Domain of metamers exciting intrinsically photosensitive retinal ganglion cells (ipRGCs) and rods

Regional response to light illuminance across the human hypothalamus

Light exerts multiple non-image-forming biological effects on physiology including the stimulation of alertness and cognition. However, the subcortical circuitry underlying the stimulating impact of light is not established in humans. We used 7 Tesla functional magnetic resonance imaging to assess the impact of variations in light illuminance on the regional activity of the hypothalamus while healthy young adults (N=26; 16 women; 24.3±2.9 y) were completing two auditory cognitive tasks. We find that, during both the executive and emotional tasks, higher illuminance triggered an activity increase over the posterior part of the hypothalamus, which includes part of the tuberomamillary nucleus and the posterior part of the lateral hypothalamus. In contrast, increasing illuminance evoked a decrease in activity over the anterior and ventral parts of the hypothalamus, encompassing notably the suprachiasmatic nucleus and another part of the tuberomammillary nucleus. Critically, the performance of the executive task was improved under higher illuminance and was negatively correlated with the activity of the posterior hypothalamus area. These findings reveal the distinct local dynamics of different hypothalamus regions that underlie the impact of light on cognition.

Impact of light on task-evoked pupil responses during cognitive tasks

Light has many non-image-forming functions including modulation of pupil size and stimulation of alertness and cognition. Part of these non-image-forming effects may be mediated by the brainstem locus coeruleus. The processing of sensory inputs can be associated with a transient pupil dilation that is likely driven in part by the phasic activity of the locus coeruleus. In the present study, we aimed to characterise the task-evoked pupil response associated with auditory inputs under different light levels and across two cognitive tasks. We continuously monitored the pupil of 20 young healthy participants (mean [SD] 24.05 [4.0] years; 14 women) whilst they completed an attentional and an emotional auditory task whilst exposed to repeated 30-40-s blocks of light interleaved with darkness periods. Blocks could either consist of monochromatic orange light (0.16 melanopic equivalent daylight illuminance (EDI) lux) or blue-enriched white light of three different levels [37, 92, 190 melanopic EDI lux; 6500 K]. For the analysis, 15 and then 14 participants were included in the attentional and emotional tasks, respectively. Generalised linear mixed models showed a significant main effect of light level on the task-evoked pupil responses triggered by the attentional and emotional tasks (p ≤ 0.0001). The impact of light was different for the target versus non-target stimulus of the attentional task but was not different for the emotional and neutral stimulus of the emotional task. There is a smaller sustained pupil size during brighter light blocks but, a higher light level triggers a stronger task-evoked pupil response to auditory stimulation, presumably through the recruitment of the locus coeruleus.

Chromatic Pupillometry as a Putative Screening Tool for Heritable Retinal Disease in Rhesus Macaques

Purpose

Non-human primates (NHPs) are useful models for human retinal disease. Chromatic pupillometry has been proposed as a noninvasive method of identifying inherited retinal diseases (IRDs) in humans; however, standard protocols employ time-consuming dark adaptation. We utilized shortened and standard dark-adaptation protocols to compare pupillary light reflex characteristics following chromatic stimulation in rhesus macaques with achromatopsia to wild-type (WT) controls with normal retinal function.

Methods

Nine rhesus macaques homozygous for the p.R656Q mutation (PDE6C HOMs) and nine WT controls were evaluated using chromatic pupillometry following 1-minute versus standard 20-minute dark adaptations. The following outcomes were measured and compared between groups: pupil constriction latency, peak constriction, pupil constriction time, and constriction velocity.

Results

Pupil constriction latency was significantly longer in PDE6C HOMs with red-light (P = 0.0002) and blue-light (P = 0.04) stimulation versus WT controls. Peak constriction was significantly less in PDE6C HOMs with all light stimulation compared to WT controls (P < 0.0001). Pupil constriction time was significantly shorter in PDE6C HOMs versus WT controls with red-light (P = 0.04) and white-light (P = 0.003) stimulation. Pupil constriction velocity was significantly slower in PDE6C HOMs versus WT controls with red-light (P < 0.0001), blue-light (P < 0.0001), and white-light (P = 0.0002) stimulation. Dark adaptation time only significantly affected peak (P = 0.008) and time of pupil constriction (P = 0.02) following blue-light stimulation.

Conclusions

Chromatic pupillometry following 1- and 20-minute dark adaptation is an effective tool for screening NHPs for achromatopsia.

Translational Relevance

Rapid identification of NHPs with IRDs will provide animal research models to advance research and treatment of achromatopia in humans.

Light as a Modulator of Non-Image-Forming Brain Functions—Positive and Negative Impacts of Increasing Light Availability

Light use is rising steeply, mainly because of the advent of light-emitting diode (LED) devices. LEDs are frequently blue-enriched light sources and may have different impacts on the non-image forming (NIF) system, which is maximally sensitive to blue-wavelength light. Most importantly, the timing of LED device use is widespread, leading to novel light exposure patterns on the NIF system. The goal of this narrative review is to discuss the multiple aspects that we think should be accounted for when attempting to predict how this situation will affect the NIF impact of light on brain functions. We first cover both the image-forming and NIF pathways of the brain. We then detail our current understanding of the impact of light on human cognition, sleep, alertness, and mood. Finally, we discuss questions concerning the adoption of LED lighting and screens, which offer new opportunities to improve well-being, but also raise concerns about increasing light exposure, which may be detrimental to health, particularly in the evening.

Optimising metameric spectra for integrative lighting to modulate the circadian system without affecting visual appearance

Smart integrative lighting systems aim to support human health and wellbeing by capitalising on the light-induced effects on circadian rhythms, sleep, and cognitive functions, while optimising the light's visual aspects like colour fidelity, visual comfort, visual preference, and visibility. Metameric spectral tuning could be an instrument to solve potential conflicts between the visual preferences of users with respect to illuminance and chromaticity and the circadian consequences of the light exposure, as metamers can selectively modulate melanopsin-based photoreception without affecting visual properties such as chromaticity or illuminance. This work uses a 6-, 8- and 11-channel LED luminaire with fixed illuminance of 250 lx to systematically investigate the metameric tuning range in melanopic equivalent daylight illuminance (EDI) and melanopic daylight efficacy ratio (melanopic DER) for 561 chromaticity coordinates as optimisation targets (2700 K to 7443 K ± Duv 0 to 0.048), while applying colour fidelity index Rf criteria from the TM-30-20 Annex E recommendations (i.e. Rf [Formula: see text] 85, Rf,h1 [Formula: see text] 85). Our results reveal that the melanopic tuning range increases with rising CCT to a maximum tuning range in melanopic DER of 0.24 (CCT: 6702 K, Duv: 0.003), 0.29 (CCT: 7443 K, Duv: 0) and 0.30 (CCT: 6702, Duv: 0.006), depending on the luminaire's channel number of 6, 8 or 11, respectively. This allows to vary the melanopic EDI from 212.5-227.5 lx up to 275-300 lx without changes in the photopic illuminance (250 lx) or chromaticity ([Formula: see text] [Formula: see text] 0.0014). The highest metameric melanopic Michelson contrast for the 6-, 8- and 11-channel luminaire is 0.16, 0.18 and 0.18, which is accomplished at a CCT of 3017 K (Duv: - 0.018), 3456 K (Duv: 0.009) and 3456 K (Duv: 0.009), respectively. By optimising ~ 490,000 multi-channel LED spectra, we identified chromaticity regions in the CIExy colour space that are of particular interest to control the melanopic efficacy with metameric spectral tuning.

Maximum possible contrast level for silent substitution: a theoretical model applied to melanopsin stimulation

For any given set of light sources stimulating the photoreceptors of the retina, the theoretical levels of illumination producing the smallest and the largest expression of one photoreceptor with fixed stimulation for the others are analytically computed. The cases of four, five, and more light sources are studied. We show that, for contrast optimization, only as many light sources as photoreceptors do matter and that, in the case of four light sources, the maximum contrast achievable for melanopsin lies at the intersection of the lines joining the sources in the CIE xy chromaticity diagram. This result is used to obtain the optimal position of four Gaussian primaries of equal bandwidth. In addition, we derive a procedure to construct level maps for melanopsin contrast overlying the diagram. In the second part of the paper, the interpersonal variability of the perceived stimulation is shown to be globally reduced if the bandwidth of the light sources is increased and, under some assumptions, if a light source is added.

Optimizing methods to isolate melanopsin-directed responses

The intrinsic melanopsin photoresponse may initiate visual signals that differ in spatiotemporal characteristics from the cone-opsin- and rhodopsin-mediated signals. Applying the CIE standard observer functions in silent-substitution methods can require individual differences in photoreceptor spectral sensitivities and pre-receptoral filtering to be corrected; failure to do so can lead to the intrusion of more sensitive cone processes with putative melanopsin-directed stimuli. Here we evaluate heterochromatic flicker photometry (HFP) and photoreceptor-directed temporal white noise as techniques to limit the effect of these individual differences. Individualized luminous efficiency functions (V(λ)) were compared to the CIE standard observer functions. We show that adapting chromaticities used in silent-substitution methods can deviate by up to 54% in luminance when estimated with the individual and standard observer functions. These deviations lead to inadvertent cone intrusions in the visual functions measured with melanopsin-directed stimuli. To eliminate the intrusions, individual HFP corrections are sufficient at low frequencies (∼1Hz) but temporal white noise is also required at higher frequencies to desensitize penumbral cones. We therefore recommend the selective application of individualized observer calibration and/or temporal white noise in silent-substitution paradigms when studying melanopsin-directed photoresponses.

Melanopsin and Cone Photoreceptor Inputs to the Afferent Pupil Light Response

Background: Retinal photoreceptors provide the main stage in the mammalian eye for regulating the retinal illumination through changes in pupil diameter, with a small population of melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) forming the primary afferent pathway for this response. The purpose of this study is to determine how melanopsin interacts with the three cone photoreceptor classes in the human eye to modulate the light-adapted pupil response.

Methods: We investigated the independent and combined contributions of the inner and outer retinal photoreceptor inputs to the afferent pupil pathway in participants with trichromatic color vision using a method to independently control the excitations of ipRGCs, cones and rods in the retina.

Results: We show that melanopsin-directed stimuli cause a transient pupil constriction generated by cones in the shadow of retinal blood vessels; desensitizing these penumbral cone signals uncovers a signature melanopsin pupil response that includes a longer latency (292 ms) and slower time (4.1x) and velocity (7.7x) to constriction than for cone-directed stimuli, and which remains sustained post-stimulus offset. Compared to melanopsin-mediated pupil responses, the cone photoreceptor-initiated pupil responses are more transient with faster constriction latencies, higher velocities and a secondary constriction at light offset. The combined pupil responses reveal that melanopsin signals are additive with the cone signals.

Conclusions: The visual system uses the L–, M–, and S–cone photoreceptor inputs to the afferent pupil pathway to accomplish the tonic modulations of pupil size to changes in image contrast. The inner retinal melanopsin-expressing ipRGCs mediate the longer-term, sustained pupil constriction to set the light-adapted pupil diameter during extended light exposures.

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.

LED Lights With Hidden Intensity-Modulated Blue Channels Aiming for Enhanced Subconscious Visual Responses

A new form of light-emitting diode (LED) light suitable for general illumination is proposed to enhance subconscious, nonimage-forming visual responses, which are essential to our well-being. Pulsing light has been shown to reduce photoreceptor adaptation and elicit stronger subconscious visual responses at an indoor illumination level. Using the silent substitution technique, a melanopsin-selective flicker can be added into white light. A linear optimization algorithm was developed to suppress any perceivable fluctuation of light intensity and colors of illuminated objects. Two examples of lights are given to illustrate the potential applications of the proposed multi-LED light for general illumination and therapeutic purposes.

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.

A five-primary photostimulator suitable for studying intrinsically photosensitive retinal ganglion cell functions in humans

Intrinsically photosensitive retinal ganglion cells (ipRGCs) can respond to light directly through self-contained photopigment, melanopsin. IpRGCs also receive synaptic inputs from rods and cones. Thus, studying ipRGC functions requires a novel photostimulating method that can account for all of the photoreceptor inputs. Here, we introduced an inexpensive LED-based five-primary photostimulator that can control the excitations of rods, S-, M-, L-cones, and melanopsin-containing ipRGCs in humans at constant background photoreceptor excitation levels, a critical requirement for studying the adaptation behavior of ipRGCs with rod, cone, or melanopsin input. We described the theory and technical aspects (including optics, electronics, software, and calibration) of the five-primary photostimulator. Then we presented two preliminary studies using the photostimulator we have implemented to measure melanopsin-mediated pupil responses and temporal contrast sensitivity function (TCSF). The results showed that the S-cone input to pupil responses was antagonistic to the L-, M- or melanopsin inputs, consistent with an S-OFF and (L + M)-ON response property of primate ipRGCs (Dacey et al., 2005). In addition, the melanopsin-mediated TCSF had a distinctive pattern compared with L + M or S-cone mediated TCSF. Other than controlling individual photoreceptor excitation independently, the five-primary photostimulator has the flexibility in presenting stimuli modulating any combination of photoreceptor excitations, which allows researchers to study the mechanisms by which ipRGCs combine various photoreceptor inputs.

Pupillary behavior in relation to wavelength and age

Pupil light reflex can be used as a non-invasive ocular predictor of cephalic autonomic nervous system integrity. Spectral sensitivity of the pupil's response to light has, for some time, been an interesting issue. It has generally, however, only been investigated with the use of white light and studies with monochromatic wavelengths are scarce. This study investigates the effects of wavelength and age within three parameters of the pupil light reflex (amplitude of response, latency, and velocity of constriction) in a large sample of younger and older adults (N = 97), in mesopic conditions. Subjects were exposed to a single light stimulus at four different wavelengths: white (5600°K), blue (450 nm), green (510 nm), and red (600 nm). Data was analyzed appropriately, and, when applicable, using the General Linear Model (GLM), Randomized Complete Block Design (RCBD), Student's t-test and/or ANCOVA. Across all subjects, pupillary response to light had the greatest amplitude and shortest latency in white and green light conditions. In regards to age, older subjects (46–78 years) showed an increased latency in white light and decreased velocity of constriction in green light compared to younger subjects (18–45 years old). This study provides data patterns on parameters of wavelength-dependent pupil reflexes to light in adults and it contributes to the large body of pupillometric research. It is hoped that this study will add to the overall evaluation of cephalic autonomic nervous system integrity.

The Verriest Lecture: Visual properties of metameric blacks beyond cone vision

The generic framework of metamerism implies that the number of sensors is smaller than the dimension of the stimulus. The metameric black paradigm was introduced by Wyszecki [Farbe2, 39 (1953)] and developed by Cohen and Kappauf [Am. J. Psychol.95, 537 (1982)]. Within a multireceptor and multiprimary scheme, we investigate how far the choice of illumination can isolate a photoreceptor response. The spectral profiles of the fundamental metamers that correspond to a collection of (x,y) values over the chromaticity diagram are shown. When the luminance is set at a fixed value, the relative excitation of the melanopsin cells and of the rods elicited by the fundamental metamers varies over the chromaticity diagram. The range of excitation of the melanopsin cells and of the rods that could be achieved at a given chromaticity, by manipulating the metameric black content, is examined. When only the melanopsin excitation is manipulated, the range of melanopsin excitation that can be achieved is rather limited. On the chromaticity diagram, the largest range of variation of the rods and the melanopsin cells excitation is obtained for (x,y) chromaticity coordinates near (1/3,1/3). Extension of Cohen's procedure to rod and cone metamers is proposed. The higher the number of spectral bands, the wider the choice of metameric lights.

Human trichromacy revisited

The presence of a photopigment (melanopsin) within certain retinal ganglion cells was a surprising and significant discovery. This pigment is routinely described as "nonvisual" to highlight its signaling role in pupil dilation and circadian rhythms. Here we asked whether light absorbed by melanopsin can be seen by healthy human subjects. To answer this requires delivering intense (above rod saturation), well-controlled lights using four independent primaries. We collected detection thresholds to many four-primary stimuli. Threshold measurements in the fovea are explained by trichromatic theory, with no need to invoke a fourth photopigment. In the periphery, where melanopsin is present, threshold measurements deviate from trichromatic theory; at high photopic levels, sensitivity is explained by absorptions in four, not three, photopigment classes. We consider a series of hypotheses to explain the tetrasensitivity at high photopic levels in the human peripheral field. The most likely hypothesis is that in healthy human subjects melanopsin absorptions influence visibility.

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.