Delayed response of human melanopsin retinal ganglion cells on the pupillary light reflex

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.

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.

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.

Correlated color temperature and light intensity: Complementary features in non-visual light field

An appropriate exposure to the light-dark cycle, with high irradiances during the day and darkness during the night is essential to keep our physiology on time. However, considering the increasing exposure to artificial light at night and its potential harmful effects on health (i.e. chronodisruption and associated health conditions), it is essential to understand the non-visual effects of light in humans. Melatonin suppression is considered the gold standard for nocturnal light effects, and the activation of intrinsically photosensitive retinal ganglion cells (ipRGCs) through the assessment of pupillary light reflex (PLR) has been recently gaining attention. Also, some theoretical models for melatonin suppression and retinal photoreceptors activation have been proposed. Our aim in this study was to determine the influence of correlated color temperature (CCT) on melatonin suppression and PLR, considering two commercial light sources, as well as to explore the possible correlation between both processes. Also, the contribution of irradiance (associated to CCT) was explored through mathematical modelling on a wider range of light sources. For that, melatonin suppression and PLR were experimentally assessed on 16 healthy and young volunteers under two light conditions (warmer, CCT 3000 K; and cooler, CCT 5700 K, at ~5·1018 photons/cm2/sec). Our experimental results yielded greater post-stimulus constriction under the cooler (5700 K, 13.3 ± 1.9%) than under the warmer light (3000 K, 8.7 ± 1.2%) (p < 0.01), although no significant differences were found between both conditions in terms of melatonin suppression. Interestingly, we failed to demonstrate correlation between PLR and melatonin suppression. Although methodological limitations cannot be discarded, this could be due to the existence of different subpopulations of Type 1 ipRGCs differentially contributing to PLR and melatonin suppression, which opens the way for further research on ipRGCs projection in humans. The application of theoretical modelling suggested that CCT should not be considered separately from irradiance when designing nocturnal/diurnal illumination systems. Further experimental studies on wider ranges of CCTs and light intensities are needed to confirm these conclusions.

Deep learning-based pupil model predicts time and spectral dependent light responses

Although research has made significant findings in the neurophysiological process behind the pupillary light reflex, the temporal prediction of the pupil diameter triggered by polychromatic or chromatic stimulus spectra is still not possible. State of the art pupil models rested in estimating a static diameter at the equilibrium-state for spectra along the Planckian locus. Neither the temporal receptor-weighting nor the spectral-dependent adaptation behaviour of the afferent pupil control path is mapped in such functions. Here we propose a deep learning-driven concept of a pupil model, which reconstructs the pupil's time course either from photometric and colourimetric or receptor-based stimulus quantities. By merging feed-forward neural networks with a biomechanical differential equation, we predict the temporal pupil light response with a mean absolute error below 0.1 mm from polychromatic (2007 [Formula: see text] 1 K, 4983 [Formula: see text] 3 K, 10,138 [Formula: see text] 22 K) and chromatic spectra (450 nm, 530 nm, 610 nm, 660 nm) at 100.01 ± 0.25 cd/m2. This non-parametric and self-learning concept could open the door to a generalized description of the pupil behaviour.

Prediction accuracy of L- and M-cone based human pupil light models

Multi-channel LED luminaires offer a powerful tool to vary retinal receptor signals while keeping visual parameters such as color or brightness perception constant. This technology could provide new fields of application in indoor lighting since the spectrum can be enhanced individually to the users' favor or task. One possible application would be to optimize a light spectrum by using the pupil diameter as a parameter to increase the visual acuity. A spectral- and time-dependent pupil model is the key requirement for this aim. We benchmarked in our work selected L- and M-cone based pupil models to find the estimation error in predicting the pupil diameter for chromatic and polychromatic spectra at 100 cd/m2. We report an increased estimation error up to 1.21 mm for 450 nm at 60-300 s exposure time. At short exposure times, the pupil diameter was approximately independent of the used spectrum, allowing to use the luminance for a pupil model. Polychromatic spectra along the Planckian locus showed at 60-300 s exposure time, a prediction error within a tolerance range of ± 0.5 mm. The time dependency seems to be more essential than the spectral dependency when using polychromatic spectra.

Relative contributions of melanopsin to brightness discrimination when hue and luminance also vary

A large number of studies have shown the effect of melanopsin-dependent retinal ganglion cells on humans performing brightness discrimination tasks. These studies often utilized targets that only differ in their melanopsin activation levels, and not in their luminance or hue, which are both factors that make large contributions to brightness discrimination. The purpose of the present study was to evaluate the relative contribution of melanopsin activation to brightness discrimination when luminance and hue are also varying in addition to melanopsin activation. Using an apparatus consisting of three separate high luminance projectors, we were able to manipulate melanopsin-isolating stimulation, and L-, M-, and S-cone stimulation separately, thus allowing us to vary stimuli in their melanopsin activation, luminance, and hue category independently. We constructed three sets of target stimuli with three different levels of melanopsin activation (100%, 131%, and 167% relative melanopsin excitation) and five levels of luminance. We then had subjects do a two-alternative forced choice task where they compared the previously described target stimuli set to a set of four comparison stimuli that varied in their hue category but had identical luminances. We found that in our stimuli set the overall contribution of melanopsin activity to brightness discrimination was small (an average of 6% increase in likelihood to call a high melanopsin activity stimulus brighter compared to a low melanopsin activity stimulus) when luminance and hue also varied. However, a significant interaction showed that when the comparison was between stimuli differing only in melanopsin stimulation (with luminance and hue unchanged) the contribution of melanopsin to brightness judgments was about 3 times larger (an average of 18% increase in likelihood to call a high melanopsin activity stimulus brighter compared to a low melanopsin activity stimulus). This suggests that although luminance and hue have large effects on brightness discrimination such that the melanopsin contribution can become hard to detect, when there are minimal cone-dependent signals available, melanopsin can make a large contribution to brightness discrimination.

Photoreceptor inputs to pupil control

The size of the pupil depends on light level. Watson and Yellott (2012) developed a unified formula to predict pupil size from luminance, field diameter, age, and number of eyes. Luminance reflects input from the L and M cones in the retina but ignores the contribution of intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing the photopigment melanopsin, which are known to control the size of the pupil. We discuss the role of melanopsin in controlling pupil size by reanalyzing an extant data set (Bouma, 1962). We confirm that melanopsin-weighted quantities, in conjunction with Watson and Yellott's formula, adequately model intensity-dependent pupil size. We discuss the contributions of other photoreceptors into pupil control.

A longitudinal study of pupillary light reflex in 6- to 24-month children

Pupillary light reflex (PLR) is an involuntary response where the pupil size changes with luminance. Studies have shown that PLR response was altered in children with autism spectrum disorders (ASDs) and other neurological disorders. However, PLR in infants and toddlers is still understudied. We conducted a longitudinal study to investigate PLR in children of 6-24 months using a remote pupillography device. The participants are categorized into two groups. The 'high risk' (HR) group includes children with one or more siblings diagnosed with ASDs; whereas the 'low risk' (LR) group includes children without an ASD diagnosis in the family history. The participants' PLR was measured every six months until the age of 24 months. The results indicated a significant age effect in multiple PLR parameters including resting pupil radius, minimal pupil radius, relative constriction, latency, and response time. In addition, the HR group had a significantly larger resting and minimal pupil size than the LR group. The experimental data acquired in this study revealed not only general age-related PLR changes in infants and toddlers, but also different PLRs in children with a higher risk of ASD.

Appearance of Maxwell's spot in images rendered using a cyan primary

The discovery of melanopsin as a third type of retinal photoreceptor, contributing to both perceptual vision and reflex light responses, represents a new opportunity to optimise the design of artificial light sources for practical applications and to generate experimental stimuli. In the case of emissive displays, multiprimary designs incorporating a cyan primary could be used to allow melanopic radiance to be controlled independent of colour and luminance. Here we explore the performance a five-primary (violet, cyan, green, yellow, red) display device and find an anomaly in colour appearance when the cyan primary is employed. The anomaly took the form of a reddish/pinkish tinge in the central visual field, consistent with descriptions of Maxwell’s spot. This effect was apparent in some full colour images and in uniform discs over a range of chromaticities. Its appearance in coloured discs correlated with differences in calculated colour coordinate between central and peripheral vision. A simulation indicated that inclusion of any primary with predominant output in the 470–500 nm range has the potential to produce such a discrepancy in central vs peripheral appearance. Applying an additional constraint in colour processing to reproduce naturally occurring differences in central vs peripheral colour coordinate eliminated appearance of the spot and produced acceptable colour images.

The flicker Pupil Light Response (fPLR)

PURPOSE

The photoreceptor classes driving the flicker pupil light response (fPLR) to monochromatic sinusoidal temporal modulation are largely unknown. Here, we determine the photoreceptor inputs to the fPLR.

METHODS

The 0.5-Hz fPLR was measured in healthy observers using a Maxwellian view (41° diameter) pupillometer at five narrowband wavelengths (short: 409 nm; intermediate: 462, 507, 530 nm; and long: 592 nm) over ∼10 log units of irradiance spanning scotopic to photopic levels (5.6 to 15.6 log quanta·cm-2·s-1; -6.9 to 3.6 log cd·m-2). The relative photoreceptor contributions to the fPLR were then derived from these amplitude-irradiance functions using a criterion fPLR.

RESULTS

The fPLR amplitude is small (≤ 3.9 ± 3.1%; mean ± SEM) below 8.0 log quanta·cm-2·s-1 then increases with retinal irradiance in accordance with a Hill function that asymptotes between 13.0 to 15.0 log quanta·cm-2·s-1 (wavelength dependent). The Hill slope is steepest for the intermediate wavelengths. Further increases in irradiance (>15.0 log quanta·cm-2·s-1) produce a distinct suppression of the fPLR for the intermediate wavelengths. The fPLR phase delay shows a linear decrease with increasing irradiance. The spectral sensitivity of the fPLR is dominated by inner retinal melanopsin ganglion cell and outer retinal rod photoreceptor inputs to the afferent pupil control pathway; the relative melanopsin : rhodopsin weighting decreases with the transition from photopic to scotopic lighting.

CONCLUSIONS

The fPLR can be used as a marker of melanopsin and rod interactions during the flicker stimulation and to quantify their contributions to the post-illumination pupil response (PIPR).

TRANSLATIONAL RELEVANCE

These irradiance and wavelength responses will be useful in standardizing the measurements of the fPLR using chromatic pupillometry.

Melanopsin contributions to non-visual and visual function

Melanopsin is a short-wavelength-sensitive photopigment that was discovered only around 20 years ago. It is expressed in the cell bodies and processes of a subset of retinal ganglion cells in the retina (the intrinsically photosensitive retinal ganglion cells; ipRGCs), thereby allowing them to signal light even in the absence of cone and rod input. Many of the fundamental properties of melanopsin signalling in humans for both visual (e.g. detection, discrimination, brightness estimation) and non-visual function (e.g. melatonin suppression, circadian phase shifting) remain to be elucidated. Here, we give an overview of what we know about melanopsin contributions in visual function and non-visual function.

How Does Light Regulate Mood and Behavioral State?

The idea that light affects mood and behavioral state is not new. However, not much is known about the particular mechanisms and circuits involved. To fully understand these, we need to know what properties of light are important for mediating changes in mood as well as what photoreceptors and pathways are responsible. Increasing evidence from both human and animal studies imply that a specialized class of retinal ganglion cells, intrinsically photosensitive retinal ganglion cells (ipRGCs), plays an important role in the light-regulated effects on mood and behavioral state, which is in line with their well-established roles in other non-visual responses (pupillary light reflex and circadian photoentrainment). This paper reviews our current understanding on the mechanisms and paths by which the light information modulates behavioral state and mood.

Colorful glares: Effects of colors on brightness illusions measured with pupillometry

We hypothesized that pupil constrictions to the glare illusion, where converging luminance gradients subjectively enhance the perception of brightness, would be stronger for 'blue' than for other colors. Such an expectation was based on reflections about the ecology of vision, where the experience of dazzling light is common when one happens to look directly at sunlight through some occluders. Thus, we hypothesized that pupil constrictions to 'blue' reflect an ecologically-based expectation of the visual system from the experience of sky's light and color, which also leads to interpret the blue gradients of illusory glare to act as effective cues to impending probable intense light. We therefore manipulated the gradients color of glare illusions and measured changes in subjective brightness of identical shape stimuli. We confirmed that the blue resulted in what was subjectively evaluated as the brightest condition, despite all colored stimuli were equiluminant. This enhanced brightness effect was observed both in a psychophysical adjustment task and in changes in pupil size, where the maximum pupil constriction peak was observed with the 'blue' converging gradients over and above to the pupil response to blue in other conditions (i.e., diverging gradients and homogeneous patches). Moreover, glare-related pupil constrictions for each participant were correlated to each individual's subjective brightness adjustments. Homogenous blue hues also constricted the pupil more than other hues, which represents a pupillometric analog of the Helmholtz-Kohlrausch effect on brightness perception. Together, these findings show that pupillometry constitutes an easy tool to assess individual differences in color brightness perception.

Effect of Stimulus Intensity and Visual Field Location on Rod- and Cone-Mediated Pupil Response to Focal Light Stimuli

Purpose

To assess the effect of stimulus intensity on rod- and cone-mediated pupil light reflex (PLR) to small stimuli presented at central and peripheral visual field (VF) locations.

Methods

The PLR to small (0.43°) chromatic stimuli was tested in the right eye of healthy subjects. Blue (485 ± 20 nm) and red (625 ± 15 nm) stimuli were presented at incremental light intensities (0.5-3.75 log cd/m2) at peripheral (21.21°) and central (4.24°) VF locations using a chromatic pupilloperimeter under mesopic or blue light adaptation conditions. The percentage of pupil contraction (PPC), maximal pupil contraction velocity (MCV), latency of MCV (LMCV) and the ratio of central to peripheral responses for PPC (QPPC value) were determined.

Results

Under mesopic light adaptation conditions, the mean PPC recorded in response to red stimuli was lower than blue stimuli in all VF locations and light intensities, and the QPPC values were higher in response to red compared with blue light stimuli across the light intensity range tested. With blue background light, the pupil responses for red and blue light stimuli were approximately the same in the peripheral VF. LMCV was nearly constant in all VF locations for blue and red stimuli, respectively.

Conclusions

The chromatic pupilloperimeter enables the assessment of rod- and cone- contribution to the PLR in different VF locations. The optimal light intensities determined here for the assessment of focal activation of the two photoreceptor systems may be used for clinical evaluation of photoreceptor health.

A quantitative analysis of the contribution of melanopsin to brightness perception

In the retina, intrinsically photosensitive retinal ganglion cells (ipRGCs) which express photopigment melanopsin have been identified as photoreceptors which differ from cones and rods. It has been established that such melanopsin-expressing RGCs are involved in the circadian photo-entrainment and pupillary light reflexes. An additional projection from ipRGCs to the lateral geniculate nucleus has been identified, which indicates the association of ipRGCs with visual perception induced by the image-forming pathway. Reportedly, ipRGCs modulate brightness perception but quantitative analysis of brightness perception involving melanopsin and cones-based signals has not been elucidated. We conducted brightness perception experiments that involved melanopsin using a novel projector with six primary colors and formulated the results for melanopsin and cone stimuli. The white visual stimuli (5 degrees in size) that we used had a single xy-chromaticity values but melanopsin stimuli were modulated by designing different spectral distributions. Perceived brightness was measured using a magnitude estimation method at several luminance levels in the near periphery (7 degrees). Additionally, pupil diameter was measured for estimating the intensity of visual stimuli on the retina. The results showed that the perceived brightness of a white visual stimulus with different spectral distributions can be described by a summation of the nearly linear melanopsin response and the non-linear cone response with weighted coefficients, and the contribution ratio of melanopsin in brightness perception increased to 50% and more with increasing visual stimulus. These suggest that melanopsin signals play a crucial role in the estimation of the absolute intensity of the light environment by obtaining absolute brightness information even when cones are adapted by light.

Pupil responses to hidden photoreceptor-specific modulations in movies

Under typical daytime light levels, the human pupillary light response (PLR) is driven by the activity of the L, M, and S cones, and melanopsin expressed in the so-called intrinsically photosensitive retinal ganglion cells (ipRGCs). However, the importance of each of these photoreceptive mechanisms in defining pupil size under real-world viewing conditions remains to be established. To address this question, we embedded photoreceptor-specific modulations in a movie displayed using a novel projector-based five-primary spatial stimulation system, which allowed for the precise control of photoreceptor activations in time and space. We measured the pupillary light response in eleven observers, who viewed short cartoon movies which contained hidden low-frequency (0.25 Hz) silent-substitution modulations of the L, M and S cones (no stimulation of melanopsin), melanopsin (no stimulation of L, M and S cones), both L, M, and S cones and melanopsin or no modulation at all. We find that all photoreceptors active at photopic light levels regulate pupil size under this condition. Our data imply that embedding modulations in photoreceptor contrast could provide a method to manipulate key adaptive aspects of the human visual system in everyday, real-world activities such as watching a movie.

Standards in Pupillography

The number of research groups studying the pupil is increasing, as is the number of publications. Consequently, new standards in pupillography are needed to formalize the methodology including recording conditions, stimulus characteristics, as well as suitable parameters of evaluation. Since the description of intrinsically photosensitive retinal ganglion cells (ipRGCs) there has been an increased interest and broader application of pupillography in ophthalmology as well as other fields including psychology and chronobiology. Color pupillography plays an important role not only in research but also in clinical observational and therapy studies like gene therapy of hereditary retinal degenerations and psychopathology. Stimuli can vary in size, brightness, duration, and wavelength. Stimulus paradigms determine whether rhodopsin-driven rod responses, opsin-driven cone responses, or melanopsin-driven ipRGC responses are primarily elicited. Background illumination, adaptation state, and instruction for the participants will furthermore influence the results. This standard recommends a minimum set of variables to be used for pupillography and specified in the publication methodologies. Initiated at the 32nd International Pupil Colloquium 2017 in Morges, Switzerland, the aim of this manuscript is to outline standards in pupillography based on current knowledge and experience of pupil experts in order to achieve greater comparability of pupillographic studies. Such standards will particularly facilitate the proper application of pupillography by researchers new to the field. First we describe general standards, followed by specific suggestions concerning the demands of different targets of pupil research: the afferent and efferent reflex arc, pharmacology, psychology, sleepiness-related research and animal studies.

Effect of Single and Combined Monochromatic Light on the Human Pupillary Light Response

The pupillary light reflex (PLR) is a neurological reflex driven by rods, cones, and melanopsin-containing retinal ganglion cells. Our aim was to achieve a more precise picture of the effects of 5-min duration monochromatic light stimuli, alone or in combination, on the human PLR, to determine its spectral sensitivity and to assess the importance of photon flux. Using pupillometry, the PLR was assessed in 13 participants (6 women) aged 27.2 ± 5.41 years (mean ± SD) during 5-min light stimuli of purple (437 nm), blue (479 nm), red (627 nm), and combinations of red+purple or red+blue light. In addition, nine 5-min, photon-matched light stimuli, ranging in 10 nm increments peaking between 420 and 500 nm were tested in 15 participants (8 women) aged 25.7 ± 8.90 years. Maximum pupil constriction, time to achieve this, constriction velocity, area under the curve (AUC) at short (0-60 s), and longer duration (240-300 s) light exposures, and 6-s post-illumination pupillary response (6-s PIPR) were assessed. Photoreceptor activation was estimated by mathematical modeling. The velocity of constriction was significantly faster with blue monochromatic light than with red or purple light. Within the blue light spectrum (between 420 and 500 nm), the velocity of constriction was significantly faster with the 480 nm light stimulus, while the slowest pupil constriction was observed with 430 nm light. Maximum pupil constriction was achieved with 470 nm light, and the greatest AUC_0-60 and AUC_240-300 was observed with 490 and 460 nm light, respectively. The 6-s PIPR was maximum after 490 nm light stimulus. Both the transient (AUC_0-60) and sustained (AUC_240-300) response was significantly correlated with melanopic activation. Higher photon fluxes for both purple and blue light produced greater amplitude sustained pupillary constriction. The findings confirm human PLR dependence on wavelength, monochromatic or bichromatic light and photon flux under 5-min duration light stimuli. Since the most rapid and high amplitude PLR occurred within the 460-490 nm light range (alone or combined), our results suggest that color discrimination should be studied under total or partial substitution of this blue light range (460-490 nm) by shorter wavelengths (~440 nm). Thus for nocturnal lighting, replacement of blue light with purple light might be a plausible solution to preserve color discrimination while minimizing melanopic activation.

The Method of Silent Substitution for Examining Melanopsin Contributions to Pupil Control

The human pupillary light response is driven by all classes of photoreceptors in the human eye-the three classes of cones, the rods, and the intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing the photopigment melanopsin. These photoreceptor classes have distinct but overlapping spectral tuning, and even a monochromatic light with a wavelength matched to the peak spectral sensitivity of a given photoreceptor will stimulate all photoreceptors. The method of silent substitution uses pairs of lights ("metamers") to selectively stimulate a given class of photoreceptors while keeping the activation of all others constant. In this primer, we describe the method of silent substitution and provide an overview of studies that have used it to examine inputs to the human pupillary light response.

Pulses of Melanopsin-Directed Contrast Produce Highly Reproducible Pupil Responses That Are Insensitive to a Change in Background Radiance

Purpose

To measure the pupil response to pulses of melanopsin-directed contrast, and compare this response to those evoked by cone-directed contrast and spectrally narrowband stimuli.

Methods

Three-second unipolar pulses were used to elicit pupil responses in human subjects across three sessions. Thirty subjects were studied in session 1, and most returned for sessions 2 and 3. The stimuli of primary interest were "silent substitution" cone- and melanopsin-directed modulations. Red and blue narrowband pulses delivered using the post-illumination pupil response (PIPR) paradigm were also studied. Sessions 1 and 2 were identical, whereas session 3 involved modulations around higher radiance backgrounds. The pupil responses were fit by a model whose parameters described response amplitude and temporal shape.

Results

Group average pupil responses for all stimuli overlapped extensively across sessions 1 and 2, indicating high reproducibility. Model fits indicate that the response to melanopsin-directed contrast is prolonged relative to that elicited by cone-directed contrast. The group average cone- and melanopsin-directed pupil responses from session 3 were highly similar to those from sessions 1 and 2, suggesting that these responses are insensitive to background radiance over the range studied. The increase in radiance enhanced persistent pupil constriction to blue light.

Conclusions

The group average pupil response to stimuli designed through silent substitution provides a reliable probe of the function of a melanopsin-mediated system in humans. As disruption of the melanopsin system may relate to clinical pathology, the reproducibility of response suggests that silent substitution pupillometry can test if melanopsin signals differ between clinical groups.

Subjective time expansion with increased stimulation of intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) contain photoreceptors that are especially sensitive to blue light. Nevertheless, how blue light and ipRGCs affect time perception remains unsolved. We used the oddball paradigm and manipulated the background light to examine whether and how blue light and ipRGCs affect perceived duration. In the oddball paradigm, participants were asked to judge the duration of the target (oddball), compared to that of the standard, with a two alternative-forced-choice procedure. When the background light was controlled to be either blue or red in Experiment 1, results showed that blue light led to longer subjective duration compared to red light. Experiment 2 further clarified the contribution of the ipRGCs. A set of multi-primary projector system that could manipulate the ipRGC stimulation were used, while the color and luminance of the background lights were kept constant throughout. Results showed that increased stimulation of ipRGCs under metameric background expanded subjective time. These results suggest that ipRGC stimulation increases arousal/attention so as to expand subjective duration.

Chromatic clocks: Color opponency in non-image-forming visual function

During dusk and dawn, the ambient illumination undergoes drastic changes in irradiance (or intensity) and spectrum (or color). While the former is a well-studied factor in synchronizing behavior and physiology to the earth's 24-h rotation, color sensitivity in the regulation of circadian rhythms has not been systematically studied. Drawing on the concept of color opponency, a well-known property of image-forming vision in many vertebrates (including humans), we consider how the spectral shifts during twilight are encoded by a color-opponent sensory system for non-image-forming (NIF) visual functions, including phase shifting and melatonin suppression. We review electrophysiological evidence for color sensitivity in the pineal/parietal organs of fish, amphibians and reptiles, color coding in neurons in the circadian pacemaker in mice as well as sporadic evidence for color sensitivity in NIF visual functions in birds and mammals. Together, these studies suggest that color opponency may be an important modulator of light-driven physiological and behavioral responses.

Rhodopsin and Melanopsin Contributions to the Early Redilation Phase of the Post-Illumination Pupil Response (PIPR)

Melanopsin expressing intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) entirely control the post-illumination pupil response (PIPR) from 6 s post-stimulus to the plateau during redilation after light offset. However, the photoreceptor contributions to the early redilation phase of the PIPR (< 6 s post-stimulus) have not been reported. Here, we evaluated the photoreceptor contributions to the early phase PIPR (0.6 s to 5.0 s) by measuring the spectral sensitivity of the criterion PIPR amplitude in response to 1 s light pulses at five narrowband stimulus wavelengths (409, 464, 508, 531 and 592 nm). The retinal irradiance producing a criterion PIPR was normalised to the peak and fitted by either a single photopigment nomogram or the combined melanopsin and rhodopsin spectral nomograms with the +L+M cone photopic luminous efficiency (Vλ) function. We show that the PIPR spectral sensitivity at times ≥ 1.7 s after light offset is best described by the melanopsin nomogram. At times < 1.7 s, the peak PIPR sensitivity shifts to longer wavelengths (range: 482 to 498 nm) and is best described by the combined photoreceptor nomogram, with major contributions from melanopsin and rhodopsin. This first report of melanopsin and rhodopsin contributions to the early phase PIPR is in line with the electrophysiological findings of ipRGC and rod signalling after the cessation of light stimuli and provides a cut-off time for isolating photoreceptor specific function in healthy and diseased eyes.

Pupillary light reflex to light inside the natural blind spot

When a light stimulus covers the human natural blind spot (BS), perceptual filling-in corrects for the missing information inside the BS. Here, we examined whether a filled-in surface of light perceived inside the BS affects the size of the short-latency pupillary light reflex (PLR), a pupil response mediated by a subcortical pathway for unconscious vision. The PLR was not induced by a red surface that was physically absent but perceptually filled-in inside the BS in the presence of a red ring surrounding it. However, a white large disk covering the BS unexpectedly induced a larger PLR than a white ring surrounding the BS border did, even though these two stimuli must be equivalent for the visual system, and trial-by-trial percepts did not predict PLR size. These results suggest that some physiological mechanism, presumably the retinal cells containing the photopigment melanopsin, receives the light projected inside the BS and enhances PLR.

Differential monocular vs. binocular pupil responses from melanopsin-based photoreception in patients with anterior ischemic optic neuropathy

We examined the effect of anterior ischemic optic neuropathy (AION) on the activity of intrinsically photosensitive retinal ganglion cells (ipRGCs) using the pupil as proxy. Eighteen patients with AION (10 unilateral, 8 bilateral) and 29 age-matched control subjects underwent chromatic pupillometry. Red and blue light stimuli increasing in 0.5 log steps were presented to each eye independently under conditions of dark and light adaptation. The recorded pupil contraction was plotted against stimulus intensity to generate scotopic and photopic response curves for assessment of synaptically-mediated ipRGC activity. Bright blue light stimuli presented monocularly and binocularly were used for melanopsin activation. The post-stimulus pupil size (PSPS) at the 6th second following stimulus offset was the marker of intrinsic ipRGC activity. Finally, questionnaires were administered to assess the influence of ipRGCs on sleep. The pupil response and PSPS to all monocularly-presented light stimuli were impaired in AION eyes, indicating ipRGC dysfunction. To binocular light stimulation, the PSPS of AION patients was similar to that of controls. There was no difference in the sleep habits of the two groups. Thus after ischemic injury to one or both optic nerves, the summated intrinsic ipRGC activity is preserved when both eyes receive adequate light exposure.

Selective stimulation of penumbral cones reveals perception in the shadow of retinal blood vessels

In 1819, Johann Purkinje described how a moving light source that displaces the shadow of the retinal blood vessels to adjacent cones can produce the entopic percept of a branching tree. Here, we describe a novel method for producing a similar percept. We used a device that mixes 56 narrowband primaries under computer control, in conjunction with the method of silent substitution, to present observers with a spectral modulation that selectively targeted penumbral cones in the shadow of the retinal blood vessels. Such a modulation elicits a clear Purkinje-tree percept. We show that the percept is specific to penumbral L and M cone stimulation and is not produced by selective penumbral S cone stimulation. The Purkinje-tree percept was strongest at 16 Hz and fell off at lower (8 Hz) and higher (32 Hz) temporal frequencies. Selective stimulation of open-field cones that are not in shadow, with penumbral cones silenced, also produced the percept, but it was not seen when penumbral and open-field cones were modulated together. This indicates the need for spatial contrast between penumbral and open-field cones to create the Purkinje-tree percept. Our observation provides a new means for studying the response of retinally stabilized images and demonstrates that penumbral cones can support spatial vision. Further, the result illustrates a way in which silent substitution techniques can fail to be silent. We show that inadvertent penumbral cone stimulation can accompany melanopsin-directed modulations that are designed only to silence open-field cones. This in turn can result in visual responses that might be mistaken as melanopsin-driven.

Assessing rod, cone, and melanopsin contributions to human pupil flicker responses

PURPOSE

We determined the relative contributions of rods, cones, and melanopsin to pupil responses in humans using temporal sinusoidal stimulation for light levels spanning the low mesopic to photopic range.

METHODS

A four-primary Ganzfeld photostimulator controlled flicker stimulations at seven light levels (-2.7 to 2 log cd/m(2)) and five frequencies (0.5-8 Hz). Pupil diameter was measured using a high-resolution eye tracker. Three kinds of sinusoidal photoreceptor modulations were generated using silent substitution: rod modulation, cone modulation, and combined rod and cone modulation in phase (experiment 1) or cone phase shifted (experiment 2) from a fixed rod phase. The melanopsin excitation was computed for each condition. A vector sum model was used to estimate the relative contribution of rods, cones, and melanopsin to the pupil response.

RESULTS

From experiment 1, the pupil frequency response peaked at 1 Hz at two mesopic light levels for the three modulation conditions. Analyzing the rod-cone phase difference for the combined modulations (experiment 2) identified a V-shaped response amplitude with a minimum between 135° and 180°. The pupil response phases increased as cone modulation phase increased. The pupil amplitude increased with increasing light level for cone, and combined (in-phase rod and cone) modulation, but not for the rod modulation.

CONCLUSIONS

These results demonstrate that cone- and rod-pathway contributions are more predominant than melanopsin contribution to the phasic pupil response. The combined rod, cone, and melanopsin inputs to the phasic state of the pupil light reflex follow linear summation.

Melanopsin gene polymorphism I394T is associated with pupillary light responses in a dose-dependent manner

BACKGROUND

Melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) play an important role in non-image forming responses to light, such as circadian photoentrainment, light-induced melatonin suppression, and pupillary light response. Although it is known that there are some single nucleotide polymorphisms (SNPs) in the melanopsin (OPN4) gene in humans, the associations of the SNPs with non-image forming responses to light remains unclear. In the present study, we examined the associations of melanopsin gene polymorphisms with pupillary light response.

METHODS

Japanese university students (mean age: 21.0 ± 1.7 years) with the genotypes of TT (n = 38), TC (n = 28) and CC (n = 7) at rs1079610 (I394T) located in the coding region participated in the present study. They were matched by age and sex ratio. Dark-adapted pupil size (<1 lx) was first measured. Then steady-state pupil size was measured during exposure to five lighting conditions (10 lx, 100 lx, 1000 lx, 3000 lx, 6000 lx in the vertical direction at eye level).

RESULTS

Significant interaction between the genotype of I394T (TT versus TC+CC) and luminance levels was found in pupil size. Under high illuminance levels (1000 lx, 3000 lx and 6000 lx), pupil sizes in subjects with the C allele were significantly smaller than those in subjects with the TT genotype. On the other hand, pupil size in subjects with the C allele under low illuminance (<1 lx) was significantly larger than that in subjects with the TT genotype. Percentages of pupil constriction under high illuminance levels were significantly greater in subjects with the C allele than in subjects with the TT genotype.

CONCLUSIONS

Human melanopsin gene polymorphism I394T interacted with irradiance in association with pupil size. This is the first evidence suggesting a functional connection between melanopsin gene polymorphism and pupillary light response as an index of non-image forming response to light.

Distinct responses of cones and melanopsin-expressing retinal ganglion cells in the human electroretinogram

BACKGROUND

The discovery of the novel photoreceptor, melanopsin-expressing retinal ganglion cells (mRGCs), has raised researchers' interest in photoreceptive tasks performed by the mRGC, especially in non-image-forming visual functions. In a prior study, we investigated the mRGC response to light stimuli independent of rods and cones with the four-primary illumination system, which modulates stimulus levels to the mRGC and cones independently, and mRGC baseline responses were recorded in the electroretinogram (ERG).

METHODS

In the present study, we used the same illumination system to compare independent responses of the mRGC and cones in five subjects (mean ± SD age, 23.0 ± 1.7 years). The ERG waveforms were examined as direct measurements of responses of the mRGCs and cones to stimulation (250 msec). Implicit times (the time taken to peaks) and peak values from 30 stimuli given to each subject were analyzed.

RESULTS

Two distinct positive peaks appeared in the mRGC response, approximately 80 msec after the onset of the stimuli and 30 msec after their offset, while no such peaks appeared in the cone response. The response to the mRGC stimulus was significantly higher than that to the cone stimulus at approximately 80 msec (P < 0.05) and tended to be higher than the cone stimulus at approximately 280 msec (P = 0.08).

CONCLUSIONS

Implicit time of the first peak was much longer than that to the b-wave and this delay might reflect mRGC's sluggish responses. This is the first report of amplitudes and implicit time in the ERG from the response of the mRGC that is independent of rods and cones, and obtained using the four-primary illumination system.

Characterization of structure and function of the mouse retina using pattern electroretinography, pupil light reflex, and optical coherence tomography

OBJECTIVE

To perform in vivo analysis of retinal functional and structural parameters in healthy mouse eyes.

ANIMAL STUDIED

Adult C57BL/6 male mice (n = 37).

PROCEDURES

Retinal function was evaluated using pattern electroretinography (pERG) and the chromatic pupil light reflex (cPLR). Structural properties of the retina and nerve fiber layer (NFL) were evaluated using spectral-domain optical coherence tomography (SD-OCT).

RESULTS

The average pERG amplitudes were found to be 11.2 ± 0.7 μV (P50-N95, mean ± SEM), with an implicit time for P50-N95 interval of 90.4 ± 5.4 ms. Total retinal thickness was 229.5 ± 1.7 μm (mean ± SEM) in the area centralis region. The thickness of the retinal nerve fiber layer (mean ± SEM) using a circular peripapillary retinal scan centered on the optic nerve was 46.7 ± 0.9 μm (temporal), 46.1 ± 0.9 μm (superior), 45.8 ± 0.9 μm (nasal), and 48.4 ± 1 μm (inferior). The baseline pupil diameter was 2.1 ± 0.05 mm in darkness, and 1.1 ± 0.05 and 0.56 ± 0.03 mm after stimulation with red (630 nm, luminance 200 kcd/m(2)) or blue (480 nm, luminance 200 kcd/m(2)) light illumination, respectively.

CONCLUSIONS

Pattern electroretinography, cPLR and SD-OCT analysis are reproducible techniques, which can provide important information about retinal and optic nerve function and structure in mice.

Melanopsin-based brightness discrimination in mice and humans

Photoreception in the mammalian retina is not restricted to rods and cones but extends to a small number of intrinsically photoreceptive retinal ganglion cells (ipRGCs), expressing the photopigment melanopsin [1–4]. ipRGCs are known to support various accessory visual functions including circadian photoentrainment and pupillary reflexes. However, despite anatomical and physiological evidence that they contribute to the thalamocortical visual projection [5–7], no aspect of visual discrimination has been shown to rely upon ipRGCs. Based on their currently known roles, we hypothesized that ipRGCs may contribute to distinguishing brightness. This percept is related to an object's luminance—a photometric measure of light intensity relevant for cone photoreceptors. However, the perceived brightness of different sources is not always predicted by their respective luminance [8–12]. Here, we used parallel behavioral and electrophysiological experiments to first show that melanopsin contributes to brightness discrimination in both retinally degenerate and fully sighted mice. We continued to use comparable paradigms in psychophysical experiments to provide evidence for a similar role in healthy human subjects. These data represent the first direct evidence that an aspect of visual discrimination in normally sighted subjects can be supported by inner retinal photoreceptors.