Repeated Divergence in Opsin Gene Expression Mirrors Photic Habitat Changes in Rapidly Evolving Crater Lake Cichlid Fishes

AbstractSelection pressures differ along environmental gradients, and traits tightly linked to fitness (e.g., the visual system) are expected to track such variation. Along gradients, adaptation to local conditions might be due to heritable and nonheritable environmentally induced variation. Disentangling these sources of phenotypic variation requires studying closely related populations in nature and in the laboratory. The Nicaraguan lakes represent an environmental gradient in photic conditions from clear crater lakes to very turbid great lakes. From two old, turbid great lakes, Midas cichlid fish (Amphilophus cf. citrinellus) independently colonized seven isolated crater lakes of varying light conditions, resulting in a small adaptive radiation. We estimated variation in visual sensitivities along this photic gradient by measuring cone opsin gene expression among lake populations. Visual sensitivities observed in all seven derived crater lake populations shifted predictably in direction and magnitude, repeatedly mirroring changes in photic conditions. Comparing wild-caught and laboratory-reared fish revealed that 48% of this phenotypic variation is genetically determined and evolved rapidly. Decreasing intrapopulation variation as environments become spectrally narrower suggests that different selective landscapes operate along the gradient. We conclude that the power to predict phenotypic evolution along gradients depends on both the magnitude of environmental change and the selective landscape shape.

Visual Field Abnormalities in Early-Stage Diabetic Retinopathy Assessed by Chromatic Perimetry

Purpose

The purpose of this study was to define the nature and extent of sensitivity loss using chromatic perimetry in diabetics who have mild or no retinopathy.

Methods

Thirty-four individuals with type II diabetes mellitus who have mild nonproliferative diabetic retinopathy (MDR; N = 17) or no diabetic retinopathy (NDR; N = 17) and 15 visually normal, non-diabetic controls participated. Sensitivity was assessed along the horizontal visual field meridian using an Octopus 900 perimeter. Measurements were performed under light- and dark-adapted conditions using long-wavelength (red) and short-wavelength (blue) Goldmann III targets. Cumulative defect curves (CDCs) were constructed to determine whether field sensitivity loss was diffuse or localized.

Results

Sensitivity was reduced significantly under light-adapted conditions for both stimulus colors for the NDR (mean defect ± SEM = -2.1 dB ± 0.6) and MDR (mean defect ± SEM = -4.0 dB ± 0.7) groups. Sensitivity was also reduced under dark-adapted conditions for both stimulus colors for the NDR (mean defect ± SEM = -1.9 dB ± 0.7) and MDR (mean defect ± SEM = -4.5 ± 1.0 dB) groups. For both diabetic groups, field loss tended to be diffuse under light-adapted conditions (up to 6.9 dB loss) and localized under dark-adapted conditions (up to 15.4 dB loss).

Conclusions

Visual field sensitivity losses suggest neural abnormalities in early stage diabetic eye disease and the pattern of the sensitivity losses differed depending on the adaptation conditions. Chromatic perimetry may be useful for subtyping individuals who have mild or no diabetic retinopathy and for better understanding their neural dysfunction.

Spectacles with highly aspherical lenslets for myopia control do not change visual sensitivity in automated static perimetry

Purpose

Spectacle lenses with arrays of lenslets have gained popularity in myopia control due to their high efficacy, low impact on visual performance, and non-invasiveness. One of the questions regarding their impact on visual performance that still remain is that: do the lenslets impact visual field sensitivity? The current study aims to investigate the impact of wearing spectacle lenses with highly aspherical lenslets (HAL) on the visual field sensitivity.

Methods

An automated static perimetry test (Goldman perimeter target III) was employed to measure the detection sensitivity in the visual field. Targets were white light dots of various luminance levels and size 0.43°, randomly appearing at 76 locations within 30° eccentricity. Twenty-one adult subjects (age 23-61, spherical equivalent refractive error (SER) -8.75 D to +0.88 D) participated in the study. Sensitivities through two lenses, HAL and a single vision lens (SVL) as the control condition, were measured in random order.

Results

The mean sensitivity differences between HAL and SVL across the 76 tested locations ranged between -1.14 decibels (dB) and 1.28 dB. Only one location at 30° in the temporal visual field reached statistical significance (p < 0.00065) whereby the sensitivity increased by 1.1 dB with HAL. No significant correlation was found between the difference in sensitivity and age or SER. Such a difference is unlikely to be clinically relevant.

Conclusion

Compared to the SVL, the HAL did not change detection sensitivity to static targets in the whole visual field within 30° eccentricity.

Retinal OFF ganglion cells allow detection of quantal shadows at starlight

Perception of light in darkness requires no more than a handful of photons, and this remarkable behavioral performance can be directly linked to a particular retinal circuit-the retinal ON pathway. However, the neural limits of shadow detection in very dim light have remained unresolved. Here, we unravel the neural mechanisms that determine the sensitivity of mice (CBA/CaJ) to light decrements at the lowest light levels by measuring signals from the most sensitive ON and OFF retinal ganglion cell types and by correlating their signals with visually guided behavior. We show that mice can detect shadows when only a few photon absorptions are missing among thousands of rods. Behavioral detection of such "quantal" shadows relies on the retinal OFF pathway and is limited by noise and loss of single-photon signals in retinal processing. Thus, in the dim-light regime, light increments and decrements are encoded separately via the ON and OFF retinal pathways, respectively.

Thyroid hormone tinkering elicits integrated phenotypic changes potentially explaining rapid adaptation of color vision in cichlid fish

Vision is critical for most vertebrates, including fish. One challenge that aquatic habitats pose is the high variability in spectral properties depending on depth and the inherent optical properties of the water. By altering opsin gene expression and chromophore usage, cichlid fish modulate visual sensitivities to maximize sensory input from the available light in their respective habitat. Thyroid hormone (TH) has been proposed to play a role in governing adaptive diversification in visual sensitivity in Nicaraguan Midas cichlids, which evolved in less than 4000 generations. As suggested by indirect measurements of TH levels (i.e., expression of deiodinases), populations adapted to short wavelength light in clear lakes have lower TH levels than ones inhabiting turbid lakes enriched in long-wavelength light. We experimentally manipulated TH levels by exposing 2-week-old Midas cichlids to exogenous TH or a TH inhibitor and measured opsin gene expression and chromophore usage (via cyp27c1 expression). Although exogenous TH induces long-wavelength sensitivity by changing opsin gene expression and chromophore usage in a concerted manner, TH-inhibited fish exhibit a visual phenotype with sensitivities shifted to shorter wavelengths. Tinkering with TH levels in eyes results in concerted phenotypic changes that can provide a rapid mechanism of adaptation to novel light environments.

Unilateral increased visual sensitivity in cluster headache: a cross-sectional study

BACKGROUND AND OBJECTIVES

Increased sensitivity to light and patterns is typically associated with migraine, but has also been anecdotally reported in cluster headache, leading to diagnostic confusion. We wanted to assess whether visual sensitivity is increased ictally and interictally in cluster headache.

METHODS

We used the validated Leiden Visual Sensitivity Scale (L-VISS) questionnaire (range 0-36 points) to measure visual sensitivity in people with episodic or chronic cluster headache: (i) during attacks; (ii) in-between attacks; and in episodic cluster headache (iii) in-between bouts. The L-VISS scores were compared with the L-VISS scores obtained in a previous study in healthy controls and participants with migraine.

RESULTS

Mean L-VISS scores were higher for: (i) ictal vs interictal cluster headache (episodic cluster headache: 11.9 ± 8.0 vs. 5.2 ± 5.5, chronic cluster headache: 13.7 ± 8.4 vs 5.6 ± 4.8; p < 0.001); (ii) interictal cluster headache vs controls (5.3 ± 5.2 vs 3.6 ± 2.8, p < 0.001); (iii) interictal chronic cluster headache vs interictal ECH in bout (5.9 ± 0.5 vs 3.8 ± 0.5, p = 0.009), and (iv) interictal episodic cluster headache in bout vs episodic cluster headache out-of-bout (5.2 ± 5.5 vs. 3.7 ± 4.3, p < 0.001). Subjective visual hypersensitivity was reported by 110/121 (91%; 9 missing) participants with cluster headache and was mostly unilateral in 70/110 (64%) and ipsilateral to the ictal pain in 69/70 (99%) participants.

CONCLUSION

Cluster headache is associated with increased ictal and interictal visual sensitivity. In contrast to migraine, this is mostly unilateral and ipsilateral on the side of the ictal pain.

Review: Use of Electrophysiological Techniques to Study Visual Functions of Aquatic Organisms

The light environments of natural water sources have specific characteristics. For the majority of aquatic organisms, vision is crucial for predation, hiding from predators, communicating information, and reproduction. Electroretinography (ERG) is a diagnostic method used for assessing visual function. An electroretinogram records the comprehensive potential response of retinal cells under light stimuli and divides it into several components. Unique wave components are derived from different retinal cells, thus retinal function can be determined by analyzing these components. This review provides an overview of the milestones of ERG technology, describing how ERG is used to study visual sensitivity (e.g., spectral sensitivity, luminous sensitivity, and temporal resolution) of fish, crustaceans, mollusks, and other aquatic organisms (seals, sea lions, sea turtles, horseshoe crabs, and jellyfish). In addition, it describes the correlations between visual sensitivity and habitat, the variation of visual sensitivity as a function of individual growth, and the diel cycle changes of visual sensitivity. Efforts to identify the visual sensitivity of different aquatic organisms are vital to understanding the environmental plasticity of biological evolution and for directing aquaculture, marine fishery, and ecosystem management.

Rich Structural Index for Stereoscopic Image Quality Assessment

The human visual system (HVS), affected by viewing distance when perceiving the stereo image information, is of great significance to study of stereoscopic image quality assessment. Many methods of stereoscopic image quality assessment do not have comprehensive consideration for human visual perception characteristics. In accordance with this, we propose a Rich Structural Index (RSI) for Stereoscopic Image objective Quality Assessment (SIQA) method based on multi-scale perception characteristics. To begin with, we put the stereo pair into the image pyramid based on Contrast Sensitivity Function (CSF) to obtain sensitive images of different resolution. Then, we obtain local Luminance and Structural Index (LSI) in a locally adaptive manner on gradient maps which consider the luminance masking and contrast masking. At the same time we use Singular Value Decomposition (SVD) to obtain the Sharpness and Intrinsic Structural Index (SISI) to effectively capture the changes introduced in the image (due to distortion). Meanwhile, considering the disparity edge structures, we use gradient cross-mapping algorithm to obtain Depth Texture Structural Index (DTSI). After that, we apply the standard deviation method for the above results to obtain contrast index of reference and distortion components. Finally, for the loss caused by the randomness of the parameters, we use Support Vector Machine Regression based on Genetic Algorithm (GA-SVR) training to obtain the final quality score. We conducted a comprehensive evaluation with state-of-the-art methods on four open databases. The experimental results show that the proposed method has stable performance and strong competitive advantage.

Age effects on saccadic suppression of luminance and color

Saccadic eye movements modulate visual perception: they initiate and terminate high acuity vision at a certain location in space, but before and during their execution visual contrast sensitivity is strongly attenuated for 100 to 200 ms. Transient perisaccadic perceptual distortions are assumed to be an important mechanism to maintain visual stability. Little is known about age effects on saccadic suppression, even though for healthy adults other major age-related changes are well documented, like a decrease of visual contrast sensitivity for intermediate and high spatial frequencies or an increase of saccade latencies. Here, we tested saccadic suppression of luminance and isoluminant chromatic flashes in 100 participants from eight to 78 years. To estimate the effect of saccadic suppression on contrast sensitivity, we used a two-alternative forced choice (2AFC) design and an adaptive staircase procedure to modulate the luminance or chromatic contrast of a flashed detection target during fixation and 15 ms after saccade onset. The target was a single horizontal luminance or chromatic line flashed 2° above or below the fixation or saccade target. Compared to fixation, average perisaccadic contrast sensitivity decreased significantly by 66% for luminance and by 36% for color. A significant correlation was found for the strength of saccadic suppression of luminance and color. However, a small age effect was found only for the strength of saccadic suppression of luminance, which increased from 64% to 70% from young to old age. We conclude that saccadic suppression for luminance and color is present in most participants independent of their age and that mechanisms of suppression stay relatively stable during healthy aging.

Characteristics of visual sensitivity in familial cortical myoclonic tremor and epilepsy

The aim of the study was to describe the electroclinical features of visual sensitivity in patients with familial cortical myoclonic tremor and epilepsy. We searched the EEG database using the terms "familial cortical myoclonic tremor and epilepsy" and "visual sensitivity" over a seven-year period from March 2013 to April 2020 in the Xijing hospital, Xi'an, China. The inclusion criteria were demonstrable electroclinical visual sensitivity in the form of eye-opening sensitivity, eye-closure sensitivity, eyes-closed sensitivity and photosensitivity. Clinical, EEG and imaging records of patients were screened, and subsequently, detailed analysis of their data was undertaken. We enrolled six patients with visual sensitivity, five of whom suffered with rare generalized tonic-clonic seizures. Neuroimaging was negative in all cases. All patients demonstrated photosensitivity; eye-opening sensitivity associated with cortical myoclonic in one patient, eyes-closed sensitivity associated with cortical myoclonic tremor status in three patients, and eye-closure sensitivity in two patients. At the last follow-up visit, cortical myoclonic tremor and epilepsy in all patients was well controlled with first-line treatment. Visual sensitivity is therefore likely to be an important reflex trait in some patients with familial cortical myoclonic tremor and epilepsy, and should be routinely evaluated in order to better define the electroclinical features in FCMTE syndrome. [Published with video sequences].

Reproductive State Modulates Retinal Sensitivity to Light in Female Túngara Frogs

Visual cues are often a vital part of animal communication and courtship. While a plethora of studies have focused on the role that hormones play in acoustic communication of anurans, relatively few have explored hormonal modulation of vision in these animals. Much of what we do know comes from behavioral studies, which show that a frog’s hormonal state can significantly affect both its visual behavior and mating decisions. However, to fully understand how frogs use visual cues to make these mating decisions, we must first understand how their visual system processes these cues, and how hormones affect these processes. To do this, we performed electroretinograms (ERGs) to measure retinal sensitivity of túngara frogs (Physalaemus pustulosus), a neotropical species whose mating behavior includes previously described visual cues. To determine the effect of hormonal state on visual sensitivity, ERGs were recorded under scotopic and photopic conditions in frogs that were either non-reproductive or hormone-treated with human chorionic gonadotropin (hCG) prior to testing. Additionally, measurements of optical anatomy determined how túngara frog eye and retina morphology related to physiological sensitivity. As expected, we found that both sexes display higher visual sensitivity under scotopic conditions compared to photopic conditions. However, hormone injections significantly increased retinal sensitivity of females under scotopic conditions. These results support the hypothesis that hormonal modulation of neural mechanisms, such as those mediating visually guided reproductive behavior in this species, include modulation of the receptor organ: the retina. Thus, our data serve as a starting point for elucidating the mechanism of hormonal modulation of visual sensitivity.

Predictable events elicit less visual and temporal information uptake in an oddball paradigm

In the visual oddball paradigm, surprising inputs can seem expanded in time relative to unsurprising repeated events. A horizontal input embedded in a train of successive vertical inputs can, for instance, seem relatively protracted in time, even if all inputs are presented for an identical duration. It is unclear if this effect results from surprising events becoming apparently protracted, or from repeated events becoming apparently contracted in time. To disambiguate, we used a non-relative duration reproduction task, in which several standards preceded a test stimulus that had to be reproduced. We manipulated the predictability of test content over successive presentations. Overall, our data suggest that predictable stimuli induce a contraction of apparent duration (Experiments 1, 3, and 4). We also examine sensitivity to test content, and find that predictable stimuli elicit less uptake of visual information (Experiments 2 and 3). We discuss these findings in relation to the predictive coding framework.

Low availability of choline in utero disrupts development and function of the retina

Adequate supply of choline, an essential nutrient, is necessary to support proper brain development. Whether prenatal choline availability plays a role in development of the visual system is currently unknown. In this study, we addressed the role of in utero choline supply for the development and later function of the retina in a mouse model. We lowered choline availability in the maternal diet during pregnancy and assessed proliferative and differentiation properties of retinal progenitor cells (RPCs) in the developing prenatal retina, as well as visual function in adult offspring. We report that low choline availability during retinogenesis leads to persistent retinal cytoarchitectural defects, ranging from focal lesions with displacement of retinal neurons into subretinal space to severe hypocellularity and ultrastructural defects in photoreceptor organization. We further show that low choline availability impairs timely differentiation of retinal neuronal cells, such that the densities of early-born retinal ganglion cells, amacrine and horizontal cells, as well as cone photoreceptor precursors, are reduced in low choline embryonic d 17.5 retinas. Maintenance of higher proportions of RPCs that fail to exit the cell cycle underlies aberrant neuronal differentiation in low choline embryos. Increased RPC cell cycle length, and associated reduction in neurofibromin 2/Merlin protein, an upstream regulator of the Hippo signaling pathway, at least in part, explain aberrant neurogenesis in low choline retinas. Furthermore, we find that animals exposed to low choline diet in utero exhibit a significant degree of intraindividual variation in vision, characterized by marked functional discrepancy between the 2 eyes in individual animals. Together, our findings demonstrate, for the first time, that choline availability plays an essential role in the regulation of temporal progression of retinogenesis and provide evidence for the importance of adequate supply of choline for proper development of the visual system.-Trujillo-Gonzalez, I., Friday, W. B., Munson, C. A., Bachleda, A., Weiss, E. R., Alam, N. M., Sha, W., Zeisel, S. H., Surzenko, N. Low availability of choline in utero disrupts development and function of the retina.

Photoreceptors and eyes of pikeperch Sander lucioperca, pike Esox lucius, perch Perca fluviatilis and roach Rutilus rutilus from a clear and a brown lake

The photoreceptors and eyes of four fish species commonly cohabiting Fennoscandian lakes with different light transmission properties were compared: pikeperch Sander lucioperca, pike Esox lucius, perch Perca fluviatilis and roach Rutilus rutilus. Each species was represented by individuals from a clear (greenish) and a humic (dark brown) lake in southern Finland: Lake Vesijärvi (LV; peak transmission around 570 nm) and Lake Tuusulanjärvi (LT; peak transmission around 630 nm). In the autumn, all species had almost purely A2-based visual pigments. Rod absorption spectra peaked at c.526 nm (S. lucioperca), c. 533 nm (E. lucius) and c. 540 nm (P. fluviatilis and R. rutilus), with no differences between the lakes. Esox lucius rods had remarkably long outer segments, 1.5-2.8-fold longer than those of the other species. All species possessed middle-wavelength-sensitive (MWS) and long-wavelength-sensitive (LWS) cone pigments in single, twin or double cones. Rutilus rutilus also had two types of short-wavelength sensitive (SWS) cones: UV-sensitive [SWS1] and blue-sensitive (SWS2) cones, although in the samples from LT no UV cones were found. No other within-species differences in photoreceptor cell complements, absorption spectra or morphologies were found between the lakes. However, E. lucius eyes had a significantly lower focal ratio in LT compared with LV, enhancing sensitivity at the expense of acuity in the dark-brown lake. Comparing species, S. lucioperca was estimated to have the highest visual sensitivity, at least two times higher than similar-sized E. lucius, thanks to the large relative size of the eye (pupil) and the presence of a reflecting tapetum behind the retina. High absolute sensitivity will give a competitive edge also in terms of short reaction times and long visual range.

Quantifying visual allodynia across migraine subtypes: the Leiden Visual Sensitivity Scale

Enhanced sensitivity to light (photophobia) and patterns is common in migraine and can be regarded as visual allodynia. We aimed to develop and validate a questionnaire to easily quantify sensitivity to light and patterns in large populations, and to assess and compare visual allodynia across different migraine subtypes and states. We developed the Leiden Visual Sensitivity Scale (L-VISS), a 9-item scale (score range 0-36 points), based on literature and patient interviews, and examined its construct validity. Furthermore, we assessed ictal and interictal visual sensitivity in episodic migraine with (n = 67) and without (n = 66) aura and chronic migraine with (n = 20) and without (n = 19) aura, and in healthy controls (n = 86). Differences between migraine subtypes and states were tested using a linear mixed model with 3 fixed factors (episodic/chronic, with/without aura, and ictal/interictal). Test-retest reliability and construct validity of L-VISS were good. Leiden Visual Sensitivity Scale scores correlated in the expected direction with light discomfort (Kendall's τ = -0.25) and pattern glare tests (τ = 0.35). Known-group comparisons confirmed its construct validity. Within migraine subtypes, L-VISS scores were higher in migraine with aura versus without aura and in chronic versus episodic migraine. The linear mixed model showed all factors affected the outcome (P < 0.001). The L-VISS is an easy-to-use scale to quantify and monitor the burden of bothersome visual sensitivity to light and patterns in large populations. There are remarkable ictal and interictal differences in visual allodynia across migraine subtypes, possibly reflecting dynamic differences in cortical excitability.

Visual detection thresholds in two trophically distinct fishes are compromised in algal compared to sedimentary turbidity

Increasing anthropogenic turbidity is among the most prevalent disturbances in freshwater ecosystems, through increases in sedimentary deposition as well as the rise of nutrient-induced algal blooms. Changes to the amount and color of light underwater as a result of elevated turbidity are likely to disrupt the visual ecology of fishes that rely on vision to survive and reproduce; however, our knowledge of the mechanisms underlying visual responses to turbidity is lacking. First, we aimed to determine the visual detection threshold, a measure of visual sensitivity, of two ecologically and economically important Lake Erie fishes, the planktivorous forage fish, emerald shiner (Notropis atherinoides), and a primary predator, the piscivorous walleye (Sander vitreus), under sedimentary and algal turbidity. Secondly, we aimed to determine if these trophically distinct species are differentially impacted by increased turbidity. We used the innate optomotor response to determine the turbidity levels at which individual fish could no longer detect a difference between a stimulus and the background (i.e. visual detection threshold). Detection thresholds were significantly higher in sedimentary compared to algal turbidity for both emerald shiner (meansediment ± SE = 79.66 ± 5.51 NTU, meanalgal ± SE = 34.41 ± 3.19 NTU) and walleye (meansediment ± SE = 99.98 ± 5.31 NTU, meanalgal ± SE = 40.35 ± 2.44 NTU). Our results suggest that across trophic levels, the visual response of fishes will be compromised under algal compared to sedimentary turbidity. The influence of altered visual environments on the ability of fish to find food and detect predators could potentially be large, leading to population- and community-level changes within the Lake Erie ecosystem.

Tobacco Use Decreases Visual Sensitivity in Schizophrenia

Smoking prevalence in patients who are diagnosed with schizophrenia (SCZ) is higher than in the general population. Chronic tobacco use in SCZ patients may reduce the side effects of antipsychotic drugs, thus serving as a self-medication for such side effects. Understanding the ways in which chronic tobacco use influences visual sensitivity has clinical implications, which may serve as a tool for non-invasively diagnosing early-stage visual processing deficits. The present study evaluated the effects of chronic tobacco use on visual sensitivity in SCZ patients. Our purpose was to provide new directions for future research, mainly psychophysical and electrophysiological studies. In the present study, 40 smoker controls (SC), 20 SCZ tobacco users, and 20 SCZ tobacco nonusers were recruited from the Psychosocial Care Center. Visual sensitivity was compared between both SCZ groups and the SC group. Patients with SCZ who were chronic tobacco users presented lower visual sensitivity for chromatic (p < 0.001) and achromatic (p < 0.001) stimuli compared with the other groups. Our findings highlight the need to evaluate possible addictive behavior in patients with SCZ, which may contribute to public policies that seek to improve the quality of life of SCZ patients and their families.

Coping with copepods: do right whales (Eubalaena glacialis) forage visually in dark waters?

North Atlantic right whales (Eubalaena glacialis) feed during the spring and early summer in marine waters off the northeast coast of North America. Their food primarily consists of planktonic copepods, Calanus finmarchicus, which they consume in large numbers by ram filter feeding. The coastal waters where these whales forage are turbid, but they successfully locate copepod swarms during the day at depths exceeding 100 m, where light is very dim and copepod patches may be difficult to see. Using models of E. glacialis visual sensitivity together with measurements of light in waters near Cape Cod where they feed and of light attenuation by living copepods in seawater, we evaluated the potential for visual foraging by these whales. Our results suggest that vision may be useful for finding copepod patches, particularly if E. glacialis searches overhead for silhouetted masses or layers of copepods. This should permit the whales to locate C. finmarchicus visually throughout most daylight hours at depths throughout their foraging range. Looking laterally, the whales might also be able to see copepod patches at short range near the surface.This article is part of the themed issue 'Vision in dim light'.

Behavioural and physiological limits to vision in mammals

Human vision is exquisitely sensitive-a dark-adapted observer is capable of reliably detecting the absorption of a few quanta of light. Such sensitivity requires that the sensory receptors of the retina, rod photoreceptors, generate a reliable signal when single photons are absorbed. In addition, the retina must be able to extract this information and relay it to higher visual centres under conditions where very few rods signal single-photon responses while the majority generate only noise. Critical to signal transmission are mechanistic optimizations within rods and their dedicated retinal circuits that enhance the discriminability of single-photon responses by mitigating photoreceptor and synaptic noise. We describe behavioural experiments over the past century that have led to the appreciation of high sensitivity near absolute visual threshold. We further consider mechanisms within rod photoreceptors and dedicated rod circuits that act to extract single-photon responses from cellular noise. We highlight how these studies have shaped our understanding of brain function and point out several unresolved questions in the processing of light near the visual threshold.This article is part of the themed issue 'Vision in dim light'.

Processing of single-photon responses in the mammalian On and Off retinal pathways at the sensitivity limit of vision

Visually guided behaviour at its sensitivity limit relies on single-photon responses originating in a small number of rod photoreceptors. For decades, researchers have debated the neural mechanisms and noise sources that underlie this striking sensitivity. To address this question, we need to understand the constraints arising from the retinal output signals provided by distinct retinal ganglion cell types. It has recently been shown in the primate retina that On and Off parasol ganglion cells, the cell types likely to underlie light detection at the absolute visual threshold, differ fundamentally not only in response polarity, but also in the way they handle single-photon responses originating in rods. The On pathway provides the brain with a thresholded, low-noise readout and the Off pathway with a noisy, linear readout. We outline the mechanistic basis of these different coding strategies and analyse their implications for detecting the weakest light signals. We show that high-fidelity, nonlinear signal processing in the On pathway comes with costs: more single-photon responses are lost and their propagation is delayed compared with the Off pathway. On the other hand, the responses of On ganglion cells allow better intensity discrimination compared with the Off ganglion cell responses near visual threshold.

This article is part of the themed issue ‘Vision in dim light’.

Macular thickness as a predictor of loss of visual sensitivity in ethambutol-induced optic neuropathy

Ethambutol is a common cause of drug-related optic neuropathy. Prediction of the onset of ethambutol-induced optic neuropathy and consequent drug withdrawal may be an effective method to stop visual loss. Previous studies have shown that structural injury to the optic nerve occurred earlier than the damage to visual function. Therefore, we decided to detect structural biomarkers marking visual field loss in early stage ethambutol-induced optic neuropathy. The thickness of peripapillary retinal nerve fiber layer, macular thickness and visual sensitivity loss would be observed in 11 ethambutol-induced optic neuropathy patients (22 eyes) using optical coherence tomography. Twenty-four healthy age- and sex-matched participants (48 eyes) were used as controls. Results demonstrated that the temporal peripapillary retinal nerve fiber layer thickness and average macular thickness were thinner in patients with ethambutol-induced optic neuropathy compared with healthy controls. The average macular thickness was strongly positively correlated with central visual sensitivity loss (r² =0.878, P=0.000). These findings suggest that optical coherence tomography can be used to efficiently screen patients. Macular thickness loss could be a potential factor for predicting the onset of ethambutol-induced optic neuropathy.

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.

Beauty and the beholder: the role of visual sensitivity in visual preference

For centuries, the essence of aesthetic experience has remained one of the most intriguing mysteries for philosophers, artists, art historians and scientists alike. Recently, views emphasizing the link between aesthetics, perception and brain function have become increasingly prevalent (Ramachandran and Hirstein, 1999; Zeki, 1999; Livingstone, 2002; Ishizu and Zeki, 2013). The link between art and the fractal-like structure of natural images has also been highlighted (Spehar et al., 2003; Graham and Field, 2007; Graham and Redies, 2010). Motivated by these claims and our previous findings that humans display a consistent preference across various images with fractal-like statistics, here we explore the possibility that observers’ preference for visual patterns might be related to their sensitivity for such patterns. We measure sensitivity to simple visual patterns (sine-wave gratings varying in spatial frequency and random textures with varying scaling exponent) and find that they are highly correlated with visual preferences exhibited by the same observers. Although we do not attempt to offer a comprehensive neural model of aesthetic experience, we demonstrate a strong relationship between visual sensitivity and preference for simple visual patterns. Broadly speaking, our results support assertions that there is a close relationship between aesthetic experience and the sensory coding of natural stimuli.

Variable light environments induce plastic spectral tuning by regional opsin coexpression in the African cichlid fish, Metriaclima zebra

Critical behaviours such as predation and mate choice often depend on vision. Visual systems are sensitive to the spectrum of light in their environment, which can vary extensively both within and among habitats. Evolutionary changes in spectral sensitivity contribute to divergence and speciation. Spectral sensitivity of the retina is primarily determined by visual pigments, which are opsin proteins bound to a chromophore. We recently discovered that photoreceptors in different regions of the retina, which view objects against distinct environmental backgrounds, coexpress different pairs of opsins in an African cichlid fish, Metriaclima zebra. This coexpression tunes the sensitivity of the retinal regions to the corresponding backgrounds and may aid in detection of dark objects, such as predators. Although intraretinal regionalization of spectral sensitivity in many animals correlates with their light environments, it is unknown whether variation in the light environment induces developmentally plastic alterations of intraretinal sensitivity regions. Here, we demonstrate with fluorescent in situ hybridization and qPCR that the spectrum and angle of environmental light both influence the development of spectral sensitivity regions by altering the distribution and level of opsins across the retina. Normally, M. zebra coexpresses LWS opsin with RH2Aα opsin in double cones of the ventral but not the dorsal retina. However, when illuminated from below throughout development, adult M. zebra coexpressed LWS and RH2Aα in double cones both dorsally and ventrally. Thus, environmental background spectra alter the spectral sensitivity pattern that develops across the retina, potentially influencing behaviours and related evolutionary processes such as courtship and speciation.

Focal relationship between structure and function within the central 10 degrees in glaucoma

PURPOSE

To investigate which measurements of inner macular thickness are the most useful for evaluating the focal relationship with visual sensitivity within the central 10° in glaucoma and which layers require correction for retinal ganglion cell (RGC) displacement.

METHODS

Sixty eyes of 60 subjects with glaucoma were included. Sensitivity of each test point of 10-2 standard automated perimetry was compared with the thickness of the retinal nerve fiber layer (RNFL), ganglion cell layer (GCL), GCL+ inner plexiform layer (IPL), and RNFL+GCL+IPL (GCC), with and without RGC displacement, using Spearman's rank correlation coefficients. Visual sensitivity was evaluated by unlogged 1/Lambert (1/L) values.

RESULTS

Retinal nerve fiber layer thickness correlated significantly with the sensitivities of all test points except for some in the papillomacular bundle region when adjusting for RGC displacement (rs = 0.287-0.767, P < 0.05). In the central 5.8°, the GCL and (GCL+IPL) thickness correlated significantly with the sensitivities of all test points when adjusting for RGC displacement (GCL: rs = 0.363-0.729, P < 0.01; (GCL+IPL): rs = 0.359-0.715, P < 0.01). The GCC thickness correlated significantly with the sensitivities of all 68 test points when adjusting for RGC displacement (rs = 0.359-0.767, P < 0.01). RGC displacement improved the correlation between sensitivity and GCL, (GCL+IPL), and GCC in the central 4 points (GCL: rs = from 0.270-0.470 to 0.421-0.540; (GCL+IPL): rs = from 0.195-0.450 to 0.381-0.549; GCC: rs = from 0.132-0.449 to 0.359-0.562).

CONCLUSIONS

The GCC is the most useful parameter to evaluate structure and function within the central 10° in glaucoma. Adjusting for RGC displacement is essential to evaluate the relationship between structure of the GCL-related layer and function at the central macula.

Visual sensitivity is a stronger determinant of illusory processes than auditory cue parameters in the sound-induced flash illusion

The sound-induced flash illusion (SIFI) is a multisensory perceptual phenomenon in which the number of brief visual stimuli perceived by an observer is influenced by the number of concurrently presented sounds. While the strength of this illusion has been shown to be modulated by the temporal congruence of the stimuli from each modality, there is conflicting evidence regarding its dependence upon their spatial congruence. We addressed this question by examining SIFIs under conditions in which the spatial reliability of the visual stimuli was degraded and different sound localization cues were presented using either free-field or closed-field stimulation. The likelihood of reporting a SIFI varied with the spatial cue composition of the auditory stimulus and was highest when binaural cues were presented over headphones. SIFIs were more common for small flashes than for large flashes, and for small flashes at peripheral locations, subjects experienced a greater number of illusory fusion events than fission events. However, the SIFI was not dependent on the spatial proximity of the audiovisual stimuli, but was instead determined primarily by differences in subjects' underlying sensitivity across the visual field to the number of flashes presented. Our findings indicate that the influence of auditory stimulation on visual numerosity judgments can occur independently of the spatial relationship between the stimuli.

Interspecific variation in Rx1 expression controls opsin expression and causes visual system diversity in African cichlid fishes

The mechanisms underlying natural phenotypic diversity are key to understanding evolution and speciation. Cichlid fishes are among the most speciose vertebrates and an ideal model for identifying genes controlling species differences. Cichlids have diverse visual sensitivities that result from species expressing subsets of seven cichlid cone opsin genes. We previously identified a quantitative trait locus (QTL) that tunes visual sensitivity by varying SWS2A (short wavelength sensitive 2A) opsin expression in a genetic cross between two Lake Malawi cichlid species. Here, we identify Rx1 (retinal and anterior neural fold homeobox) as the causative gene for the QTL using fine mapping and RNAseq in retinal transcriptomes. Rx1 is differentially expressed between the parental species and correlated with SWS2A expression in the F2 progeny. Expression of Rx1 and SWS2A is also correlated in a panel of 16 Lake Malawi cichlid species. Association mapping in this panel identified a 413-bp deletion located 2.5-kb upstream of the Rx1 translation start site that is correlated with decreased Rx1 expression. This deletion explains 62% of the variance in SWS2A expression across 53 cichlid species in 29 genera. The deletion occurs in both the sand and rock-dwelling cichlid clades, suggesting that it is an ancestral polymorphism. Our finding supports the hypothesis that mixing and matching of ancestral polymorphisms can explain the diversity of present day cichlid phenotypes.

The visual ecology of a deep-sea fish, the escolar Lepidocybium flavobrunneum (Smith, 1843)

Escolar (Lepidocybium flavobrunneum, family Gempylidae) are large and darkly coloured deep-sea predatory fish found in the cold depths (more than 200 m) during the day and in warm surface waters at night. They have large eyes and an overall low density of retinal ganglion cells that endow them with a very high optical sensitivity. Escolar have banked retinae comprising six to eight layers of rods to increase the optical path length for maximal absorption of the incoming light. Their retinae possess two main areae of higher ganglion cell density, one in the ventral retina viewing the dorsal world above (with a moderate acuity of 4.6 cycles deg(-1)), and the second in the temporal retina viewing the frontal world ahead. Electrophysiological recordings of the flicker fusion frequency (FFF) in isolated retinas indicate that escolar have slow vision, with maximal FFF at the highest light levels and temperatures (around 9 Hz at 23°C) which fall to 1-2 Hz in dim light or cooler temperatures. Our results suggest that escolar are slowly moving sit-and-wait predators. In dim, warm surface waters at night, their slow vision, moderate dorsal resolution and highly sensitive eyes may allow them to surprise prey from below that are silhouetted in the downwelling light.

Melatonin modulates visual function and cell viability in the mouse retina via the MT1 melatonin receptor

A clear demonstration of the role of melatonin and its receptors in specific retinal functions is lacking. The present study investigated the distribution of MT1 receptors within the retina, and the scotopic and photopic electroretinograms (ERG) and retinal morphology in wild-type (WT) and MT1 receptor-deficient mice. MT1 receptor transcripts were localized in photoreceptor cells and in some inner retinal neurons. A diurnal rhythm in the dark-adapted ERG responses was observed in WT mice, with higher a- and b-wave amplitudes at night, but this rhythm was absent in mice lacking MT1 receptors. Injection of melatonin during the day decreased the scotopic response threshold and the amplitude of the a- and b-waves in the WT mice, but not in the MT1(-/-) mice. The effects of MT1 receptor deficiency on retinal morphology was investigated at three different ages (3, 12, and 18 months). No differences between MT1(-/-) and WT mice were observed at 3 months of age, whereas at 12 months MT1(-/-) mice have a significant reduction in the number of photoreceptor nuclei in the outer nuclear layer compared with WT controls. No differences were observed in the number of cells in inner nuclear layer or in ganglion cells at 12 months of age. At 18 months, the loss of photoreceptor nuclei in the outer nuclear layer was further accentuated and the number of ganglion cells was also significantly lower than that of controls. These data demonstrate the functional significance of melatonin and MT1 receptors in the mammalian retina and create the basis for future studies on the therapeutic use of melatonin in retinal degeneration.

Effects of dopamine depletion on visual sensitivity of zebrafish

The visual sensitivity of zebrafish in which the retinal dopaminergic interplexiform cells (DA-IPCs) were destroyed by 6-hydroxydopamine was measured behaviorally. During the first 6-8 min of dark adaptation, visual thresholds of DA-IPC-depleted animals were similar to those of control animals. Thereafter, their visual thresholds were elevated so that by 14-18 min of dark adaptation, they were 2-3 log units above those of control animals. In DA-IPC-depleted animals, the electroretinogram was normal in terms of light sensitivity and waveform, but the light threshold for eliciting a ganglion cell discharge was raised by 1.8 log units as compared with control animals. No obvious rod system function was detected in DA-IPC-depleted animals as measured behaviorally. Partial rescue of the behavioral visual sensitivity loss in DA-IPC-depleted animals occurred when dopamine or a long-acting dopamine agonist (2-amino-6, 7-dihydroxy-1, 2, 3, 4-tetrahydronaphthalene hydrobromide) were injected intraocularly. Our data suggest that the principal visual defect shown by DA-IPC-depleted animals is attributable to effects occurring in the inner retina, mainly on rod signals. We also show that dopamine is involved in mediating the effect of the circadian clock on visual sensitivity.

Disruption of the olfactoretinal centrifugal pathway may relate to the visual system defect in night blindness b mutant zebrafish

We describe here a dominant mutation, night blindness b (nbb), which causes an age-related visual system defect in zebrafish. At 4-5 months of age, dark-adapted nbb(+/-) mutants show abnormal visual threshold fluctuations when measured behaviorally. Light sensitizes the animals; thus early dark adaptation of nbb(+/-) fish is normal. After 2 hr of dark adaptation, however, visual thresholds of nbb(+/-) mutants are raised on average 2-3 log units, and rod system function is not detectable. Electroretinograms recorded from nbb(+/-) mutants are normal, but ganglion cell thresholds are raised in prolonged darkness, suggesting an inner retinal defect. The visual defect of nbb(+/-) mutants may be likely caused by an abnormal olfactoretinal centrifugal innervation; in nbb(+/-) mutants, the olfactoretinal centrifugal projection to the retina is disrupted, and the number of retinal dopaminergic interplexiform cells is reduced. A similar visual defect as shown by nbb(+/-) mutants is observed in zebrafish in which the olfactory epithelium and olfactory bulb have been excised. Homozygous nbb fish display an early onset neural degeneration throughout the CNS and die by 7-8 d of age.

Human cone pigment expressed in transgenic mice yields altered vision

Genetically driven alterations in the complement of retinal photopigments are fundamental steps in the evolution of vision. We sought to determine how a newly added photopigment might impact vision by studying a transgenic mouse that expresses a human cone photopigment. Electroretinogram (ERG) measurements indicate that the added pigment works well, significantly changing spectral sensitivity without deleteriously affecting the operation of the native cone pigments. Visual capacities of the transgenic mice were established in behavioral tests. The new pigment was found to provide a significant expansion of the spectral range over which mice can perceive light, thus underlining the immediate utility of acquiring a new photopigment. The transgenic mouse also has the receptor basis for a novel color vision capacity, but tests show that potential was not realized. This failure likely reflects limitations in the organizational arrangement of the mouse retina.