The photoreceptor process in vision

Brightness and hue judgment of dyed hair color

This study aims to investigate human hair color perception through two empirical studies in the context of colored hair. The preliminary study was intended to establish a numerical representation of perceptually meaningful brightness levels. It identified that the brightness level was proportional to the power of 0.766 of L*. In the visual assessment, participants (N = 47) categorized 246 hair color samples into eight color hue groups aligned with the Munsell system. Hue judgment was conducted by visually comparing dyed hair tresses with natural black hair. Based on the L*, a*, and b* values of hair tresses and visual assessments thereof, we observed the ranges of hue categories for hair color alongside the brightness levels. Additionally, the differences between the Munsell hue names and the assessment results were compared. Predominantly influenced by the dark brown hair color, the neutral orientation was shifted to the first quadrant of the a*-b* plane. The study contributes to an understanding of human hair color perception and provides insights into color categorization and labeling, especially when the context is confined.

Vitamin A fortification: Recent advances in encapsulation technologies

Vitamin A is an essential micronutrient whose deficiency is still a major health concern in many regions of the world. It plays an essential role in human growth and development, immunity, and vision, but may also help prevent several other chronic diseases. The total amount of vitamin A in the human diet often falls below the recommended dietary allowance of approximately 900-1000 μ $ \umu $ g/day for a healthy adult. Moreover, a significant proportion of vitamin A may be degraded during food processing, storage, and distribution, thereby reducing its bioactivity. Finally, the vitamin A in some foods has a relatively low bioavailability, which further reduces its efficacy. The World Health Organization has recommended fortification of foods and beverages as a safe and cost-effective means of addressing vitamin A deficiency. However, there are several factors that must be overcome before effective fortified foods can be developed, including the low solubility, chemical stability, and bioavailability of this oil-soluble vitamin. Consequently, strategies are required to evenly disperse the vitamin throughout food matrices, to inhibit its chemical degradation, to avoid any adverse interactions with any other food components, to ensure the food is palatable, and to increase its bioavailability. In this review article, we discuss the chemical, physical, and nutritional attributes of vitamin A, its main dietary sources, the factors contributing to its current deficiency, and various strategies to address these deficiencies, including diet diversification, biofortification, and food fortification.

Gold nanostructures: synthesis, properties, and neurological applications

Recent advances in technology are expected to increase our current understanding of neuroscience. Nanotechnology and nanomaterials can alter and control neural functionality in both in vitro and in vivo experimental setups. The intersection between neuroscience and nanoscience may generate long-term neural interfaces adapted at the molecular level. Owing to their intrinsic physicochemical characteristics, gold nanostructures (GNSs) have received much attention in neuroscience, especially for combined diagnostic and therapeutic (theragnostic) purposes. GNSs have been successfully employed to stimulate and monitor neurophysiological signals. Hence, GNSs could provide a promising solution for the regeneration and recovery of neural tissue, novel neuroprotective strategies, and integrated implantable materials. This review covers the broad range of neurological applications of GNS-based materials to improve clinical diagnosis and therapy. Sub-topics include neurotoxicity, targeted delivery of therapeutics to the central nervous system (CNS), neurochemical sensing, neuromodulation, neuroimaging, neurotherapy, tissue engineering, and neural regeneration. It focuses on core concepts of GNSs in neurology, to circumvent the limitations and significant obstacles of innovative approaches in neurobiology and neurochemistry, including theragnostics. We will discuss recent advances in the use of GNSs to overcome current bottlenecks and tackle technical and conceptual challenges.

Adaptations and evolutionary trajectories of the snake rod and cone photoreceptors

Most vertebrates have duplex retinas, with two classes of photoreceptors, rods and cones. In the group of Snakes, however, distinct patterns of retinal morphology are associated with transitions between diurnal-nocturnal habits and reflect important adaptations of their visual system. Pure-cone, pure-rod and duplex retinas were described in different species, and this variability led Gordon Walls (1934) to formulate the transmutation theory, which suggests that rods and cones are not fixed entities, but can assume transitional states. Three opsin genes are expressed in retinas of most snake species, lws, rh1, and sws1, and recent studies have shown that the rhodopsin gene, rh1, is expressed in pure-cone retinas of diurnal snakes. This expression raised many questions about the nature of transmutation and functional aspects of the rhodopsin in a cone-like photoreceptor. Extreme differences in the retinal architecture of diurnal and nocturnal snakes also highlight the complexity of adaptations of their visual structures, which might have contributed to the adaptive radiation of this group and will be discussed in this review.

Origin and adaptation of green-sensitive (RH2) pigments in vertebrates

One of the critical times for the survival of animals is twilight where the most abundant visible lights are between 400 and 550 nanometres (nm). Green-sensitive RH2 pigments help nonmammalian vertebrate species to better discriminate wavelengths in this blue-green region. Here, evaluation of the wavelengths of maximal absorption (λmax s) of genetically engineered RH2 pigments representing 13 critical stages of vertebrate evolution revealed that the RH2 pigment of the most recent common ancestor of vertebrates had a λmax of 503 nm, while the 12 ancestral pigments exhibited an expanded range in λmax s between 474 and 524 nm, and present-day RH2 pigments have further expanded the range to ~ 450-530 nm. During vertebrate evolution, eight out of the 16 significant λmax shifts (or |Δλmax | ≥ 10 nm) of RH2 pigments identified were fully explained by the repeated mutations E122Q (twice), Q122E (thrice) and M207L (twice), and A292S (once). Our data indicated that the highly variable λmax s of teleost RH2 pigments arose from gene duplications followed by accelerated amino acid substitution.

Reduced Fading of Visual Afterimages after Transcranial Magnetic Stimulation over Early Visual Cortex

In the complete absence of small transients in visual inputs (e.g., by experimentally stabilizing an image on the retina or in everyday life during intent staring), information perceived by the eyes will fade from the perceptual experience. Although the mechanisms of visual fading remain poorly understood, one possibility is that higher level brain regions actively suppress the stable visual signals via targeted feedback onto early visual cortex (EVC). Here, we used positive afterimages and multisensory conflict to induce gestalt-like fading of participants' own hands. In two separate experiments, participants rated the perceived quality of their hands both before and after transcranial magnetic stimulation (TMS) was applied over EVC. In a first experiment, triple-pulse TMS was able to make a faded hand appear less faded after the pulses were applied, compared with placebo pulses. A second experiment demonstrated that this was because triple-pulse TMS slowed down fading of the removed hand that otherwise occurs naturally over time. Interestingly, TMS similarly affected the left and right hands, despite being applied only over the right EVC. Together, our results suggest that TMS over EVC attenuates the effects of visual fading in positive afterimages, and it might do so by crossing transcollosal connections or via multimodal integration sites in which both hands are represented.

Nutritional Status Measures Are Correlated with Pupillary Responsiveness in Zambian Children

Background

Impairments in visual function have been well characterized in vitamin A deficiency. However, eye function may also be sensitive to other nutrient deficiencies.

Objective

We examined associations between visual function-characterized by pupillary threshold or pupillary responsiveness-and nutritional status in Zambian children.

Methods

We used digital pupillometry to measure visual responses to calibrated light stimuli (-2.9 to 0.1 log cd/m2) among dark-adapted children aged 4-8 y (n = 542). We defined pupillary threshold as the first light stimulus at which pupil diameter decreased by ≥10% and considered a pupillary threshold ≥-0.9 log cd/m2 as impaired. Pupillary responsiveness was defined by absolute percentage of change in pupil diameter from pre- to poststimulus. We tested associations between these measures and serum concentrations of retinol, β-carotene, ferritin, soluble transferrin receptor, and hemoglobin (Hb <11.0 or 11.5 g/dL were used to define anemia, depending on age), as well as anthropometric indexes, with the use multilevel mixed-effects models.

Results

Pupillary threshold was correlated only with serum retinol (r = 0.12, P < 0.05). The strongest correlates of pupillary responsiveness were Hb (r = -0.16, P < 0.01), height-for-age z score (r = 0.14, P < 0.05), weight-for-age z score (r = 0.14, P < 0.05), and soluble transferrin receptor (r = 0.12, P < 0.05). In multivariate models, anemia was positively associated with pupillary responsiveness (β = 2.99; 95% CI: 1.26, 4.72).

Conclusions

In this marginally nourished population, we found positive correlations between vitamin A status, iron status, or anthropometric indexes and visual function. Hb was negatively associated with visual function, with greater pupillary responsiveness among anemic children. We posit that this may signal altered parasympathetic activity, possibly driven by infection. Future studies should consider a broader range of indicators to better characterize the relation between nutrition and visual function. This trial was registered at clinicaltrials.gov as NCT01695148.

Flexible Neural Hardware Supports Dynamic Computations in Retina

The ability of the retina to adapt to changes in mean light intensity and contrast is well known. Classically, however, adaptation is thought to affect gain but not to change the visual modality encoded by a given type of retinal neuron. Recent findings reveal unexpected dynamic properties in mouse retinal neurons that challenge this view. Specifically, certain cell types change the visual modality they encode with variations in ambient illumination or following repetitive visual stimulation. These discoveries demonstrate that computations performed by retinal circuits with defined architecture can change with visual input. Moreover, they pose a major challenge for central circuits that must decode properties of the dynamic visual signal from retinal outputs.

A simple method for studying the molecular mechanisms of ultraviolet and violet reception in vertebrates

BACKGROUND

Many vertebrate species use ultraviolet (UV) reception for such basic behaviors as foraging and mating, but many others switched to violet reception and improved their visual resolution. The respective phenotypes are regulated by the short wavelength-sensitive (SWS1) pigments that absorb light maximally (λmax) at ~360 and 395-440 nm. Because of strong epistatic interactions, the biological significance of the extensive mutagenesis results on the molecular basis of spectral tuning in SWS1 pigments and the mechanisms of their phenotypic adaptations remains uncertain.

RESULTS

The magnitudes of the λmax-shifts caused by mutations in a present-day SWS1 pigment and by the corresponding forward mutations in its ancestral pigment are often dramatically different. To resolve these mutagenesis results, the A/B ratio, in which A and B are the areas formed by amino acids at sites 90, 113 and 118 and by those at sites 86, 90 and 118 and 295, respectively, becomes indispensable. Then, all critical mutations that generated the λmax of a SWS1 pigment can be identified by establishing that 1) the difference between the λmax of the ancestral pigment with these mutations and that of the present-day pigment is small (3 ~ 5 nm, depending on the entire λmax-shift) and 2) the difference between the corresponding A/B ratios is < 0.002.

CONCLUSION

Molecular adaptation has been studied mostly by using comparative sequence analyses. These statistical results provide biological hypotheses and need to be tested using experimental means. This is an opportune time to explore the currently available and new genetic systems and test these statistical hypotheses. Evaluating the λmaxs and A/B ratios of mutagenized present-day and their ancestral pigments, we now have a method to identify all critical mutations that are responsible for phenotypic adaptation of SWS1 pigments. The result also explains spectral tuning of the same pigments, a central unanswered question in phototransduction.

Recurrent episodes of night blindness in a patient with short bowel syndrome

PURPOSE

To describe clinical characteristics in a patient with recurrent episodes of night blindness due to vitamin A deficiency caused by short bowel syndrome in Crohn disease.

METHODS

Retrospective analysis of best-corrected visual acuity (BCVA), kinetic perimetry, slit-lamp biomicroscopy, ophthalmoscopy, fundus photography, fundus autofluorescence (FAF), spectral domain optical coherence tomography (SD-OCT), dark adaptometry (DA) and electroretinography (ERG). Serum vitamin A level was measured.

RESULTS

A 44-year-old man with a 3-year history of night blindness suffered from a short bowel syndrome with chronic malabsorption due to ileocecal resection in Crohn disease. Both eyes had a BCVA of 0.9, Bitot's spots of the conjunctiva and no significant fundus abnormalities. SD-OCT showed no remarkable changes, whereas FAF was brighter than normal in the center of the fovea. DA showed normal cone and a lack of rod function. The dark-adapted 0.01 ERG was non-detectable, the dark-adapted 3.0 ERG severely diminished, but the light-adapted 3.0 and 30 Hz flicker ERGs were within normal limits. Serum vitamin A level was 0.11 µg/ml (normal 0.30-0.65 µg/ml). Treatment with intravenous vitamin A caused a rapid recovery of night vision and ERG. However, during the following 3 years, he had three further episodes of night blindness with loss of rod function. During each period, parenteral vitamin A substitution brought complete recovery of night vision and rod function. BCVA, fundus, FAF findings and SD-OCT remained unchanged during the course.

CONCLUSIONS

In patients with known chronic malabsorption serum vitamin A level should be regularly checked to avoid recurrent night blindness episodes. ERG might be more sensitive than serum vitamin A level and is recommended in case of night blindness but still normal vitamin A level.

On the 'discovery' of vitamin A

Vitamin A is essential for normal growth, reproduction, immunity, and vision. The characterization of vitamin A spanned a period of about 130 years. During this long, incremental process, there is no single event that can be called the 'discovery' of vitamin A. The physiologist François Magendie conducted nutritional deprivation experiments with dogs in 1816 that resulted in corneal ulcers and high mortality - a finding similar to the common clinical situation in poorly fed, abandoned infants in Paris. In the 1880s, Nicolai Lunin showed that there was an unknown substance in milk that was essential for nutrition. Carl Socin suggested that an unknown substance for growth in egg yolk was fat soluble. Frederick Gowland Hopkins proposed in 1906 that there were 'unsuspected dietetic factors' that were necessary for life. In 1911, Wilhelm Stepp demonstrated that this essential substance in milk was fat soluble. The following year, Hopkins showed that there were 'accessory factors' present in 'astonishingly small amounts' in milk that supported life. Contrary to the dogma that all fats had similar nutritional value, in 1913, Elmer McCollum and Marguerite Davis at Wisconsin and Thomas Osborne and Lafayette Mendel at Yale showed butter and egg yolk were not equivalent to lard and olive oil in supporting the growth and survival of rats. The growth-supporting 'accessory factor' became known as 'fat-soluble A' in 1918 and then 'vitamin A' in 1920. Paul Karrer described the chemical structure of vitamin A in 1932. Harry Holmes and Ruth Corbet isolated and crystallized vitamin A in 1937. Methods for the synthesis of vitamin A came with the work of David Adriaan van Dorp and Jozef Ferdinand Arens in 1946 and Otto Isler and colleagues in 1947. Further work on the role of vitamin A in immunity and child survival continued until through the 1990s.

Photoreceptor structure and development analyses using GFP transgenes

In recent years, studies of zebrafish rod and cone photoreceptors have yielded novel insights into the differentiation of distinct photoreceptor cell types and the mechanisms guiding photoreceptor regeneration following cell death, and they have provided models of human retinal degeneration. These studies were facilitated by the use of transgenic zebrafish expressing fluorescent reporter genes under the control of various cell-specific promoters. Improvements in transgenesis techniques (e.g., Tol2 transposition), the availability of numerous fluorescent reporter genes with different localization properties, and the ability to generate transgenes via recombineering (e.g., Gateway technology) have enabled researchers to quickly develop transgenic lines that improve our understanding of the causes of human blindness and ways to mitigate its effects.

The interphotoreceptor retinoid binding (IRBP) is essential for normal retinoid processing in cone photoreceptors

11-cis Retinal is the light-sensitive component in rod and cone photoreceptors, and its isomerization to all-trans retinal in the presence of light initiates the visual response. For photoreceptors to function normally, all-trans retinal must be converted back into 11-cis retinal through the visual cycle. While rods are primarily responsible for dim light vision, the ability of cones to function in constant light is essential to human vision and may be facilitated by cone-specific visual cycle pathways. The interphotoreceptor retinoid-binding protein (IRBP) is a proposed retinoid transporter in the visual cycle, but rods in Irbp ( -/- ) mice have a normal visual cycle. However, there is evidence that IRBP has cone-specific functions. Cone electroretinogram (ERG) responses are reduced, despite having cone densities and opsin levels similar to C57Bl/6 (WT) mice. Treatment with 9-cis retinal rescues the cone response in Irbp ( -/- ) mice and shows that retinoid deficiency underlies cone dysfunction. These data indicate that IRBP is essential to normal cone function and demonstrate that differences exist in the visual cycle of rods and cones.

Normal cone function requires the interphotoreceptor retinoid binding protein

11-cis-retinal is the light-sensitive component in rod and cone photoreceptors, and its isomerization to all-trans retinal in the presence of light initiates the visual response. For photoreceptors to function normally, all-trans retinal must be converted back into 11-cis-retinal through a series of enzymatic steps known as the visual cycle. The interphotoreceptor retinoid-binding protein (IRBP) is a proposed retinoid transporter in the visual cycle, but rods in Irbp(-/-) mice have a normal visual cycle. While rods are primarily responsible for dim light vision, the ability of cones to function in constant light is essential to human vision and may be facilitated by cone-specific visual cycle pathways. We analyzed the cones in Irbp(-/-) mice to determine whether IRBP has a cone-specific visual cycle function. Cone electroretinogram (ERG) responses were reduced in Irbp(-/-) mice, but similar responses from Irbp(-/-) mice at all ages suggest that degeneration does not underlie cone dysfunction. Furthermore, cone densities and opsin levels in Irbp(-/-) mice were similar to C57BL/6 (wild-type) mice, and both cone opsins were properly localized to the cone outer segments. To test for retinoid deficiency in Irbp(-/-) mice, ERGs were analyzed before and after intraperitoneal injections of 9-cis-retinal. Treatment with 9-cis-retinal produced a significant recovery of the cone response in Irbp(-/-) mice and shows that retinoid deficiency underlies cone dysfunction. These data indicate that IRBP is essential to normal cone function and demonstrate that differences exist in the visual cycle of rods and cones.

Vitamin a deficiency and clinical disease: an historical overview

Vitamin A deficiency has a plethora of clinical manifestations, ranging from xerophthalmia (practically pathognomonic) to disturbances in growth and susceptibility to severe infection (far more protean). Like other classical vitamin deficiency states (scurvy, rickets), some of the signs and symptoms of xerophthalmia were recognized long ago. Reports related to vitamin A and/or manifestations of deficiency might conveniently be divided into "ancient" accounts; eighteenth to nineteenth century clinical descriptions (and their purported etiologic associations); early twentieth century laboratory animal experiments and clinical and epidemiologic observations that identified the existence of this unique nutrient and manifestations of its deficiency; and, most recently, a flowering of carefully conducted clinical studies and field-based randomized trials that documented the full extent and impact of deficiency among the poor of low- and middle-income countries, which in turn changed global health policy.

Submicroscopic organization of retinal cones of the rabbit

The fine structure of the cone cell of the rabbit is described and compared wtih that of the rod. The cone outer segment consists of a pile of flattened sacs with two membranes 30 A thick and a regular clear space in between of about 30 A. The membrane of the rod sacs is slightly thicker (∼40 A) and the clear space is less regular and frequently absent in the deeper regions. The distance between sacs is from 85 to 95 A in the cone and from 110 to 120 A in the rod, and the total repeating period is about 190 A and 210 A, respectively. These results are discussed in relation to the concentration of solids in both photoreceptors. A connecting cilium was observed in the cone cell and compared with that previously described in rods (4). This finding suggests that morphogenetically the cone may also result of the differentiation of a primitive cilium (5). The inner segment of the cone shows a distal portion with large concentration of elongated mitochondria and a proximal one with a large Golgi complex in the axis surrounded by components of the endoplasmic reticulum. It is concluded that both photoreceptors have a similar general plan of submicroscopic organization, with some minor difference in fine structure probably related to their specific chemical composition and function.

Molecular evolution of vertebrate visual pigments

Dramatic improvement of our understanding of the genetic basis of vision was brought by the molecular characterization of the bovine rhodopsin gene and the human rhodopsin and color opsin genes (Nathans and Hogness, 1983; Nathans et al., 1984, 1986a,b). The availability of cDNA clones from these studies has facilitated the isolation of retinal and nonretinal opsin genes and cDNA clones from a large variety of species. Today, the number of genomic and cDNA clones of opsin genes isolated from different vertebrate species exceeds 100 and is increasing rapidly. The opsin gene sequences reveal the importance of the origin and differentiation of various opsins and visual pigments. To understand the molecular genetic basis of spectral tuning of visual pigments, it is essential to establish correlations between a series of the sequences of visual pigments and their lambda(max) values. The potentially important amino acid changes identified in this way have to be tested whether they are in fact responsible for the lambda(max)-shifts using site-directed mutagenesis and cultured cells. A major goal of molecular evolutionary genetics is to understand the molecular mechanisms involved in functional adaptations of organisms to different environments, including the mechanisms of the regulation of the spectral absorption. Therefore, both molecular evolutionary analyses of visual pigments and vision science have an important common goal.

Molecular genetic basis of adaptive selection: examples from color vision in vertebrates

A central unanswered question in phototransduction is how photosensitive molecules, visual pigments, regulate their absorption spectra. In nature, there exist various types of visual pigments that are adapted to diverse photic environments. To elucidate the molecular mechanisms involved in the adaptive selection of these pigments, we have to identify amino acid changes of pigments that are potentially important in changing the wavelength of maximal absorption (lambda max) and then determine the effects of these mutations on the shift in lambda max. The desired mutants can be constructed using site-directed mutagenesis, expressed in tissue culture cells, and the functional effect of virtually any such mutant can be rigorously determined. The availability of these cell/molecular methods makes vision an ideal model system in studying adaptive mechanisms at the molecular level. The identification of potentially important amino acid changes using evolutionary biological means is an indispensable step in elucidating the molecular mechanisms that underlie the spectral tuning of visual pigments.

LIGHT ADAPTATION KINETICS: THE INFLUENCE OF SPATIAL FACTORS

Reducing the target diameter of an adapting (conditioning) flash of light results in a progressive rise in the conventional light adaptation curve, as measured with a small superimposed test flash presented at the end of adapting flashes of variable duration. When both targets are the same size, an abrupt and marked rise in threshold is obtained, resulting from a unique effect that occurs near the termination of the adapting flash. This effect can be demonstrated by means of a variable delay procedure, and it indicates that neural as well as photo-chemical processes limit the time course of light adaptation.

VEP pattern after photostress: an index of macular function

Eighty-seven subjects were examined by means of visual evoked potentials (VEPs) to evaluate the recovery time of the amplitude of the pattern VEP after photostress. Twenty-eight subjects were normal and 59 patients were affected by carotid occlusive disease, vascular retinopathies with and without macular involvement, and connective tissue diseases treated with antimalarial drugs. This examination method emphasizes visual dysfunction in a stage where ophthalmological changes are absent or minimal and therefore may be a useful test for the investigation of macular function.

Nutritional blindness in Africa

Nutritional blindness is loss of useful vision resulting from vitamin deficiency. A malnutrition Xerophthalmia means all the ocular manifestations of inadequate metabolism of vitamin A, nutritional blindness being the end result of the most severe cases. The estimated overall prevalence of nutritional blindness in Africa is very low, below the WHO levels of significance, although isolated clusters of locally high prevalence exist, usually in arid, sparsely-populated regions. The peak age group affected is 2-year olds, with most nutritional blindness having its effect before age 6 years. Xerophthalmia may be considered as a serious side effect of protein-energy malnutrition (PEM). When associated with corneal sequelae of xerophthalmia, PEM has an estimated overall mortality of 50%. Intervention programs, therefore, are more appropriately aimed at the broader condition of life-threatening PEM than at the specifically vision-threatening problem of xerophthalmia. Parameters of significant prevalence detection are discussed, and the relative merits of different forms of vitamin A-specific intervention programs are weighed.