Can We See with Melanopsin?

Melanopsin retinal ganglion cell function in Alzheimer's vs. Parkinson's disease an exploratory meta-analysis and review of pupillometry protocols

BACKGROUND

Neurodegenerative diseases share retinal abnormalities. Chromatic pupillometry allows in vivo assessment of photoreceptor functional integrity, including melanopsin-expressing retinal ganglion cells. This exploratory meta-analysis assesses retinal photoreceptor functionality in Alzheimer's vs. Parkinson's disease and conducts an in-depth review of applied pupillometric protocols.

METHODS

Literature reviews on PubMed and Scopus from 1991 to August 2023 identified chromatic pupillometry studies on Alzheimer's disease (AD; n = 42 patients from 2 studies) and Parkinson's disease (PD; n = 66 from 3 studies). Additionally, a pre-AD study (n = 10) and an isolated REM Sleep Behavior Disorder study (iRBD; n = 10) were found, but their results were not included in the meta-analysis statistics.

RESULTS

Melanopsin-mediated post-illumination pupil response to blue light was not significantly impaired in Alzheimer's (weighted mean difference = -1.54, 95% CI: 4.57 to 1.49, z = -1.00, p = 0.319) but was in Parkinson's (weighted mean difference = -9.14, 95% CI: 14.19 to -4.08, z = -3.54, p < 0.001). Other pupil light reflex metrics showed no significant differences compared to controls. Studies adhered to international standards of pupillometry with moderate to low bias. All studies used full-field stimulation. Alzheimer's studies used direct while Parkinson's studies used consensual measurement. Notably, studies did not control for circadian timing and Parkinson's patients were on dopaminergic treatment.

CONCLUSION AND RELEVANCE

Results affirm chromatic pupillometry as a useful method to assess melanopsin-related retinal cell dysfunction in Parkinson's but not in Alzheimer's disease. While adhering to international standards, future studies may analyze the effects of local field stimulation, dopaminergic treatment, and longitudinal design to elucidate melanopsin dysfunction in Parkinson's disease.

Basics, benefits, and pitfalls of pupillometers assessing visual function

Numerous commercially and non-commercially available pupillometers are nowadays able to assess various biological functions in humans, by evaluating pupils' dynamics in response to specific stimuli. However, the use of pupillometers for ophthalmic afferent evaluations (i.e., photoreceptoral responses) in real-world settings is relatively limited. Recent scientific and technological advances, coupled with artificial intelligence methods have improved the performance of such devices to objectively detect, quantify, and classify functional disturbances of the retina and the optic nerve. This review aims to summarize the scientific principles, indications, outcomes, and current limitations of pupillometry used for evaluation of afferent pathways in ophthalmic clinical settings.

A pupillary contrast response in mice and humans: Neural mechanisms and visual functions

In the pupillary light response (PLR), increases in ambient light constrict the pupil to dampen increases in retinal illuminance. Here, we report that the pupillary reflex arc implements a second input-output transformation; it senses temporal contrast to enhance spatial contrast in the retinal image and increase visual acuity. The pupillary contrast response (PCoR) is driven by rod photoreceptors via type 6 bipolar cells and M1 ganglion cells. Temporal contrast is transformed into sustained pupil constriction by the M1's conversion of excitatory input into spike output. Computational modeling explains how the PCoR shapes retinal images. Pupil constriction improves acuity in gaze stabilization and predation in mice. Humans exhibit a PCoR with similar tuning properties to mice, which interacts with eye movements to optimize the statistics of the visual input for retinal encoding. Thus, we uncover a conserved component of active vision, its cell-type-specific pathway, computational mechanisms, and optical and behavioral significance.

Recommendations for measuring and standardizing light for laboratory mammals to improve welfare and reproducibility in animal research

Light enables vision and exerts widespread effects on physiology and behavior, including regulating circadian rhythms, sleep, hormone synthesis, affective state, and cognitive processes. Appropriate lighting in animal facilities may support welfare and ensure that animals enter experiments in an appropriate physiological and behavioral state. Furthermore, proper consideration of light during experimentation is important both when it is explicitly employed as an independent variable and as a general feature of the environment. This Consensus View discusses metrics to use for the quantification of light appropriate for nonhuman mammals and their application to improve animal welfare and the quality of animal research. It provides methods for measuring these metrics, practical guidance for their implementation in husbandry and experimentation, and quantitative guidance on appropriate light exposure for laboratory mammals. The guidance provided has the potential to improve data quality and contribute to reduction and refinement, helping to ensure more ethical animal use.

Non-image-forming vision as measured through ipRGC-mediated pupil constriction is not modulated by covert visual attention

In brightness, the pupil constricts, while in darkness, the pupil dilates; this is known as the pupillary light response (PLR). The PLR is driven by all photoreceptors: rods and cones, which contribute to image-forming vision, and intrinsically photosensitive retinal ganglion cells (ipRGCs), which mainly contribute to non-image-forming vision. Rods and cones cause immediate pupil constriction upon light exposure, whereas ipRGCs cause sustained constriction throughout light exposure. Recent studies have shown that covert attention modulated the initial PLR; however, it remains unclear whether the same holds for the sustained PLR. We tested this by leveraging ipRGCs' responsiveness to blue light, causing the most prominent sustained constriction. While replicating previous studies by showing that pupils constricted more when either directly looking at, or covertly attending to, bright as compared to dim stimuli (with the same color), we also found that the pupil constricted more when directly looking at blue as compared to red stimuli (with the same luminosity). Crucially, however, in two high-powered studies (n = 60), we did not find any pupil-size difference when covertly attending to blue as compared to red stimuli. This suggests that ipRGC-mediated pupil constriction, and possibly non-image-forming vision more generally, is not modulated by covert attention.

Coexistence within one cell of microvillous and ciliary phototransductions across M1- through M6-IpRGCs

Intrinsically photosensitive retinal ganglion cells (ipRGCs) serve as primary photoceptors by expressing the photopigment, melanopsin, and also as retinal relay neurons for rod and cone signals en route to the brain, in both cases for the purpose of non-image vision as well as aspects of image vision. So far, six subtypes of ipRGCs (M1 through M6) have been characterized. Regarding their phototransduction mechanisms, we have previously found that, unconventionally, rhabdomeric (microvillous) and ciliary signaling motifs co-exist within a given M1-, M2-, and M4-ipRGC, with the first mechanism involving PLCβ4 and TRPC6,7 channels and the second involving cAMP and HCN channels. We have now examined M3-, M5-, and M6-cells and found that each cell likewise uses both signaling pathways for phototransduction, despite differences in the percentage representation by each pathway in a given ipRGC subtype for bright-flash responses (and saturated except for M6-cells). Generally, M3- and M5-cells show responses quite similar in kinetics to M2-responses, and M6-cell responses resemble broadly those of M1-cells although much lower in absolute sensitivity and amplitude. Therefore, similar to rod and cone subtypes in image vision, ipRGC subtypes possess the same phototransduction mechanism(s) even though they do not show microvilli or cilia morphologically.

A new method to quantify the human visual threshold from melanopsin sensitive ganglion cells

Traditional photoreceptors utilize the chromophore retinal to absorb light coupled with a unique opsin protein to specify receptor spectral sensitivity. Light absorption triggers a cascade of events transducing light energy to neural signals beginning with graded potentials in receptors (rods and cones) and bipolar cells in outer and middle retina eventuating in action potentials at the inner retinal amacrine and ganglion cell levels. Unlike traditional photoreceptors, ganglion cells in the inner retina (intrinsically photosensitive retinal ganglion cells, ipRGCs) absorb short wavelength, blue light utilizing their photopigment melanopsin. Assessment across multiple species show that the ipRGCs mediate myriad visual and non-visual functions including photo-entrainment and circadian rhythms, the pupillary light reflex, sleep, alertness, cognition, mood, and even conscious visual perception. Some ipRGC functions can persist despite blindness in animal models and humans exemplifying their multidisciplinary control of visual and non-visual functions. In previous research we used selective chromatic adaptation (blue stimulus on a bright amber field) to suppress input from rods, red and green sensitive cones to identify retinal and cortical responses from ipRGCs. Herein we used a similar approach, coupled with a filter to block input from blue sensitive cones, to develop a clinically expedient method to measure the full-field, putative visual threshold from human ipRGCs. This metric may expand our ability to detect, diagnose and monitor ocular and neurologic disease and provide a global retinal metric of ipRGCs as a potential outcome measure for studies using gene therapy to arrest and/or improve vision in hereditary retinal diseases.

Calibrating Vision: Concepts and Questions

The idea that visual coding and perception are shaped by experience and adjust to changes in the environment or the observer is universally recognized as a cornerstone of visual processing, yet the functions and processes mediating these calibrations remain in many ways poorly understood. In this article we review a number of facets and issues surrounding the general notion of calibration, with a focus on plasticity within the encoding and representational stages of visual processing. These include how many types of calibrations there are - and how we decide; how plasticity for encoding is intertwined with other principles of sensory coding; how it is instantiated at the level of the dynamic networks mediating vision; how it varies with development or between individuals; and the factors that may limit the form or degree of the adjustments. Our goal is to give a small glimpse of an enormous and fundamental dimension of vision, and to point to some of the unresolved questions in our understanding of how and why ongoing calibrations are a pervasive and essential element of vision.

A non-canonical retina-ipRGCs-SCN-PVT visual pathway for mediating contagious itch behavior

Contagious itch behavior informs conspecifics of adverse environment and is crucial for the survival of social animals. Gastrin-releasing peptide (GRP) and its receptor (GRPR) in the suprachiasmatic nucleus (SCN) of the hypothalamus mediates contagious itch behavior in mice. Here, we show that intrinsically photosensitive retina ganglion cells (ipRGCs) convey visual itch information, independently of melanopsin, from the retina to GRP neurons via PACAP-PAC1R signaling. Moreover, GRPR neurons relay itch information to the paraventricular nucleus of the thalamus (PVT). Surprisingly, neither the visual cortex nor superior colliculus is involved in contagious itch. In vivo calcium imaging and extracellular recordings reveal contagious itch-specific neural dynamics of GRPR neurons. Thus, we propose that the retina-ipRGC-SCN-PVT pathway constitutes a previously unknown visual pathway that probably evolved for motion vision that encodes salient environmental cues and enables animals to imitate behaviors of conspecifics as an anticipatory mechanism to cope with adverse conditions.

It has been shown that GRP-GRPR neuropeptide signaling in the SCN is important for contagious itch behavior in mice. Gao et al. find that SCN-projecting ipRGCs are sufficient to relay itch information from the retina to the SCN by releasing neuropeptide PACAP to activate the GRP-GRPR pathway.

Look-up and look-down neurons in the mouse visual thalamus during freely moving exploration

Visual information reaches cortex via the thalamic dorsal lateral geniculate nucleus (dLGN). dLGN activity is modulated by global sleep/wake states and arousal, indicating that it is not simply a passive relay station. However, its potential for more specific visuomotor integration is largely unexplored. We addressed this question by developing robust 3D video reconstruction of mouse head and body during spontaneous exploration paired with simultaneous neuronal recordings from dLGN. Unbiased evaluation of a wide range of postures and movements revealed a widespread coupling between neuronal activity and few behavioral parameters. In particular, postures associated with the animal looking up/down correlated with activity in >50% neurons, and the extent of this effect was comparable with that induced by full-body movements (typically locomotion). By contrast, thalamic activity was minimally correlated with other postures or movements (e.g., left/right head and body torsions). Importantly, up/down postures and full-body movements were largely independent and jointly coupled to neuronal activity. Thus, although most units were excited during full-body movements, some expressed highest firing when the animal was looking up ("look-up" neurons), whereas others expressed highest firing when the animal was looking down ("look-down" neurons). These results were observed in the dark, thus representing a genuine behavioral modulation, and were amplified in a lit arena. Our results demonstrate that the primary visual thalamus, beyond global modulations by sleep/awake states, is potentially involved in specific visuomotor integration and reveal two distinct couplings between up/down postures and neuronal activity.

Intrinsically photosensitive retinal ganglion cells in glaucoma

Glaucoma is a group of eye diseases afflicting more than 70 million people worldwide. It is characterized by damage to retinal ganglion cells (RGCs) that ultimately leads to the death of the cells and vision loss. The diversity of RGC types has been appreciated for decades, and studies, including ours, have shown that RGCs degenerate and die in a type-specific manner in rodent models of glaucoma. The type-specific loss of RGCs results in differential damage to visual and non-visual functions. One type of RGC, the intrinsically photosensitive retinal ganglion cell (ipRGC), expressing the photopigment melanopsin, serves a broad array of non-visual responses to light. Since its discovery, six subtypes of ipRGC have been described, each contributing to various image-forming and non-image-forming functions such as circadian photoentrainment, the pupillary light reflex, the photic control of mood and sleep, and visual contrast sensitivity. We recently demonstrated a link between type-specific ipRGC survival and behavioral deficits in a mouse model of chronic ocular hypertension. This review focuses on the type-specific ipRGC degeneration and associated behavioral changes in animal models and glaucoma patients. A better understanding of how glaucomatous insult impacts the ipRGC-based circuits will have broad impacts on improving the treatment of glaucoma-associated non-visual disorders.

The role of ipRGCs in ocular growth and myopia development

The increasing global prevalence of myopia calls for elaboration of the pathogenesis of this disease. Here, we show that selective ablation and activation of intrinsically photosensitive retinal ganglion cells (ipRGCs) in developing mice induced myopic and hyperopic refractive shifts by modulating the corneal radius of curvature (CRC) and axial length (AL) in an opposite way. Melanopsin- and rod/cone-driven signals of ipRGCs were found to influence refractive development by affecting the AL and CRC, respectively. The role of ipRGCs in myopia progression is evidenced by attenuated form-deprivation myopia magnitudes in ipRGC-ablated and melanopsin-deficient animals and by enhanced melanopsin expression/photoresponses in form-deprived eyes. Cell subtype-specific ablation showed that M1 subtype cells, and probably M2/M3 subtype cells, are involved in ocular development. Thus, ipRGCs contribute substantially to mouse eye growth and myopia development, which may inspire novel strategies for myopia intervention.

Differential effects of experimental glaucoma on intrinsically photosensitive retinal ganglion cells in mice

Glaucoma is a group of eye diseases characterized by retinal ganglion cell (RGC) loss and optic nerve damage. Studies, including this study, support that RGCs degenerate and die in a type-specific manner following the disease insult. Here we specifically examined one RGC type, the intrinsically photosensitive retinal ganglion cell (ipRGC), and its associated functional deficits in a mouse model of experimental glaucoma. We induced chronic ocular hypertension (OHT) by laser photocoagulation and then characterized the survival of ipRGC subtypes. We found that ipRGCs suffer significant loss, similar to the general RGC population, but ipRGC subtypes are differentially affected following chronic OHT. M4 ipRGCs, which are involved in pattern vision, are susceptible to chronic OHT. Correspondingly, mice with chronic OHT experience reduced contrast sensitivity and visual acuity. By comparison, M1 ipRGCs, which project to the suprachiasmatic nuclei to regulate circadian rhythmicity, exhibit almost no cell loss following chronic OHT. Accordingly, we observed that circadian re-entrainment and circadian rhythmicity are largely not disrupted in OHT mice. Our study demonstrates the link between subtype-specific ipRGC survival and behavioral deficits in glaucomatous mice. These findings provide insight into glaucoma-induced visual behavioral deficits and their underlying mechanisms.

Is melanopsin activation affecting large field color-matching functions?

Color theory is based on the exclusive activation of cones. However, since the discovery of melanopsin expressing cells in the human retina, evidence of its intrusion in brightness and color vision is increasing. We aimed to assess if differences between peripheral or large field and foveal color matches can be accounted for by melanopsin activation or rod intrusion. Photopic color matches by young observers showed that differences between extrafoveal and foveal results cannot be explained by rod intrusion. Furthermore, statistical analyses on existing color-matching functions suggest a role of melanopsin activation, particularly, in large field S fundamentals.

Study of display white point based on mixed chromatic adaptation

In this study, the settings of the display white points were investigated, which presented the color appearance matched with a neutral surface as observed in the state of mixed chromatic adaptation. A psychophysical experiment was conducted under 20 illumination and viewing conditions via successive binocular color matching. It is discovered that the metameric light sources have generally equivalent effects on the observers' adaptation states and the resulting white points. The correlated color temperature (CCT) of the illumination and the adapting luminance, both with a significant influence on the mixed chromatic adaptation, exhibit a positive and a negative relation to the white point CCT, respectively. The immersive illumination affects the white point through the adaptation ratio and the baseline illuminant. Finally, the experimental results were verified to be predictable with an amended mixed chromatic adaptation model, which produced a mean chromaticity error of only 0.0027 units of CIE 1976 u'v'.

Rodent models in translational circadian photobiology

Over the last decades remarkable advances have been made in the understanding of the photobiology of circadian rhythms. The identification of a third photoreceptive system in the mammalian eye, in addition to the rods and cones that mediate vision, has transformed our appreciation of the role of light in regulating physiology and behavior. These photosensitive retinal ganglion cells (pRGCs) express the blue-light sensitive photopigment melanopsin and project to the suprachiasmatic nuclei (SCN)-the master circadian pacemaker-as well as many other brain regions. Much of our understanding of the fundamental mechanisms of the pRGCs, and the processes that they regulate, comes from mouse and other rodent models. Here we describe the contribution of rodent models to circadian photobiology, including both their strengths and limitations. In addition, we discuss how an appreciation of both rodent and human data is important for translational circadian photobiology. Such an approach enables a bi-directional flow of information whereby an understanding of basic mechanisms derived from mice can be integrated with studies from humans. Progress in this field is being driven forward at several levels of analysis, not least by the use of personalized light measurements and photoreceptor specific stimuli in human studies, and by studying the impact of environmental, rather than laboratory, lighting on different rodent models.

Rhodopsins at a glance

Rhodopsins are photoreceptive membrane proteins consisting of a common heptahelical transmembrane architecture that contains a retinal chromophore. Rhodopsin was first discovered in the animal retina in 1876, but a different type of rhodopsin, bacteriorhodopsin, was reported to be present in the cell membrane of an extreme halophilic archaeon, Halobacterium salinarum, 95 years later. Although these findings were made by physiological observation of pigmented tissue and cell bodies, recent progress in genomic and metagenomic analyses has revealed that there are more than 10,000 microbial rhodopsins and 9000 animal rhodopsins with large diversity and tremendous new functionality. In this Cell Science at a Glance article and accompanying poster, we provide an overview of the diversity of functions, structures, color discrimination mechanisms and optogenetic applications of these two rhodopsin families, and will also highlight the third distinctive rhodopsin family, heliorhodopsin.

Melanopic stimulation does not alter psychophysical threshold sensitivity for luminance flicker

In addition to the rod and cone photoreceptors the retina contains intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells express the photopigment melanopsin and are known to be involved in reflexive visual functions such as pupil response and photo-entrainment of the circadian rhythm. It is possible that the ipRGCs contribute to conscious visual perception, either by providing an independent signal to the geniculo-striate pathway, or by interacting with and thus modifying signals arising from “classical” retinal ganglion cells that combine and contrast cone input. Here, we tested for the existence of an interaction by asking if a 350% change in melanopsin stimulation alters psychophysical sensitivity for the detection of luminance flicker. In Experiment 1, we tested for a change in the threshold for detecting luminance flicker in three participants after they adapted to backgrounds with different degrees of tonic melanopsin stimulation. In Experiments 2 and 3, this test was repeated, but now for luminance flicker presented on a transient pedestal of melanopsin stimulation. Across the three experiments, no effect of melanopsin stimulation upon threshold flicker sensitivity was found. Our results suggest that even large changes in melanopsin stimulation do not affect near-threshold, cone-mediated visual perception.

Bongard and Smirnov on the tetrachromacy of extra-foveal vision

In Moscow in the 1950's, the physicist M. M. Bongard developed the use of silent substitution to establish the number of dimensions of human or animal colour vision and to derive colour-matching functions either for whole organisms or for individual neuronal channels. In 1956, he and his colleague M. S. Smirnov reported that extra-foveal human vision was tetrachromatic when tested by the silent-substitution method that they called 'replacement colorimetry'. In the steady state, trichromatic matches were possible in extra-foveal regions, but transients were visible when one such match was replaced by another. If, however, a match was made with four primaries, then a silent substitution was possible; and such matches - unlike trichromatic ones - were stable with light level and with changes in the state of chromatic adaptation. Bongard and Smirnov believed that the fourth receptor had the spectral sensitivity of the rods, but of course they were working long before the discovery of intrinsically photosensitive retinal ganglion cells. On the fiftieth anniversary of Bongard's grievous death, we provide a translation of Bongard and Smirnov's paper on the tetrachromacy of extra-foveal vision. In a commentary, we give the background to their work and provide further details of their apparatus and procedure. We briefly discuss related research and the reception in the West of Bongard and Smirnov's claims. We suggest that an analogy can be made between the tetrachromacy of the parafovea and the 'weak tetrachromacy' of heterozygotes for anomalous colour vision, whose trichromatic matches are not stable with chromatic adaptation.

PupilEXT: Flexible Open-Source Platform for High-Resolution Pupillometry in Vision Research

The human pupil behavior has gained increased attention due to the discovery of the intrinsically photosensitive retinal ganglion cells and the afferent pupil control path's role as a biomarker for cognitive processes. Diameter changes in the range of 10-2 mm are of interest, requiring reliable and characterized measurement equipment to accurately detect neurocognitive effects on the pupil. Mostly commercial solutions are used as measurement devices in pupillometry which is associated with high investments. Moreover, commercial systems rely on closed software, restricting conclusions about the used pupil-tracking algorithms. Here, we developed an open-source pupillometry platform consisting of hardware and software competitive with high-end commercial stereo eye-tracking systems. Our goal was to make a professional remote pupil measurement pipeline for laboratory conditions accessible for everyone. This work's core outcome is an integrated cross-platform (macOS, Windows and Linux) pupillometry software called PupilEXT, featuring a user-friendly graphical interface covering the relevant requirements of professional pupil response research. We offer a selection of six state-of-the-art open-source pupil detection algorithms (Starburst, Swirski, ExCuSe, ElSe, PuRe and PuReST) to perform the pupil measurement. A developed 120-fps pupillometry demo system was able to achieve a calibration accuracy of 0.003 mm and an averaged temporal pupil measurement detection accuracy of 0.0059 mm in stereo mode. The PupilEXT software has extended features in pupil detection, measurement validation, image acquisition, data acquisition, offline pupil measurement, camera calibration, stereo vision, data visualization and system independence, all combined in a single open-source interface, available at https://github.com/openPupil/Open-PupilEXT.

Spherical arena reveals optokinetic response tuning to stimulus location, size, and frequency across entire visual field of larval zebrafish

Many animals have large visual fields, and sensory circuits may sample those regions of visual space most relevant to behaviours such as gaze stabilisation and hunting. Despite this, relatively small displays are often used in vision neuroscience. To sample stimulus locations across most of the visual field, we built a spherical stimulus arena with 14,848 independently controllable LEDs. We measured the optokinetic response gain of immobilised zebrafish larvae to stimuli of different steradian size and visual field locations. We find that the two eyes are less yoked than previously thought and that spatial frequency tuning is similar across visual field positions. However, zebrafish react most strongly to lateral, nearly equatorial stimuli, consistent with previously reported spatial densities of red, green, and blue photoreceptors. Upside-down experiments suggest further extra-retinal processing. Our results demonstrate that motion vision circuits in zebrafish are anisotropic, and preferentially monitor areas with putative behavioural relevance.

Evening preference correlates with regional brain volumes in the anterior occipital lobe

Chronotype or diurnal preference is a questionnaire-based measure influenced both by circadian period and by the sleep homeostat. In order to further characterize the biological determinants of these measures, we used a hypothesis-free approach to investigate the association between the score of the morningness-eveningness questionnaire (MEQ) and the Munich chronotype questionnaire (MCTQ), as continuous variables, and volumetric measures of brain regions acquired by magnetic resonance imaging (MRI). Data were collected from the Baependi Heart Study cohort, based in a rural town in South-Eastern Brazil. MEQ and anatomical 1.5-T MRI scan data were available from 410 individuals, and MCTQ scores were available from a subset of 198 of them. The average MEQ (62.2 ± 10.6) and MCTQ (average MSFsc 201 ± 85 min) scores were suggestive of a previously reported strong general tendency toward morningness in this community. Setting the significance threshold at P > .002 to account for multiple comparisons, we observed a significant association between lower MEQ score (eveningness) and greater volume of the left anterior occipital sulcus (β = -0.163, p = .001) of the occipital lobe. No significant associations were observed for MCTQ. This may reflect the smaller dataset for MCTQ, and/or the fact that MEQ, which asks questions about preferred timings, is more trait-like than the MCTQ, which asks questions about actual timings. The association between MEQ and a brain region dedicated to visual information processing is suggestive of the increasingly recognized fluidity in the interaction between visual and nonvisual photoreception and the circadian system, and the possibility that chronotype includes an element of masking.

Intrinsically Photosensitive Retinal Ganglion Cells of the Human Retina

Light profoundly affects our mental and physical health. In particular, light, when not delivered at the appropriate time, may have detrimental effects. In mammals, light is perceived not only by rods and cones but also by a subset of retinal ganglion cells that express the photopigment melanopsin that renders them intrinsically photosensitive (ipRGCs). ipRGCs participate in contrast detection and play critical roles in non-image-forming vision, a set of light responses that include circadian entrainment, pupillary light reflex (PLR), and the modulation of sleep/alertness, and mood. ipRGCs are also found in the human retina, and their response to light has been characterized indirectly through the suppression of nocturnal melatonin and PLR. However, until recently, human ipRGCs had rarely been investigated directly. This gap is progressively being filled as, over the last years, an increasing number of studies provided descriptions of their morphology, responses to light, and gene expression. Here, I review the progress in our knowledge of human ipRGCs, in particular, the different morphological and functional subtypes described so far and how they match the murine subtypes. I also highlight questions that remain to be addressed. Investigating ipRGCs is critical as these few cells play a major role in our well-being. Additionally, as ipRGCs display increased vulnerability or resilience to certain disorders compared to conventional RGCs, a deeper knowledge of their function could help identify therapeutic approaches or develop diagnostic tools. Overall, a better understanding of how light is perceived by the human eye will help deliver precise light usage recommendations and implement light-based therapeutic interventions to improve cognitive performance, mood, and life quality.

Photosensitive ganglion cells: A diminutive, yet essential population

Our visual system has evolved to provide us with an image of the scene that surrounds us, informing us of its texture, colour, movement, and depth with an enormous spatial and temporal resolution, and for this purpose, the image formation (IF) dedicates the vast majority of our retinal ganglion cell (RGC) population and much of our cerebral cortex. On the other hand, a minuscule proportion of RGCs, in addition to receiving information from classic cone and rod photoreceptors, express melanopsin and are intrinsically photosensitive (ipRGC). These ipRGC are dedicated to non-image-forming (NIF) visual functions, of which we are unaware, but which are essential for aspects related to our daily physiology, such as the timing of our circadian rhythms and our pupillary light reflex, among many others. Before the discovery of ipRGCs, it was thought that the IF and NIF functions were distinct compartments regulated by different RGCs, but this concept has evolved in recent years with the discovery of new types of ipRGCs that innervate subcortical IF regions, and therefore have IF visual functions. Six different types of ipRGCs are currently known. These are termed M1-M6, and differ in their morphological, functional, molecular properties, central projections, and visual behaviour responsibilities. A review is presented on the melanopsin visual system, the most active field of research in vision, for which knowledge has grown exponentially during the last two decades, when RGCs giving rise to this pathway were first discovered.

Plasticity in perception: insights from color vision deficiencies

Inherited color vision deficiencies typically result from a loss or alteration of the visual photopigments absorbing light and thus impact the very first step of seeing. There is growing interest in how subsequent steps in the visual pathway might be calibrated to compensate for the altered receptor signals, with the possibility that color coding and color percepts might be less severely impacted than the receptor differences predict. These compensatory adjustments provide important insights into general questions about sensory plasticity and the sensory and cognitive processes underlying how we experience color.