Cone Photopigments Research Articles

Poster Session I: Vernier thresholds of a Poisson-noise-limited computational observer with and without fixational eye movements.

Vernier acuity is a fundamental measure of spatial vision. We modeled how stimulus encoding by the cones limits Vernier acuity. We determined Vernier thresholds for a computational observer that had access to the Poisson-distributed cone photopigment excitations. The observer also had access to the cone mosaic layout and the stimulus possibilities on each trial. We varied stimulus contrast (100%, 50%, 22%, 11% Michelson contrast) and duration (2, 4, 9, 18 stimulus frames; frame duration 8.33 ms) while fixing other stimulus properties (foveal viewing; two achromatic vertical bars; length 6.2 arcmin; width 1 arcmin; vertical gap 0.1 arcmin). When the retinal image is stationary, Vernier thresholds depend jointly on contrast and duration through contrast energy: squared contrast times duration. Introducing fixational drift eye movements impairs performance, when the information about eye path is not accounted for by the computational observer. When the path of fixational drift is made available and used ideally, there is no noticeable difference with the stationary case. The lack of improvement when the path of fixational drift is known exactly may reflect the high-density of foveal cones relative to the optical point spread function and the fact that we did not introduce temporal filtering by the visual system. Our results suggest the possibility of a rich interaction between optics, cone sampling, fixational eye movements, post-receptoral filtering and visual performance.

Boosting 2-photon vision with adaptive optics.

The 2-photon effect in vision occurs when two photons of the same wavelength are absorbed by cone photopigment in the retina and create a visual sensation matching the appearance of light close to half their wavelength. This effect is especially salient for infrared light, where humans are mostly insensitive to 1-photon isomerizations and thus any perception is dominated by 2-photon isomerizations. This phenomenon can be made more readily visible using short-pulsed lasers, which increase the likelihood of 2-photon excitation by making photon arrivals at the retina more concentrated in time. Adaptive optics provides another avenue for enhancing the 2-photon effect by focusing light more tightly at the retina, thereby increasing the spatial concentration of incident photons. This article makes three contributions. First, we demonstrate through color-matching experiments that an adaptive optics correction can provide a 25-fold increase in the luminance of the 2-photon effect-a boost equivalent to reducing pulse width by 96%. Second, we provide image-based evidence that the 2-photon effect occurs at the photoreceptor level. Third, we use our results to compute the specifications for a system that could utilize 2-photon vision and adaptive optics to image and stimulate the retina using a single infrared wavelength and reach luminance levels comparable to conventional displays.

Award Lecture: 2021 Tillyer Award Lecture: The Early Visual Encoding: Computations and Consequences.

The visual system takes sensory measurements of the light incident at the eyes and uses these to make perceptual inferences about external world. The sensory measurements do not preserve all of the information available in the light signal. One approach to understanding the implications of the initial stages of visual processing is ideal observer analysis, which evaluates the information available to support psychophysical discriminations at various stages of the early visual representation. We are interested in extending this type of analysis to take into account the statistical structure of natural images. To do so, we developed an open-source computational model of the initial visual encoding, ISETBio (isetbio.org). ISETBio incorporates specification of visual displays, retinal image formation through the eye's optics, spatio-spectral sampling by the retinal cone mosaic, Poisson noise in the cone photopigment excitations, transduction of excitations to photocurrent, and fixational eye movements. In this talk, I will introduce ISETBio and illustrate a set of insights it enables about visual processing by reviewing a number of computational examples. These examples will include ways that combining ISETBio with Bayesian image-reconstruction methods helps us understand how the interaction of the visual encoding and the statistical structure of natural images shapes visual performance.

Formulae for generating standard and individual human cone spectral sensitivities.

Normal color perception is complicated. But at its initial stage it is relatively simple, since at photopic levels it depends on the activations of just three photoreceptor types: the long- (L-), middle- (M-) and short- (S-) wavelength-sensitive cones. Knowledge of how each type responds to different wavelengths-the three cone spectral sensitivities-can be used to model human color vision and in practical applications to specify color and predict color matches. The CIE has sanctioned the cone spectral sensitivity estimates of Stockman and Sharpe (Stockman and Sharpe, 2000, Vision Res) and their associated measures of luminous efficiency as "physiologically-relevant" standards for color vision (CIE, 2006; 2015). These LMS cone spectral sensitivities are specified at 5- and 1-nm steps for mean "standard" observers with normal cone photopigments and average ocular transparencies, both of which can vary in the population. Here, we provide formulae for the three cone spectral sensitivities as well as for macular and lens pigment density spectra, all as continuous functions of wavelength from 360 to 850 nm. These functions reproduce the tabulated discrete CIE LMS cone spectral sensitivities for 2-deg and 10-deg with little error in both linear and logarithmic units. Furthermore, these formulae allow the easy computation of non-standard cone spectral sensitivities (and other color matching functions) with individual differences in macular, lens and photopigment optical densities, and with spectrally shifted hybrid or polymorphic L- and M-cone photopigments appropriate for either normal or red-green color vision deficient observers.

Functional Duplication of the Short-Wavelength-Sensitive Opsin in Sea Snakes: Evidence for Reexpanded Color Sensitivity Following Ancestral Regression

Abstract Color vision is mediated by ancient and spectrally distinct cone opsins. Yet, while there have been multiple losses of opsin genes during the evolution of tetrapods, evidence for opsin gains via functional duplication is extremely scarce. Previous studies have shown that some secondarily marine elapid snakes have acquired expanded “UV–blue” sensitivity via changes at key spectral tuning amino acid sites of the Short-Wavelength Opsin 1 (SWS1) gene. Here, we use elapid reference genomes to show that the molecular origin of this adaptation involved repeated, proximal duplications of the SWS1 gene in the fully marine Hydrophis cyanocinctus. This species possesses four intact SWS1 genes; two of these genes have the ancestral UV sensitivity, and two have a derived sensitivity to the longer wavelengths that dominate marine habitats. We suggest that this remarkable expansion of the opsin repertoire of sea snakes functionally compensates for the ancestral losses of two middle-wavelength opsins in the earliest (dim-light adapted) snakes. This provides a striking contrast to the evolution of opsins during ecological transitions in mammals. Like snakes, early mammals lost two cone photopigments; however, lineages such as bats and cetaceans underwent further opsin losses during their adaptation to dim-light environments.

Characterizing cone spectral classification by optoretinography.

Light propagation in photoreceptor outer segments is affected by photopigment absorption and the phototransduction amplification cascade. Photopigment absorption has been studied using retinal densitometry, while recently, optoretinography (ORG) has provided an avenue to probe changes in outer segment optical path length due to phototransduction. With adaptive optics (AO), both densitometry and ORG have been used for cone spectral classification based on the differential bleaching signatures of the three cone types. Here, we characterize cone classification by ORG, implemented in an AO line-scan optical coherence tomography (OCT), and compare it against densitometry. The cone mosaics of five color normal subjects were classified using ORG showing high probability (∼0.99), low error (<0.22%), high test-retest reliability (∼97%), and short imaging durations (< 1 hour). Of these, the cone spectral assignments in two subjects were compared against AO-scanning laser opthalmoscope densitometry. High agreement (mean: 91%) was observed between the two modalities in these two subjects, with measurements conducted 6-7 years apart. Overall, ORG benefits from higher sensitivity and dynamic range to probe cone photopigments compared to densitometry, and thus provides greater fidelity for cone spectral classification.

Smartphone Color Vision Testing as an Alternative to the Conventional Ishihara Booklet.

Introduction Color vision testing was first seen as a parameter to be tested in the 1700s. Nowadays, it is a well-known phenomenon with significant quality-of-life implications. Structures involved in color vision include the lens, pupil, retinal cone photopigments, and several photoreceptor processes that translate the incoming spectrum of different light wavelengths into a processed colored image. An initial color vision assessment was made simply by comparing the color perception of the individual to that of the examiner. The most commonly used tools to screen for color vision defects today are color plates, such as the Ishihara color plates. In the modern age, smartphones have evolved to become an essential part of our everyday lives with applications such as Eye Handbook, which allow easier access to color vision testing using color plates displayed on smartphone screens. In this study, we compared color vision testing on Android and iOS devices to the standard Ishihara booklet. Materials and methods A cross-sectional validation study was performed on patients presenting to the Armed Forces Institute of Ophthalmology, Rawalpindi, Pakistan for six months. The sample size collected was 162 with a 95% confidence interval. The age range of the sample population was kept at 12-70 years. A patient was selected for participation in the study, and a color vision assessment was performed using the Ishihara color plates and Android and iOS smartphones. The collected data was then entered into IBM SPSS (Statistical Package for the Social Sciences) Statistics 25 for analysis, with the p-value being kept at 0.05. Results The sample size was 162, with the gender distribution being predominantly male (69.14%). The average age of the participants was 35.94 (SD = 12.04). The result of the two-tailed paired sample z-testwas not significant based on a p-value of 0.565, indicating the null hypothesis cannot be rejected. This finding suggests the difference between the mean of Ishihara and the mean of the iPhone was not significantly different from zero. Similar results were found for comparisons between Android smartphones and the Ishihara booklet. Conclusions Previous studies conducted showed nearly 60% of subjects with normal color vision correctly identified all colors on standard Ishihara color plates. The two-tailed paired sample t-test conducted in our study showed no significant difference between either of the smartphone groups (iPhone or Android) and the Ishihara booklet group, indicating that smartphones present a viable alternative to standard Ishihara booklet testing. However, there are certain limitations to our study. Different types of smartphone screens present a challenge in standardization while testing color vision, something that is not a problem when using the Ishihara booklet. However, smartphones are more widely available, more versatile, and present far greater ease of access. Both these factors should be considered when comparing the two in future studies.

Assessment of photoreceptor function with ultrafast retinal densitometry.

The optical density of visual pigment can be measured by imaging the dark-adapted eye while bleaching with visible light. This measurement can be made for individual photoreceptor cells using adaptive optics; however, activation of the phototransduction cascade imparts rapid changes in phase that modulate the signal via optical interference. This limits utility because data must be averaged over many experimental runs. Here we used a "flood" illuminated adaptive optics system at 4000 fps, bright light to achieve rapid bleaching, and broad illumination bandwidth to mitigate interference effects. Data were super-resolved using the natural motion of the eye to overcome the reduced pixel resolution of the ultrafast camera. This approach was applied to classify the trichromatic cone photoreceptor mosaic at a single fixation locus within the foveal region of 3 healthy subjects. Subjects were dark adapted for 6 minutes to replenish cone photopigment. This was followed either directly by imaging at 555 ± 50 nm, or by first pre-adapting the retina to 700 nm light to preferentially deplete "L" cone pigment. A total of 3,252 cones were classified as either "S", "M", or "L" type based on clustering of the intensity data observed under these two conditions. Mean classification probability ranged from 99.3 to 99.8%, with individual cell probabilities exceeding 95% in 97.0 to 99.2% of cones. Accuracy of cone classification peaked when using the first 10-30 ms of data, with significant reductions in accuracy noted with the inclusion of data from later times. Our results show that rapid bleaching and data acquisition significantly improve the robustness of cell-resolved densitometry.

Do You See What I See? Diversity in Human Color Perception.

In our tendency to discuss the objective properties of the external world, we may fail to notice that our subjective perceptions of those properties differ between individuals. Variability at all levels of the color vision system creates diversity in color perception, from discrimination to color matching, appearance, and subjective experience, such that each of us lives in a unique perceptual world. In this review, I discuss what is known about individual differences in color perception and its determinants, particularly considering genetically mediated variability in cone photopigments and the paradoxical effects of visual environments in both contributing to and counteracting individual differences. I make the case that, as well as being of interest in their own right and crucial for a complete account of color vision, individual differences can be used as a methodological tool in color science for the insights that they offer about the underlying mechanisms of perception.

Human retinal dark adaptation tracked in vivo with the electroretinogram: insights into processes underlying recovery of cone- and rod-mediated vision.

The substantial time taken for regaining visual sensitivity (dark adaptation) following bleaching exposures has been investigated for over a century. Psychophysical studies yielded the classic biphasic curve representing recovery of cone-driven and rod-driven vision. The electroretinogram (ERG) permits direct assessment of recovery at the level of the retina (photoreceptors, bipolar cells), with the first report over 70 years ago. Over the last two decades, ERG studies of dark adaptation have generated insights into underlying physiological processes. After large bleaches, rod photoreceptor circulating current, estimated from the rod-isolated bright-flash ERG a-wave, takes 30min to recover, indicating that products of bleaching, thought to be free opsin (unbound to 11-cis-retinal), continue to activate phototransduction, shutting off rod circulating current. In contrast, cone current, assessed with cone-driven bright-flash ERG a-waves, recovers within 100ms following similar exposures, suggesting that free opsin is less able to shut off cone current. The cone-driven dim-flash a-wave can be used to track recovery of cone photopigment, showing regeneration is 'rate-limited' rather than first order. Recoveries of the dim-flash ERG b-wave are consistent also with rate-limited rod photopigment regeneration (where free opsin, desensitising the visual system as an 'equivalent background', is removed by rate-limited delivery of 11-cis-retinal). These findings agree with psychophysical and retinal densitometry studies, although there are unexplained points of divergence. Post-bleach ERG recovery has been explored in age-related macular degeneration and in trials of visual cycle inhibitors for retinal diseases. ERG tracking of dark adaptation may prove useful in future clinical contexts.

IMPAIRMENTS OF L-CONE/M-CONE AND S-CONE-MEDIATED COLOR DISCRIMINATION IN MACULAR TELANGIECTASIA TYPE II.

To characterize red-green and tritan color discrimination in eyes with macular telangiectasia Type II (MacTel). Color discrimination was assessed by metameric matching methods using an Oculus MR Anomaloscope. Red-green color discrimination was assessed using the Rayleigh equation, and tritan color discrimination was assessed using the Moreland equation. Results were expressed as anomalquotient (AQ) and tritanomalquotient (TAQ) units, respectively. Seventeen eyes with MacTel were compared with 16 control eyes with normal vision. Twelve eyes with MacTel demonstrated abnormal color matches; except for two eyes with red-shifted Rayleigh matches, the primary abnormality evident was reduced color discrimination. On average, Rayleigh matching ranges were significantly widened in MacTel (0.518 ± 0.066 AQ units) compared with normal (0.14 ± 0.03 AQ units; P < 0.0001). Similarly, Moreland matching ranges were significantly wider (0.794 ± 0.109 TAQ units) than normal control subjects (0.204 ± 0.070 TAQ units; P < 0.0001). Losses in color discrimination did not correlate significantly with the best-corrected visual acuity, although Moreland matching ranges were significantly correlated to Rayleigh matching ranges. MacTel results in a combined acquired red-green and tritan color vision deficiency. A minority of eyes demonstrated red-shifted Rayleigh matches, consistent with decreases in cone photopigment optical density.

IMPAIRMENTS IN CONE PIGMENT REGENERATION AND ABSOLUTE THRESHOLD IN MACULAR TELANGIECTASIA TYPE 2.

To test the hypothesis that Müller cell dysfunction in macular telangiectasia type 2 (MacTel) results in delayed cone adaptation kinetics and to assess absolute cone and rod thresholds in this condition. Adaptation after an approximate 63.5% full-field cone photopigment bleach was assessed for Goldmann size V (1.7° diameter) 640 nm (red) and 480 nm (blue) targets presented at a retinal locus corresponding to 2° temporal to fixation. The cone time constant of adaptation and absolute cone and rod thresholds were calculated from exponential functions fitted to the resultant dark adaptation curves. Eighteen eyes with MacTel (from 11 patients) were compared with 19 control eyes (from 16 normal subjects). Cone adaptation kinetics were significantly impaired in MacTel, as was the absolute cone threshold. Final thresholds for blue targets were also significantly elevated in MacTel, consistent with impaired rod absolute threshold. Losses in sensitivity observed in MacTel were consistent with a so-called d1/2 mechanism (i.e., receptoral) site of sensitivity loss. In addition to previously documented impairments in rod dark adaptation, MacTel results in a significant elevation in cone thresholds because of pathology at the level of the photoreceptors. The delays in cone adaptation that we found in eyes with MacTel may reflect impairment of the Müller cell-mediated cone-specific visual cycle.

Distinct type II opsins in the eye decode light properties for background adaptation and behavioural background preference.

Crypsis increases survival by reducing predator detection. Xenopus laevis tadpoles decode light properties from the substrate to induce two responses: a cryptic coloration response where dorsal skin pigmentation is adjusted to the colour of the substrate (background adaptation) and a behavioural crypsis where organisms move to align with a specific colour surface (background preference). Both processes require organisms to detect reflected light from the substrate. We explored the relationship between background adaptation and preference and the light properties able to trigger both responses. We also analysed which retinal photosensor (type II opsin) is involved. Our results showed that these two processes are segregated mechanistically, as there is no correlation between the preference for a specific background with the level of skin pigmentation, and different dorsal retina-localized type II opsins appear to underlie the two crypsis modes. Indeed, inhibition of melanopsin affects background adaptation but not background preference. Instead, we propose pinopsin is the photosensor involved in background preference. pinopsin mRNA is co-expressed with mRNA for the sws1 cone photopigment in dorsally located photoreceptors. Importantly, the developmental onset of pinopsin expression aligns with the emergence of the preference for a white background, but after the background adaptation phenotype appears. Furthermore, white background preference of tadpoles is associated with increased pinopsin expression, a feature that is lost in premetamorphic froglets along with a preference for a white background. Thus, our data show a mechanistic dissociation between background adaptation and background preference, and we suggest melanopsin and pinopsin, respectively, initiate the two responses.

Intraperitoneal chromophore injections delay early-onset and rapid retinal cone degeneration in a mouse model of Leber congenital amaurosis

Highly expressed in the retinal pigment epithelium (RPE), the RPE-specific 65-kDa (RPE65) enzyme is indispensable to generate 11-cis-retinal (11cRAL), a chromophore for rhodopsin and cone photopigments. RPE65 deficiency can lead to Leber congenital amaurosis type 2 (LCA2), in which the isomerization of photobleached all-trans-retinal into photosensitive 11cRAL is blocked, ultimately causing severe retinal dysfunction and degeneration. The related mouse models, which are constructed through gene knockout or caused by spontaneous mutations, morphologically present with early-onset and rapid retinal cone cells degeneration, including loss of short-wavelength-sensitive cone opsins (S-opsins) and mislocalization of medium-wavelength-sensitive cone opsins (M-opsins). Studies have shown that routine Rpe65 gene replacement therapy, mediated by an adeno-associated virus (AAV) vector, can restore RPE65 protein. However, AAV transfection and Rpe65 transgene expression require at least one to two weeks, and the treatment cannot fully block the early-onset cone degeneration. To determine the feasibility of delaying cone degeneration before gene therapy, we investigated the impact of 11cRAL treatment in an early-age LCA2 retinal degeneration 12 (rd12) mouse model. Similar to human patients, the mouse model carries a spontaneous mutation in the Rpe65 gene, which results in disrupted endogenous 11cRAL regeneration. We found that RPE65 deficiency did not notably affect rodent retinal vessels. Under red light illumination, the rd12 mice were intraperitoneally injected with exogenous 11cRAL from postnatal day (P) 14 to P21. Three days after the last injection, a notable recovery of retinal function was observed using scotopic and photopic electroretinograms. Using optical coherence tomography and histological analyses of the deficient retinas, we found changes in the thickness of the photoreceptor outer segment (OS); this change could be rescued by early 11cRAL treatment. In addition, the treatment notably preserved M- and S-opsins, both of which maintained appropriate localization inside cone cells, as shown by the wild-type mice. In contrast, the age-matched untreated rd12 mice were characterized by retinal S-opsin loss and M-opsin mislocalization from the photoreceptor OS to the inner segment, outer nuclear layer, or outer plexiform layer. Notably, 11cRAL treatment could not maintain retinal function for a long time. Ten days after the last injection, the rod and M-cone electroretinograms significantly decreased, and S-cone responses almost extinguished. Our findings suggest that early 11cRAL treatment is useful for restoring retinal function and rescuing morphology in the rd12 mouse model, and the early-onset and rapid cone degeneration can be delayed before gene therapy.

Modeling individual variations in equiluminance settings.

Recently, we reported measurements of heterochromatic flicker photometry (HFP) in 22 young observers, with stimuli that (nominally) modulated only L- and M-cones and were kept at (approximately) a constant multiple of detection threshold. These equiluminance settings were represented as the angle in the (L, M) cone contrast plane, with the greenish peak of the flicker in quadrant II and the reddish peak in quadrant IV; equiluminance settings were reported as the greenish angle. The mean equiluminance angle was 116.3° (an M:L cone contrast ratio of −2 at equiluminance), but individual differences in the settings were substantial, with the variation across individuals almost five times larger than the within-subject precision in the settings. In the present study we sought to determine the degree to which we could account for our observers’ HFP settings by plausible variations in the macular pigment optical density (MPOD), the lens pigment optical density (LPOD), the cone photopigment optical densities (PPOD), and serine/alanine polymorphism in L-cone opsin (λmax shift). Most of the range of our measured equiluminance angles could be accounted for by these factors, although the largest two angles (smallest |ΔM/M: ΔL/L| ratio at equiluminance) could not. Individual differences in HFP have sometimes been taken to indicate variations in the ratio of L:M cone number; our results suggest that most of the individual differences in HFP might be equally well ascribed to physiological factors other than cone number. Simple linear models allow predictions of equiluminance angle, cone adapting level, and artifactual S-cone contrast from the values of the four factors considered here.

Short-wavelength-sensitive 2 (Sws2) visual photopigment models combined with atomistic molecular simulations to predict spectral peaks of absorbance.

For many species, vision is one of the most important sensory modalities for mediating essential tasks that include navigation, predation and foraging, predator avoidance, and numerous social behaviors. The vertebrate visual process begins when photons of the light interact with rod and cone photoreceptors that are present in the neural retina. Vertebrate visual photopigments are housed within these photoreceptor cells and are sensitive to a wide range of wavelengths that peak within the light spectrum, the latter of which is a function of the type of chromophore used and how it interacts with specific amino acid residues found within the opsin protein sequence. Minor differences in the amino acid sequences of the opsins are known to lead to large differences in the spectral peak of absorbance (i.e. the λmax value). In our prior studies, we developed a new approach that combined homology modeling and molecular dynamics simulations to gather structural information associated with chromophore conformation, then used it to generate statistical models for the accurate prediction of λmax values for photopigments derived from Rh1 and Rh2 amino acid sequences. In the present study, we test our novel approach to predict the λmax of phylogenetically distant Sws2 cone opsins. To build a model that can predict the λmax using our approach presented in our prior studies, we selected a spectrally-diverse set of 11 teleost Sws2 photopigments for which both amino acid sequence information and experimentally measured λmax values are known. The final first-order regression model, consisting of three terms associated with chromophore conformation, was sufficient to predict the λmax of Sws2 photopigments with high accuracy. This study further highlights the breadth of our approach in reliably predicting λmax values of Sws2 cone photopigments, evolutionary-more distant from template bovine RH1, and provided mechanistic insights into the role of known spectral tuning sites.

A computational observer model of spatial contrast sensitivity: Effects of photocurrent encoding, fixational eye movements, and inference engine.

We have recently shown that the relative spatial contrast sensitivity function (CSF) of a computational observer operating on the cone mosaic photopigment excitations of a stationary retina has the same shape as human subjects. Absolute human sensitivity, however, is 5- to 10-fold lower than the computational observer. Here we model how additional known features of early vision affect the CSF: fixational eye movements and the conversion of cone photopigment excitations to cone photocurrents (phototransduction). For a computational observer that uses a linear classifier applied to the responses of a stimulus-matched linear filter, fixational eye movements substantially change the shape of the CSF by reducing sensitivity above 10 c/deg. For a translation-invariant computational observer that operates on the squared response of a quadrature-pair of linear filters, the CSF shape is little changed by eye movements, but there is a two fold reduction in sensitivity. Phototransduction dynamics introduce an additional two fold sensitivity decrease. Hence, the combined effects of fixational eye movements and phototransduction bring the absolute CSF of the translation-invariant computational observer to within a factor of 1 to 2 of the human CSF. We note that the human CSF depends on processing of the retinal representation by many thalamo-cortical neurons, which are individually quite noisy. Our modeling suggests that the net effect of post-retinal noise on contrast-detection performance, when considered at the neural population and behavioral level, is quite small: The inference mechanisms that determine the CSF, presumably in cortex, make efficient use of the information carried by the cone photocurrents of the fixating eye.

Non-photopic and photopic visual cycles differentially regulate immediate, early, and late phases of cone photoreceptor-mediated vision

Cone photoreceptors in the retina enable vision over a wide range of light intensities. However, the processes enabling cone vision in bright light (i.e. photopic vision) are not adequately understood. Chromophore regeneration of cone photopigments may require the retinal pigment epithelium (RPE) and/or retinal Müller glia. In the RPE, isomerization of all-trans-retinyl esters to 11-cis-retinol is mediated by the retinoid isomerohydrolase Rpe65. A putative alternative retinoid isomerase, dihydroceramide desaturase-1 (DES1), is expressed in RPE and Müller cells. The retinol-isomerase activities of Rpe65 and Des1 are inhibited by emixustat and fenretinide, respectively. Here, we tested the effects of these visual cycle inhibitors on immediate, early, and late phases of cone photopic vision. In zebrafish larvae raised under cyclic light conditions, fenretinide impaired late cone photopic vision, while the emixustat-treated zebrafish unexpectedly had normal vision. In contrast, emixustat-treated larvae raised under extensive dark-adaptation displayed significantly attenuated immediate photopic vision concomitant with significantly reduced 11-cis-retinaldehyde (11cRAL). Following 30 min of light, early photopic vision was recovered, despite 11cRAL levels remaining significantly reduced. Defects in immediate cone photopic vision were rescued in emixustat- or fenretinide-treated larvae following exogenous 9-cis-retinaldehyde supplementation. Genetic knockout of Des1 (degs1) or retinaldehyde-binding protein 1b (rlbp1b) did not eliminate photopic vision in zebrafish. Our findings define molecular and temporal requirements of the nonphotopic or photopic visual cycles for mediating vision in bright light.

An Alternative Psychophysical Diagnostic Indicator of the Aging Eye.

Purpose Impaired adaptation to changes in lighting levels as well as mesopic visual function is a common complaint in those over the age of 65. The use of photostress is a well-established method to test the adaption rate and the response of the visual cycle. In this study, we test visual function recovery to mesopic luminance stimuli following a long duration photostress in young and elderly subjects. If successful in strongly differentiating aging macular function, these methods may also be useful in the study of pathologies such as age-related macular degeneration. Methods A group of 12 older normal subjects (mean age 75.1 ± 4.79) and a control group of 5 younger normal subjects (mean age 26.2 ± 4.19) were subjected to macular photostress using the OraLux photostress system. The OraLux system provides a diffuse light source bleaching 84% of cone photopigment while maintaining an exposure safety factor of 200 times less than the maximum safe exposure. After each photostressing session, macular recovery was tracked using a foveal, variable contrast, flickering stimulus of mean luminance in the high mesopic range. Recovery was tracked for 300 seconds. The endpoint was time to recovery to each individual's baseline sensitivity as determined by two static sensitivity trials prior to photostress. Results Proportional hazards analysis of recovery time yielded a statistically significant difference between the older group and the young group (HR = 0.181; p=0.0289). The estimated hazard ratio of 0.181 indicates that older subjects return to baseline at less than one-fifth the rate of younger subjects. The hazards ratio remained statistically significant after adjusting for visual acuity (HR = 0.093; p=0.0424). Conclusion Photostress recovery of flicker sensitivity under mesopic conditions is a strong differentiator of aging macular function. This agrees with subject-reported complaints in reduced luminance conditions after exposure to bright lights such as night driving. The qualitative similarity between the aging retina and changes in early AMD suggests that flicker recovery following photostress may be useful as a surrogate endpoint in AMD clinical trials.

Initiation of feeding by four sympatric Neotropical primates (Ateles belzebuth, Lagothrix lagotricha poeppigii, Plecturocebus (Callicebus) discolor, and Pithecia aequatorialis) in Amazonian Ecuador: Relationships to photic and ecological factors.

We examined photic and ecological factors related to initiation of feeding by four sympatric primates in the rain forest of Amazonian Ecuador. With rare exceptions, morning activities of all taxa began only after the onset of nautical twilight, which occurred 47–48 min before sunrise. The larger spider and woolly monkeys, Ateles belzebuth and Lagothrix lagotricha poeppigii, left their sleeping trees before sunrise about half the time, while the smaller sakis and titi monkeys, Pithecia aequatorialis and Plecturocebus (formerly Callicebus) discolor, did not emerge until sunrise or later. None of the four taxa routinely began feeding before sunrise. Pithecia began feeding a median 2.17 h after sunrise, at least 0.8 h later than the median feeding times of the other three taxa. The early movement of Ateles and Lagothrix, and late initiation of feeding by Pithecia are consistent with temporal niche partitioning. Among most New World primate species, all males and many females, have dichromatic color vision, with only two cone photopigments, while some females are trichromats with three cone photopigments. Current evidence indicates that the dichromats have a foraging advantage in dim light, which could facilitate utilization of twilight periods and contribute to temporal niche partitioning. However, in our study, dichromatic males did not differentially exploit the dim light of twilight, and times of first feeding bouts of female Ateles and Lagothrix were similar to those of males. First feeding bouts followed a seasonal pattern, occurring latest in May-August, when ripe fruit abundance and ambient temperature were both relatively low. The most frugivorous taxon, Ateles, exhibited the greatest seasonality, initiating feeding 1.4 h later in May-August than in January-April. This pattern may imply a strategy of conserving energy when ripe fruit is scarcer, but starting earlier to compete successfully when fruit is more abundant. Lower temperatures were associated with later feeding of Ateles (by 26 min / °C) and perhaps Pithecia, but not Lagothrix or Plecturocebus. The potential for modification of temporal activity patterns and temporal niche partitioning by relatively small changes in temperature should be considered when predicting the effects of climate change.