Melanopsin photoreception differentially modulates rod-mediated and cone-mediated human temporal vision

Measuring contrast processing in the visual system using the steady state visually evoked potential (SSVEP)

Contrast is the currency of the early visual system. Measuring the way that the computations underlying contrast processing depend on factors such as spatial and temporal frequency, age, clinical conditions, eccentricity, chromaticity and the presence of other stimuli has been a focus of vision science for over a century. One of the most productive experimental approaches in this field has been the use of the 'steady-state visually-evoked potential' (SSVEP): a technique where contrast modulating inputs are 'frequency tagged' (presented at well-defined frequencies and phases) and the electrical signals that they generate in the brain are analyzed in the temporal frequency domain. SSVEPs have several advantages over conventional measures of visually-evoked responses: they have relatively unambiguous ouput measures, a high signal to noise ratio (SNR), and they allow us to analyze interactions between stimulus components using a convenient mathematical framework. Here we describe how SSVEPs have been used to study visual contrast over the past 70 years. Because our thinking about SSVEPs is well-described by simple mathematical models, we embed code that illustrates key steps in the modelling and analysis. This paper can therefore be used both as a review of the use of SSVEP in measuring human contrast processing, and as an interactive learning aid.

Within-subjects ultra-short sleep-wake protocol for characterising circadian variations in retinal function

Prior studies suggest that visual functions undergo time-of-day variations. Under naturalistic entrainment, diurnal changes in physiology may be driven by circadian and/or homeostatic processes, and repeated measurements at different times of day are thus not suitable to draw unambiguous conclusions about circadian effects on visual function. In this study, we disentangle circadian and homeostatic effects on variations of retinal function. We examine the earliest stages of image-forming (temporal contrast sensitivity of the post-receptoral channels) and non-image forming visual functions (pupillary light response) by employing a short forced-desynchrony multiple-naps protocol lasting 40 hours. Participants (n = 12, 50% female) will stay in a controlled time-isolating environment under dim-light conditions and adhere to an ultra-short sleep-wake cycle, alternating between 2h30m of wake time in dim light and hour of sleep in no light. During eleven intervals of wakefulness, participants will undergo psychophysical and pupillometric assessments with silent-substitution stimuli. We hypothesize that the sensitivity of retinal mechanisms undergoes circadian variations. This hypothesis will be investigated by separately determining psychophysical contrast thresholds to silent-substitution stimuli targeting the post-receptoral (consistency) pathways (isoluminant red-green, L-M; isoluminant blue-yellow, S; luminance, L+M+S). We will furthermore measure the pupillary light response to peripheral stimuli (annulus 10∘-30∘) in comparison to the response to stimuli isolating or including melanopsin stimulation. All stimuli will be delivered at constant retinal irradiance using a Maxwellian view system or artificially restricting pupil size. Additionally, we will quantify and report effects of our test stimuli on the circadian system by comparing the dim light melatonin onset (DLMO) timing during two supplementary evening sessions, comparing dim-light conditions to such with experimental light exposure. Our work informs the fundamental biological mechanisms underlying the influence of light on the human circadian system. Based on our findings, current models about the sensitivity of the circadian system may need to be modified in order to account for the bidirectional influence of circadian function and photoreception.

Increased brightness assimilation in rod vision

Our visual system uses contextual cues to estimate the brightness of surfaces: brightness can shift toward (assimilation) or away from (contrast) the brightness of the surroundings. We investigated brightness induction at different light levels and found a potential influence of rod photoreceptors on brightness induction. We then used a novel tetrachromatic display to generate stimuli differentially exciting rods or cones at a fixed light adaptation level. Under rod vision, brightness assimilation was enhanced while brightness contrast was not altered in comparison to cone vision. We ruled out that this effect was mediated by the low resolution of night vision. Our findings suggest that rod vision affects the high-level interpretation of visual scenes that results in differences in brightness assimilation but not contrast. Our results imply that the visual system employs more perceptual inferences under rod vision than under cone vision to solve visual ambiguities in complex spatial displays.

What can the eye see with melanopsin?

A subpopulation of human retinal ganglion cells contains the melanopsin photopigment, allowing them to act as a fifth photoreceptor class. These ganglion cells project to the visual cortex, but to reveal its intrinsic contribution to conscious vision is technically challenging as it requires melanopsin to be separated from the responses originating in the rods and three cone classes. Using a display engineered to isolate the melanopic visual response, we show that it detects lowpass spatial (≤0.35 cycles per degree) and temporal image content (≤1 Hz) but cannot reconstruct the stimulus form necessary for object recognition. We demonstrate that a model of the spatially diffuse intrinsically-photosensitive retinal ganglion cells' sampling structure is predictive of the measured image reconstruction limits of melanopic spatial vision. Separately, we find that under five-photoreceptor silent substitution conditions, rod pathways alone can support form vision in bright lighting when typically thought to be in saturation. Form vision that is absent from melanopsin can be only perceived in mixtures of both melanopsin and rod signals because it is the rod pathway that sees the form. Our findings show that melanopsin's unique tuning to the diffuse and slow-changing elements in the world provides a stabilized reference point for vision.

Investigating light sensitivity in bipolar disorder (HELIOS-BD)

Many people with bipolar disorder have disrupted circadian rhythms. This means that the timing of sleep and wake activities becomes out-of-sync with the standard 24-hour cycle. Circadian rhythms are strongly influenced by light levels and previous research suggests that people with bipolar disorder might have a heightened sensitivity to light, causing more circadian rhythm disruption, increasing the potential for triggering a mood switch into mania or depression. Lithium has been in clinical use for over 70 years and is acknowledged to be the most effective long-term treatment for bipolar disorder. Lithium has many reported actions in the body but the precise mechanism of action in bipolar disorder remains an active area of research. Central to this project is recent evidence that lithium may work by stabilising circadian rhythms of mood, cognition and rest/activity. Our primary hypothesis is that people with bipolar disorder have some pathophysiological change at the level of the retina which makes them hypersensitive to the visual and non-visual effects of light, and therefore more susceptible to circadian rhythm dysfunction. We additionally hypothesise that the mood-stabilising medication lithium is effective in bipolar disorder because it reduces this hypersensitivity, making individuals less vulnerable to light-induced circadian disruption. We will recruit 180 participants into the HELIOS-BD study. Over an 18-month period, we will assess visual and non-visual responses to light, as well as retinal microstructure, in people with bipolar disorder compared to healthy controls. Further, we will assess whether individuals with bipolar disorder who are being treated with lithium have less pronounced light responses and attenuated retinal changes compared to individuals with bipolar disorder not being treated with lithium. This study represents a comprehensive investigation of visual and non-visual light responses in a large bipolar disorder population, with great translational potential for patient stratification and treatment innovation.

Illumination by short-wavelength light inside the blind spot decreases light detectability

Although the optic disk corresponding to the blind spot contains no classical photoreceptors, it contains photopigment melanopsin. To clarify whether melanopsin is involved in light detection, we conducted detection tasks for light stimuli presented in the normal visual field, with and without another illumination inside the blind spot. We found that a blue blind-spot illumination decreased the light detectability on a dark background. This effect was replicable when it was determined immediately after the blind-spot illumination was turned off, suggesting the contribution of a sluggish system rather than scattering. Moreover, the aforementioned effect was not observed when the blind-spot illumination was in red, indicating wavelength specificity in favor of melanopsin's sensitivity profile. These findings suggest that melanopsin is activated by the blind-spot illumination and thereby interferes with light detection near the absolute threshold. Light detection originating from conventional photoreceptors is modulated by melanopsin-based computation presumably estimating a baseline noise level.

Melanopsin-mediated amplification of cone signals in the human visual cortex

The ambient daylight variation is coded by melanopsin photoreceptors and their luxotonic activity increases towards midday when colour temperatures are cooler, and irradiances are higher. Although melanopsin and cone photoresponses can be mediated via separate pathways, the connectivity of melanopsin cells across all levels of the retina enables them to modify cone signals. The downstream effects of melanopsin-cone interactions on human vision are however, incompletely understood. Here, we determined how the change in daytime melanopsin activation affects the human cone pathway signals in the visual cortex. A 5-primary silent-substitution method was developed to evaluate the dependence of cone-mediated signals on melanopsin activation by spectrally tuning the lights and stabilizing the rhodopsin activation under a constant cone photometric luminance. The retinal (white noise electroretinogram) and cortical responses (visual evoked potential) were simultaneously recorded with the photoreceptor-directed lights in 10 observers. By increasing the melanopsin activation, a reverse response pattern was observed with cone signals being supressed in the retina by 27% (p = 0.03) and subsequently amplified by 16% (p = 0.01) as they reach the cortex. We infer that melanopsin activity can amplify cone signals at sites distal to retinal bipolar cells to cause a decrease in the psychophysical Weber fraction for cone vision.

Are ipRGCs involved in human color vision? Hints from physiology, psychophysics, and natural image statistics

Human photoreceptors consist of cones, rods, and melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs). First studied in circadian regulation and pupillary control, ipRGCs project to a variety of brain centers suggesting a broader involvement beyond non-visual functions. IpRGC responses are stable, long-lasting, and with a particular codification of photoreceptor signals. In comparison with the transient and adaptive nature of cone and rod signals, ipRGCs' signaling might provide an ecological advantage to different attributes of color vision. Previous studies have indicated melanopsin's influence on visual responses yet its contribution to color perception in humans remains debated. We summarized evidence and hypotheses (from physiology, psychophysics, and natural image statistics) about direct and indirect involvement of ipRGCs in human color vision, by first briefly assessing the current knowledge about the role of melanopsin and ipRGCs in vision and codification of spectral signals. We then approached the question about melanopsin activation eliciting a color percept, discussing studies using the silent substitution method. Finally, we explore various avenues through which ipRGCs might impact color perception indirectly, such as through involvement in peripheral color matching, post-receptoral pathways, color constancy, long-term chromatic adaptation, and chromatic induction. While there is consensus about the role of ipRGCs in brightness perception, confirming its direct contribution to human color perception requires further investigation. We proposed potential approaches for future research, emphasizing the need for empirical validation and methodological thoroughness to elucidate the exact role of ipRGCs in human color vision.

Targeting sleep and the circadian system as a novel treatment strategy for Parkinson's disease

There is a growing appreciation of the wide range of sleep-wake disturbances that occur frequently in Parkinson's disease. These are known to be associated with a range of motor and non-motor symptoms and significantly impact not only on the quality of life of the patient, but also on their bed partner. The underlying causes for fragmented sleep and daytime somnolence are no doubt multifactorial but there is clear evidence for circadian disruption in Parkinson's disease. This appears to be occurring not only as a result of the neuropathological changes that occur across a distributed neural network, but even down to the cellular level. Such observations indicate that circadian changes may in fact be a driver of neurodegeneration, as well as a cause for some of the sleep-wake symptoms observed in Parkinson's disease. Thus, efforts are now required to evaluate approaches including the prescription of precision medicine to modulate photoreceptor activation ratios that reflect daylight inputs to the circadian pacemaker, the use of small molecules to target clock genes, the manipulation of orexin pathways that could help restore the circadian system, to offer novel symptomatic and novel disease modifying strategies.

Melanopsin enhances image persistence

Contributions of the inner retinal photopigment melanopsin to human visual perception are incompletely understood. Here, we use a four-primary display to produce stimuli differing in melanopsin versus cone contrast in psychophysical paradigms in eight subjects with normal color vision. We address two predictions from electrophysiological recordings of the melanopsin system in non-human mammals: melanopsin influences color and/or supports image persistence under visual fixation. We first construct chromatic contrast sensitivity contours for stimuli differing in melanopsin excitation presented as a central annulus (10°) or peripheral (22.5°) spot. We find that although including melanopsin contrast produces modest changes in the average chromatic coordinates in both eccentricities, this occurs equally at low (0.5 Hz) and higher (3.75 Hz) temporal frequencies, arguing that it reflects divergence in cone spectral sensitivity in our participants from that captured in standardized cone fundamentals rather than a melanopsin contribution to color. We continue to ask whether the established ability of melanopsin to sustain firing of visual neurons under extended light exposure has a visual correlate, using the optical illusion of Troxler fading in which blurred spots in periphery disappear during visual fixation. We find that introducing additional melanopsin contrast (+28% Michelson contrast) to either bright or dark spots increases fading latency by 35% ± 8.8% and 41% ± 13.6%, respectively. Our data argue that the primary contribution of melanopsin to perception under these conditions is not to provide a color percept but rather to enhance persistence of low spatial frequency patterns during visual fixation.

Enhanced human contrast sensitivity with increased stimulation of melanopsin in intrinsically photosensitive retinal ganglion cells

The intrinsically photosensitive retinal ganglion cells (ipRGCs) are known to serve non-image-forming functions, such as photoentrainment of the circadian rhythm and pupillary light reflex. However, how they affect human spatial vision is largely unknown. The spatial contrast sensitivity function (CSF), which measures contrast sensitivity as a function of spatial frequency, was used in the current study to investigate the function of ipRGCs in pattern vision. To compare the effects of different background lights on the CSF, we utilized the silent substitution technique. We manipulated the stimulation level of melanopsin (i.e., the visual pigment of ipRGCs) from the background light while keeping the cone stimulations constant, or vice versa. We conducted four experiments to measure the CSFs at various spatial frequencies, eccentricities, and levels of background luminance. Results showed that melanopsin stimulation from the background light enhances spatial contrast sensitivity across different eccentricities and luminance levels. Our finding that melanopsin contributes to CSF, combined with the receptive field analysis, suggests a role for the magnocellular pathway and challenges the conventional view that ipRGCs are primarily responsible for non-visual functions.

PySilSub: An open-source Python toolbox for implementing the method of silent substitution in vision and nonvisual photoreception research

The normal human retina contains several classes of photosensitive cell-rods for low-light vision, three cone classes for daylight vision, and intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing melanopsin for non-image-forming functions, including pupil control, melatonin suppression, and circadian photoentrainment. The spectral sensitivities of the photoreceptors overlap significantly, which means that most lights will stimulate all photoreceptors to varying degrees. The method of silent substitution is a powerful tool for stimulating individual photoreceptor classes selectively and has found much use in research and clinical settings. The main hardware requirement for silent substitution is a spectrally calibrated light stimulation system with at least as many primaries as there are photoreceptors under consideration. Device settings that will produce lights to selectively stimulate the photoreceptor(s) of interest can be found using a variety of analytic and algorithmic approaches. Here we present PySilSub (https://github.com/PySilentSubstitution/pysilsub), a novel Python package for silent substitution featuring flexible support for individual colorimetric observer models (including human and mouse observers), multiprimary stimulation devices, and solving silent substitution problems with linear algebra and constrained numerical optimization. The toolbox is registered with the Python Package Index and includes example data sets from various multiprimary systems. We hope that PySilSub will facilitate the application of silent substitution in research and clinical settings.

Protocol for isolation of melanopsin and rhodopsin in the human eye using silent substitution

Melanopsin-mediated visual and non-visual functions are difficult to study in vivo. To isolate melanopsin responses, non-standard light stimulation instruments are required, with at least as many primaries as photoreceptor classes in the eye. In this protocol, we describe the physical light calibrations of the display instrumentation, control of stimulus artefacts, and correction of individual between-eye differences in human observers. The protocol achieves complete photoreceptor silent substitution in psychophysical, pupillometry, and electroretinographic experiments for probing melanopsin, rod, and cone function. For complete details on the use and execution of this protocol, please refer to Uprety et al. (2022).1.

Melanopsin-driven surround induction on the red/green balance of yellow

To test the potential role of melanopsin-dependent ipRGCs in surround induction effects, we used a four-channel projector apparatus to hold the cone activity in a surround constant while varying the amount of melanopsin activity between two levels: low (baseline) and high (136% of the baseline). Rods were partially controlled by having the subjects complete conditions after either adapting to a bright field or darkness. The subjects adjusted the red/green balance of a 2.5° central target that varied in its ratio of L and M cones, but was equiluminant with the surround, to a perceptual null point (neither reddish nor greenish). When the surround melanopsin activity was higher, the subjects set their yellow balances at significantly higher L/(L+M) ratios, suggesting the high melanopsin surround was inducing greenishness into the central yellow stimulus. This is consistent with surround brightness effects that show the induction of greenishness into a central yellow test by high luminance surrounds. This potentially provides further evidence for a general role of melanopsin activity in brightness perception.