these effects, whether systemic or a more central (i.e. brain) is a question that warrants further study.

Serum and retinal responses to three different doses of macular carotenoids over 12 weeks of supplementation.

The macular carotenoids lutein (L), zeaxanthin (Z), and mesozeaxanthin (MZ) have been shown to have neuroprotective and visual performance benefits once deposited in retinal tissues. The purpose of this 12-week trial was to determine biweekly the absorption kinetics, efficiency of retinal deposition, and effects on the spatial profile of macular pigment for three levels of L + Z + MZ supplement. This study was a double-blind, placebo-controlled 12-week trial. Twenty-eight healthy subjects, aged 18-25 yrs participated. Subjects were randomly assigned to one of four daily supplementation groups: placebo (safflower oil; n = 5), 7.44 mg total macular carotenoid (n = 7), 13.13 mg total macular carotenoid (n = 8), and 27.03 (n = 8) mg total macular carotenoid. Ratios of the three carotenoids were virtually identical for the three levels of supplement (83% L, 10% Z, 7% MZ). At baseline and every two weeks thereafter over the 12-week study period, a fasting blood draw was conducted and, via heterochromatic flicker photometry, spatial profiles of macular pigment optical density (MPOD) were determined. Compared to placebo, serum concentrations of both L and total Z, for each of the supplement levels, were found to increase significantly from baseline after two weeks of daily ingestion (p < 0.001). Likewise, MPOD increased significantly in all treatment groups (p < 0.001) compared to placebo. Serum responses (L, Z, and L + Z) were linearly related to dose (p < 0.001 for all), but not to retinal response. L: Z serum response ratios decreased exponentially with increases in dose (p = 0.008). The ratio of MPOD change: total serum response was found to be highest for the 13.13 mg level of supplement (p = 0.021), followed by 27.03- and 7.44-mg doses. The very center of the spatial profile of MPOD increased in a fashion commensurate with dose level. Although L serum responses increased with dose, the slope of increase was shallower than for Z. Given the higher levels of L in the supplements, this is suggestive of a compressed response with relatively high doses of L. Although all three doses significantly augmented MPOD, the 13.13 mg/day L + Z supplement level was the most efficient in doing so. The data regarding efficiency may inform recommendations regarding macular carotenoid supplementation for age-related macular degeneration. Lastly (although not statistically significant), the shift toward a more pronounced central peak in the spatial profile of MPOD in all treatment groups suggests that central retinal deposition of Z and MZ was efficient and can be seen after a short period of supplementation, especially with higher (e.g. 27.03 mg) daily doses of macular carotenoids. ISRCTN trial registration number: ISRCTN54990825.

Visual Performance in the "Real World": Contrast Sensitivity, Visual Acuity, and Effects of Macular Carotenoids.

Visual acuity (VA) is compared to contrast sensitivity (CS) testing in assessing "real-world" visual performance, and it is recommended that both should be measured routinely in the clinic. The role of nutritional intervention in improving visual performance is reviewed and emphasized. A brief history and illustration of both VA and CS, within the scope of visual performance, is presented. Parameters for effective CS testing in the clinic, and guidelines for interpretation of results, including a new model for understanding the visual impact of changes in CS, are also presented. Relevant research that supports the use of the macular carotenoids lutein, zeaxanthin, and meso-zeaxanthin to enhance visual performance is reviewed with suggested guidelines for supplementation. CS testing is easily performed at a single intermediate target size and is an excellent tool for the accurate assessment of a patient's overall visual experience. Research continues to uncover the strong link between nutrition and visual performance; the macular carotenoids appear to be especially effective in this regard, and their benefits to visual performance now importantly include contrast sensitivity. Clinicians can provide an improved level of care by incorporating into the examination protocol CS testing and, where appropriate, nutritional counseling and intervention.

Lutein across the Lifespan: From Childhood Cognitive Performance to the Aging Eye and Brain.

Lutein is a non-provitamin A dietary carotenoid found in dark green leafy vegetables, corn, eggs, and avocados. Among the carotenoids, lutein and its isomer, zeaxanthin, are the only 2 that cross the blood-retina barrier to form macular pigment in the retina. Lutein also preferentially accumulates in the human brain across multiple life stages. A variety of scientific evidence supports a role for lutein in visual as well as cognitive function across the lifespan. The purpose of this review is to summarize the latest science on lutein's role in the eye and the brain across different ages.

Effects of macular xanthophyll supplementation on brain-derived neurotrophic factor, pro-inflammatory cytokines, and cognitive performance.

PURPOSE: Oxidative and inflammatory processes play a major role in stress-induced neural atrophy. There is a wide body of literature linking oxidative and inflammatory stress with reductions in neurotrophic factors, stress resilience, and cognitive function. Based on their antioxidant and anti-inflammatory capacity, we investigated the effect of the dietary carotenoids lutein and zeaxanthin, along with the zeaxanthin isomer meso-zeaxanthin (collectively the "macular xanthophylls" [MXans]) on systemic brain-derived neurotrophic factor (BDNF) and anti-oxidant capacity (AOC), and the pro-inflammatory cytokines TNF-α, IL-6, and IL-1ß. To investigate higher-order effects, we assessed cognitive performance. METHODS: 59 young (18-25 yrs.), healthy subjects participated in a 6-month, double-blind, placebo-controlled trial to evaluate the effects of MXan supplementation on the aforementioned serum parameters and cognitive performance. Subjects were randomly assigned to one of three groups: placebo, 13 mg, or 27 mg/day total MXans; all measures were taken at baseline and 6 months. Blood was obtained via fasting blood draw, and MXan concentration in the retina (termed macular pigment optical density [MPOD]) was measured via customized heterochromatic flicker photometry. Serum BDNF and cytokines were assessed via ELISA. Serum antioxidant capacity (AOC) and serum MXan concentrations were quantified via colorimetric microplate assay, and high-performance liquid chromatography, respectively. Cognitive performance was measured via a computer-based assessment tool (CNS Vital Signs). RESULTS: BDNF, MPOD, serum MXans, and AOC all increased significantly versus placebo in both treatment groups over the 6-month study period (p < .05 for all). IL-1ß decreased significantly versus placebo in both treatment groups (p = .0036 and p = .006, respectively). For cognitive measures, scores for composite memory, verbal memory, sustained attention, psychomotor speed, and processing speed all improved significantly in treatment groups (p < .05 for all) and remained unchanged in the placebo group. Several measures were found to be significantly associated in terms of relational changes over the course of the study. Notably, change in BDNF was related to change in IL-1ß (r = -0.47; p < .001) and MPOD (r = 0.44; p = .0086). Additionally, changes in serum MXans were strongly related to AOC (r = 0.79 & 0.61 for lutein and zeaxanthin isomers respectively; p < .001). For cognitive scores, change in BDNF was correlated to change in composite memory (r = 0.32; p = .014) and verbal memory (r = 0.35; p = .007), whereas change in MPOD was correlated with change in both psychomotor speed (r = 0.38; p = .003), and processing speed (r = 0.35; p = .007). Change in serum lutein was found to be significantly correlated to change in verbal memory (r = 0.41; p < .001), composite memory (r = 0.31; p = .009), and sustained attention (r = 0.28; p = .036). Change in serum zeaxanthin isomers was significantly correlated with change in verbal memory (r = 0.33; p = .017). Lastly, change in AOC was significantly associated with verbal memory (r = 0.34; p = .021), composite memory (r = 0.29; p = .03), and sustained attention (r = 0.35; p = .016). No significant relational changes in any cognitive parameter were found for the placebo group. CONCLUSIONS: Six months of daily supplementation with at least 13 mg of MXans significantly reduces serum IL-1ß, significantly increases serum MXans, BDNF, MPOD, and AOC, and improves several parameters of cognitive performance. Findings suggest that increased systemic antioxidant/anti-inflammatory capacity (and not necessarily deposition of the carotenoids in neural tissues), may explain many of the effects determined in this study. The significant relationship between change in BDNF and IL-1ß over the course of the study suggests that regular consumption of MXans interrupts the inflammatory cascade that can lead to reduction of BDNF. Changes in MPOD and BDNF appear to account for enhancement in cognitive parameters that involve speed of processing and complex processing, respectively. ISRCTN Clinical Trial Registration: ISRCTN16156382.

Visual Crowding in Glaucoma.

Purpose: Crowding refers to the phenomenon in which objects that can be recognized when viewed in isolation are unrecognizable in clutter. Crowding sets a fundamental limit to the capabilities of the peripheral vision and is essential in explaining performance in a broad array of daily tasks. Due to the effects of glaucoma on peripheral vision, we hypothesized that neural loss in the disease would lead to stronger effects of visual crowding. Methods: Subjects were asked to discriminate the orientation of a target letter when presented with surrounding flankers. The critical spacing value (scritical), which was required for correct discrimination of letter orientation, was obtained for each quadrant of the visual field. scritical values were correlated with standard automated perimetry (SAP) mean sensitivity (MS) and optical coherence tomography (OCT) retinal nerve fiber layer (RNFL) thickness measurements. Results: The study involved 13 subjects with mild glaucomatous visual field loss and 13 healthy controls. Glaucomatous eyes had significantly greater (worse) scritical than controls (170.4 ± 27.1 vs. 145.8 ± 28.0 minimum of visual angle, respectively; P = 0.007). scritical measurements were significantly associated with RNFL thickness measurements (R2 = 26%; P < 0.001) but not with SAP MS (P = 0.947). Conclusions: In glaucoma patients, a pronounced visual crowding effect is observed, even in the presence of mild visual field loss on standard perimetry. scritical was associated with the amount of neural loss quantified by OCT. These results may have implications for understanding how glaucoma patients are affected in daily tasks where crowding effects may be significant.

Mobile Telephone Use and Reaction Time in Drivers With Glaucoma.

Importance: Combining mobile telephone use with driving is not unusual. However, distracted driving limits driving performance because of limited capacity for persons to divide attention. Objectives: To investigate the frequency of mobile telephone use while driving and to assess whether patients with glaucoma had a disproportionate decrease in driving performance while conversing on a mobile telephone compared with healthy participants. Design, Setting, and Participants: Cross-sectional study of surveys collected from 112 patients with glaucoma and 70 control participants investigating mobile telephone use while driving. A randomly selected subgroup of 37 patients with glaucoma and 28 controls drove in a driving simulator to investigate peripheral event detection performance during distracted driving at the Visual Performance Laboratory, Duke University, Durham, North Carolina. Data collection was performed from December 1, 2016, through April 30, 2017. Exposures: Participants answered a survey and submitted to a driving simulation test with and without mobile telephone use. Main Outcomes and Measures: Survey answers were collected, and distracted driving performance, assessed by reaction time to peripheral stimuli, was analyzed. Results: Of the 182 participants who answered the survey, the 112 participants with glaucoma included 56 women (50.0%) and had a mean (SD) age of 73.6 (9.6) years. The 70 controls included 49 women (70.0%) and had a mean (SD) age of 68.4 (10.9) years. When asked about mobile telephone use while driving, 30 patients with glaucoma (26.8%) admitted rarely using and 2 (1.8%) sometimes using it. In the control group, 20 participants (28.6%) admitted rarely using and 2 (2.9%) sometimes using the telephone while driving (P = .80). Reaction times to peripheral stimuli were significantly longer among patients with glaucoma compared with controls during mobile telephone use (median [interquartile range], 1.86 [1.42-2.29] seconds vs 1.14 [0.98-1.59] seconds; P = .02). Compared with driving performance while not using a mobile telephone, the mean (SD) increase of 0.85 (0.60) second in reaction time while conversing on the mobile telephone among patients with glaucoma was significantly greater than the mean (SD) increase of 0.68 (0.83) second for controls (P = .03). Conclusions and Relevance: This study's findings indicate that patients with glaucoma use mobile telephones while driving as frequently as healthy participants. However, the findings also suggest that patients with glaucoma may experience a greater decline than healthy participants in their ability to detect peripheral events while driving when also talking on a mobile telephone. Patients with glaucoma should be informed that they may have a higher driving risk that may be worsened by distractions, such as mobile telephone use.

Spatial profile of macular pigment and its relationship to foveal architecture.

PURPOSE: Macular pigment (MP) is composed of two dietary carotenoids, lutein and zeaxanthin, and a carotenoid generated by the retina, meso-zeaxanthin. There is large intersubject variability in peak optical density, spatial profile, and lateral extent of macular pigment, and it has been suggested that foveal architecture may play a role in this variability. This study is an initial investigation of the relationship between the spatial profile of macular pigment and foveal architecture. METHODS: Sixty normal subjects were enrolled (one was eventually excluded). The spatial profile of macular pigment optical density (MPOD) was measured by customized heterochromatic flicker photometry (cHFP). High-resolution macular thickness maps were obtained by optical coherence tomography. Four parameters were analyzed: (1) minimum foveal thickness (MFT) at the intersection of six radial scans; (2) central foveal thickness (CFT) averaged over the central 1 mm of the fovea; (3) foveal width identified as the region lacking a nerve fiber layer; and (4) foveal width measured from crest to crest. Lifestyle and vision information were obtained by questionnaire. RESULTS: The mean +/- SD MPOD at 0.25 degrees eccentricity was 0.49 +/- 0.23 and at 0.5 degrees eccentricity, 0.41 +/- 0.21. A first-order decreasing exponential function accounted for most of the variance of the MP profile averaged across subjects (r(2) = 0.99). MPOD measured at 0.25 degrees was unrelated to both measures of foveal thickness for the entire study group (r = 0.03, P = 0.81, and r = -0.08, P = 0.57, respectively). Similarly, MPOD measured at 0.5 degrees was unrelated to foveal thickness in the entire study group (r = 0.12, P = 0.36 and r = -0.05, P = 0.71, respectively). However, when analyzed separately in the nonwhite subjects, the relationship between MPOD at 0.25 degrees and MFT was positive and significant (r = 0.59, P = 0.01), but remained unrelated to CFT (r = 0.20, P = 0.41). Similarly, in the nonwhite subjects, the relationship between MPOD at 0.5 degrees and MFT was positive and significant (r = 0.68, P < 0.01), but again was unrelated to CFT (r = 0.23, P = 0.32). There was no significant relationship between MPOD and either measure of foveal thickness in the white subjects. In the entire study group, there was a positive and significant relationship between foveal width and MPOD averaged across the fovea (r = 0.41, P < 0.01) and between foveal width and MP integrated across the fovea (r = 0.41, P < 0.01). CONCLUSIONS: Foveal MP was positively and significantly related to foveal width in the entire study group. This relationship may be determined by the greater length of the cone axons (Henle fibers) in wider foveas. MPOD was unrelated to foveal thickness in the white subjects. However, in the nonwhite subjects there was a positive association between MFT and MPOD at the 0.25 degrees and 0.5 degrees eccentricities, suggesting that other personal characteristics modulate the MPOD-retinal thickness relationship.

Macular Carotenoid Supplementation Improves Visual Performance, Sleep Quality, and Adverse Physical Symptoms in Those with High Screen Time Exposure.

The dramatic rise in the use of smartphones, tablets, and laptop computers over the past decade has raised concerns about potentially deleterious health effects of increased "screen time" (ST) and associated short-wavelength (blue) light exposure. We determined baseline associations and effects of 6 months' supplementation with the macular carotenoids (MC) lutein, zeaxanthin, and mesozeaxanthin on the blue-absorbing macular pigment (MP) and measures of sleep quality, visual performance, and physical indicators of excessive ST. Forty-eight healthy young adults with at least 6 h of daily near-field ST exposure participated in this placebo-controlled trial. Visual performance measures included contrast sensitivity, critical flicker fusion, disability glare, and photostress recovery. Physical indicators of excessive screen time and sleep quality were assessed via questionnaire. MP optical density (MPOD) was assessed via heterochromatic flicker photometry. At baseline, MPOD was correlated significantly with all visual performance measures (p < 0.05 for all). MC supplementation (24 mg daily) yielded significant improvement in MPOD, overall sleep quality, headache frequency, eye strain, eye fatigue, and all visual performance measures, versus placebo (p < 0.05 for all). Increased MPOD significantly improves visual performance and, in turn, improves several undesirable physical outcomes associated with excessive ST. The improvement in sleep quality was not directly related to increases in MPOD, and may be due to systemic reduction in oxidative stress and inflammation.

Contrast Sensitivity and Lateral Inhibition Are Enhanced With Macular Carotenoid Supplementation.

Purpose: Once deposited in the retina, the so-called macular carotenoids lutein (L), zeaxanthin (Z), and mesozeaxanthin (MZ) have been shown to enhance visual performance. The purpose of our study was to investigate whether increasing macular pigment optical density (MPOD) could enhance lateral inhibitory processes, and thereby improve contrast sensitivity (CS). Methods: A total of 59 young (18-25 years), healthy individuals participated in this 1-year, double-masked, placebo-controlled study. MPOD was assessed via heterochromatic flicker photometry. Lateral inhibition sensitivity (LIS) was determined with a computer-based, user-adjustable Hermann grid. CS (at 8 cycles/degree) was determined with a two-alternative, forced-choice procedure. Subjects received either the placebo (n = 10), 12 mg total macular carotenoids (n = 24), or 24 mg total macular carotenoids (n = 25). Results: MPOD, LIS, and CS increased significantly in treatment groups between baseline and 6 months, and between 6 and 12 months (P < 0.05 for all) versus placebo. The relationships between changes in MPOD and both LIS and CS were significant at 6 and 12 months (P < 0.05 for both). Changes in CS and LIS over the 12-month study period were found to be significantly related (r = 0.41; P = 0.0014). Conclusions: Increases in MPOD led to enhanced lateral inhibitory processes, which correspond to improved CS. Because optical filtering has the same net effect on dark versus light bars, it cannot explain these improvements. Improvement in CS with increases in MPOD therefore appears to involve enhancement of the fundamental physiological systems that give rise to edge detection.

Macular pigment optical density and photophobia light threshold.

Light absorption by macular pigment may attenuate visual discomfort, or photophobia, for targets composed of short-wavelength light. Macular pigment optical density (MPOD) and photophobia light thresholds were measured psychophysically in 10 subjects. The energy necessary to induce photophobia for a short-wavelength target relative to a long-wavelength target was linearly related to MPOD, as well as estimates of peak MPOD and integrated macular pigment. In four subjects who consumed lutein supplements, increases in MPOD corresponded to increases in photophobia light thresholds. Light absorption by macular pigment appears to influence the amount of short-wavelength light necessary to elicit photophobia.

Macular carotenoid supplementation improves disability glare performance and dynamics of photostress recovery.

BACKGROUND: The so-called macular carotenoids (MC) lutein (L), zeaxanthin (Z), and meso-zeaxanthin (MZ) comprise the diet-derived macular pigment (MP). The purpose of this study was to determine effects of MC supplementation on the optical density of MP (MPOD), repeated-exposure photostress recovery (PSR), and disability glare (DG) thresholds. METHODS: This was a double-blind, placebo-controlled trial. Fifty-nine young (mean age = 21.7), healthy volunteers participated in this study. Subjects supplemented their daily diet with either 10 mg L + 2 mg total Z (1 mg Z + 1 mg MZ; n = 24), 20 mg L + 4 mg total Z (2 mg Z + 2 mg MZ; n = 25), or placebo (n = 10) for 12 months. The primary outcome was a composite measure of visual performance in glare, defined by change in DG and PSR. Secondary outcomes included MPOD and visual fatigue. The primary endpoint for outcomes was 12 months. MPOD was assessed with customized heterochromatic flicker photometry. PSR times for an 8 cycle /degree, 15 % contrast Gabor patch target were determined after each of five successive exposures to intense LED lights. DG threshold was defined as the intensity of a ring of lights through which subjects were able to maintain visibility of the aforementioned target. Measures of all parameters were conducted at baseline, 6 months, and 12 months. Repeated-measures ANOVA, and Pearson product-moment correlations were used to determine statistically significant correlations, and changes within and between groups. RESULTS: MPOD for subjects in both supplementation groups increased significantly versus placebo at both 6- and 12-month visits (p < 0.001 for all). Additionally, PSR times and DG thresholds improved significantly from baseline compared to placebo at 6- and 12-month visits (p < 0.001 for all). At baseline, MPOD was significantly related to both DG thresholds (r = 0.444; p = 0.0021) and PSR times (r = -0.56; p < 0.001). As a function of MPOD, the repeated-exposure PSR curves became more asymptotic, as opposed to linear. The change in subjects' DG thresholds were significantly related to changes in PSR times across the study period (r = -0.534; p < 0.001). CONCLUSIONS: Increases in MPOD lead to significant improvements in PSR times and DG thresholds. The asymptotic shape of the repeated-exposure PSR curves suggests that increases in MPOD produce more consistent steady-state visual performance in bright light conditions. The mechanism for this effect may involve both the optical filtering and biochemical (antioxidant) properties of MP. TRIAL REGISTRATION: ISRCTN trial registration number: ISRCTN54990825. Data reported in this manuscript represent secondary outcome measures from the registered trial.

Dietary lutein and zeaxanthin: possible effects on visual function.

Of the many carotenoids circulating in human sera, only lutein and zeaxanthin are accumulated throughout the tissues of the eye. Within the eye, they reach their highest concentration in the central retina, where they are clinically referred to as the macula lutea. Lutein and zeaxanthin, more commonly referred to as macular pigments, may serve a variety of roles in the specialized vision of higher primates. This paper reviews recent studies investigating the influence of macular pigments on human visual performance. Such studies have offered insight into why lutein and zeaxanthin are uniquely concentrated in ocular tissues.

Temporal Visual Mechanisms May Mediate Compensation for Macular Pigment.

Macular pigment (MP) is a pre-receptoral filter that is diet derived and deposited in relatively high optical density in the foveal region of the retina. Due to its yellow coloration, MP absorbs light of relatively short wavelengths, ranging from 400 nm to 520 nm. Despite the spectral and spatial nonuniformity imposed upon the sensory retina by MP, perception appears to be relatively uniform across the central visual field. MP therefore offers an opportunity to determine experimentally potential mechanisms responsible for mediating this uniformity. After assessing, in 14 subjects, MP's effects on the temporal sensitivity of both the short-wavelength- and middle-/long-wavelength-sensitive visual pathways, it appears that the visual system compensates for absorption of short-wavelength light by MP by slowing the sampling rate of short-wavelength cones and by increasing the processing speed of middle-/long-wavelength-sensitive cones. This mechanism could work via temporal summation or a temporal neural code, whereby slower response dynamics lead to amplification of relatively weak signals.

Nitric Oxide and Lutein: Function, Performance, and Protection of Neural Tissue.

The soluble gas neurotransmitter nitric oxide (NO) serves many important metabolic and neuroregulatory functions in the retina and brain. Although it is necessary for normal neural function, NO can play a significant role in neurotoxicity. This is often seen in disease states that involve oxidative stress and inflammation of neural tissues, such as age-related macular degeneration and Alzheimer's disease. The dietary xanthophyll carotenoid lutein (L) is a potent antioxidant and anti-inflammatory agent that, if consumed in sufficient amounts, is deposited in neural tissues that require substantial metabolic demand. Some of these specific tissues, such as the central retina and frontal lobes of the brain, are impacted by age-related diseases such as those noted above. The conspicuous correspondence between metabolic demand, NO, and L is suggestive of a homeostatic relationship that serves to facilitate normal function, enhance performance, and protect vulnerable neural tissues. The purpose of this paper is to review the literature on these points.

Macular Pigment and Visual Performance in Low-Light Conditions.

PURPOSE: By reducing rod intrusion and improving efficiency of neural signaling throughout the visual system, macular pigment (MP) could improve many aspects of visual performance in low-light level conditions. Our study examined this possibility for a variety of visual performance parameters, including spatial resolution, dark adaptation kinetics, and color detection. METHODS: Twenty-seven subjects participated in the study. Spatial profiles of MP optical density (MPOD) were determined by using heterochromatic flicker photometry. Mesopic- and scotopic-adaptation level experiments were conducted in Maxwellian view. RESULTS: Subjects with higher MPOD required significantly lower contrast to detect the mesopic-level resolution targets; this effect became stronger with increasing spatial frequency. Dark adaptation recovery times were significantly faster as a function of MPOD (by nearly 2 minutes for the lowest mesopic-level task [high versus low MPOD]; P < 0.001). Absolute scotopic thresholds were also significantly associated with MPOD (P < 0.001). Macular pigment optical density was inversely associated with detection of yellow (P < 0.001), and, paradoxically, approached a significant positive correlation with the detection of blue (P = 0.06). CONCLUSIONS: Macular pigment appears to enhance visual function in low-light conditions. Based on the results of this study, it can be said that MP extends the range of foveal vision into lower light. Additionally, MP appears to enhance dark adaptation kinetics, which suggests that increased MPOD leads to more efficient photopigment regeneration. The findings of the color detection portion of the study are suggestive of an active compensatory mechanism that offsets absorption by MP in order to maintain normal color perception.

Spatial properties of photophobia.

PURPOSE: To determine the spatial properties of stimuli that elicit photophobia (PP) in normal subjects: Does PP exhibit spatial summation? Are different parafoveal quadrants (superior, inferior, temporal, and nasal) of the retina differentially sensitive in PP? What is the relationship between PP sensitivity and retinal eccentricity? What is the relationship between the spatial properties of PP and the spatial distribution of macular pigment (MP)? METHODS: A Maxwellian-view optical system with a xenon light source was used to present the stimuli. Four normal subjects viewed stimuli of various sizes, retinal locations, and one of two chromatic contents: xenon-white and a broadband orange. The intensity of the test stimulus was increased between trials until the PP threshold was reached. The squinting response corresponding to PP was assessed by electromyography and used as an objective criterion of PP. Three parameters were examined: stimulus size, parafoveal retinal locus (superior, inferior, temporal, and nasal), and retinal eccentricity (extending into the perifovea). Spatial profiles of MP were measured psychophysically using heterochromatic flicker photometry (HFP). RESULTS: Spatial summation for PP was found essentially to adhere to Piper's law (radiance proportional to square root of stimulus area). The PP response was greater to centrally than peripherally viewed targets. In this regard, MP acted as a spatially integrated filter in the attenuation of PP. CONCLUSIONS: The degree of spatial summation found for PP indicates that an increase of 1.0 log unit in field area results in an approximately 0.57-log-unit decrease in the radiance required to elicit PP. PP appears to serve the function of retinal photoprotection.

Light exposure and macular pigment optical density.

PURPOSE: Biochemical research has demonstrated that lutein and zeaxanthin, the two macular carotenoids, are bleachable pigments. Further, evidence suggests that exposure to UV light can degrade plasma carotenoid levels in vivo. The present study investigated the effects of exposure to normal levels of light on the levels of lutein and zeaxanthin in the retina. METHODS: The optical density of macular pigment (MPOD) was measured in two male subjects under four different light-adaptation conditions for 20 days. Heterochromatic flicker photometry was used to measure MPOD at 0.5 degrees eccentricity. RESULTS: The four conditions of light adaptation did not significantly affect MPOD. As in previous studies, however, a significant day-to-day difference was observed for both subjects. CONCLUSIONS: The results suggest that lutein and zeaxanthin levels in the eye are unaffected by light and oxidation throughout the day. This justifies current research methods in which MPOD measures are made regardless of the time of day. However, significant between-day variance indicates that multiple MPOD measures may be necessary to evaluate lutein and zeaxanthin levels in the retina accurately.

Enhancing performance while avoiding damage: a contribution of macular pigment.

PURPOSE: To compare action spectra for visual discomfort in the fovea and the parafovea and to determine the effect of macular pigment (MP). METHODS: Visual discomfort thresholds to lights from 440 to 600 nm were obtained for six young (<35 y), visually normal subjects with a wide range of MP densities (0.10-0.71 at 30' eccentricity). Foveal and parafoveal conditions were assessed. Discomfort thresholds were also obtained for xenon-white light (partially absorbed by MP), and a broadband yellow (outside the absorption band of MP). MP was measured psychophysically using heterochromatic flicker photometry (HFP). RESULTS: For the parafovea, discomfort sensitivity (1/threshold) increased sharply with decreasing wavelength for all subjects. Commensurate with a subject's MP level, MP significantly reduced visual discomfort to short wavelengths (including xenon-white light) for central viewing. CONCLUSIONS: MP simultaneously reduces visual discomfort and protects from light damage at short wavelengths. As a result, MP increases the range of safe and comfortable light levels. Because higher light levels enable improved visual sensitivity for fine detail, these findings indicate that the spectral absorption properties and spatial distribution of MP combine to protect the retina while enhancing visual performance. The action spectrum for visual discomfort closely matches the risk for acute light damage to the retinal pigment epithelium, and it is consistent with a major influence from the intrinsically photosensitive retinal ganglion cells containing melanopsin. We suggest that MP interacts with nonimage-forming retinal input to achieve the dual outcomes of visual discomfort reduction and protection from light damage.

A potential mechanism for compensation in the blue-yellow visual channel.

Due to their unique contribution to human vision, the short (S)-wavelength sensitive cones, their anatomical projections and, more recently, the cortical representation of their function, have motivated intense scientific interest. The principal study of the visual channel associated with S-cone projections has been conducted using psychophysical, neurophysiological, and ex vivo anatomical techniques, whereas more recent research on the pathway has employed functional magnetic resonance imaging (fMRI). The purpose of this manuscript is to present a perspective regarding the means by which color signals within this visual channel are processed in the brain, namely how differences in short-wavelength light transmission caused by intraocular, pre-receptoral filtration are compensated for. Recent results from fMRI and psychophysical studies indicate the existence of a frequency-dependent signal amplification mechanism, whereby lower frequencies result in an amplification of S-cone signals. This finding could motivate a future research direction for determining the localization of blue-yellow color processing and neural compensation in the blue-yellow visual channel.