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.

Efficacy of biologically-directed daylight therapy on sleep and circadian rhythm in Parkinson's disease: a randomised, double-blind, parallel-group, active-controlled, phase 2 clinical trial.

Background: New non-pharmacological treatments for improving non-motor symptoms in Parkinson's disease (PD) are urgently needed. Previous light therapies for modifying sleep behaviour lacked standardised protocols and were not personalised for an individual patient chronotype. We aimed to assess the efficacy of a biologically-directed light therapy in PD that targets retinal inputs to the circadian system on sleep, as well as other non-motor and motor functions. Methods: In this randomised, double-blind, parallel-group, active-controlled trial at the Queensland University of Technology, Australia, participants with mild to moderate PD were computer randomised (1:1) to receive one of two light therapies that had the same photometric luminance and visual appearance to allow blinding of investigators and participants to the intervention. One of these biologically-directed lights matched natural daylight (Day Mel), which is known to stimulate melanopsin cells. The light therapy of the other treatment arm of the study, specifically supplemented the stimulation of retinal melanopsin cells (Enhanced Mel), targeting deficits to the circadian system. Both lights were administered 30 min per day over 4-weeks and personalised to an individual patient's chronotype, while monitoring environmental light exposure with actigraphy. Co-primary endpoints were a change from baseline in mean sleep macrostructure (polysomnography, PSG) and an endocrine biomarker of circadian phase (dim light melatonin secretion onset, DLMO) at weeks 4 and 6. Participants data were analysed using an intention to treat principle. All endpoints were evaluated by applying a mixed model analysis. The trial is registered with the Australian New Zealand Clinical Trials Registry, ACTRN12621000077864. Findings: Between February 4, 2021 and August 8, 2022, 144 participants with PD were consecutively screened, 60 enrolled and randomly assigned to a light intervention. There was no significant difference in co-primary outcomes between randomised groups overall or at any individual timepoint during follow-up. The mean (95% CI) for PSG, N3% was 24.15 (19.82-28.48) for Day Mel (n = 23) and 19.34 (15.20-23.47) for the Enhanced Mel group (n = 25) in week 4 (p = 0.12); and 21.13 (16.99-25.28) for Day Mel (n = 26) and 18.48 (14.34-22.62) for the Enhanced Mel group (n = 25) in week 6, (p = 0.37). The mean (95% CI) DLMO (decimal time) was 19.82 (19.20-20.44) for Day Mel (n = 22) and 19.44 (18.85-20.04) for the Enhanced Mel group (n = 24) in week 4 (p = 0.38); and 19.90 (19.27-20.53) for Day Mel (n = 23) and 19.04 (18.44-19.64) for the Enhanced Mel group (n = 25) in week 6 (p = 0.05). However, both the controlled daylight (Day Mel) and the enhanced melanopsin (Enhanced Mel) interventions demonstrated significant improvement in primary PSG sleep macrostructure. The restorative deep sleep phase (PSG, N3) significantly improved at week 6 in both groups [model-based mean difference to baseline (95% CI): -3.87 (-6.91 to -0.83), p = 0.04]. There was a phase-advance in DLMO in both groups which did not reach statistical significance between groups at any time-point. There were no safety concerns or severe adverse events related to the intervention. Interpretation: Both the controlled daylight and melanopsin booster light showed efficacy in improving measures of restorative deep sleep in people with mild to moderate PD. That there was no significant difference between the two intervention groups may be due to the early disease stage. The findings suggest that controlled indoor daylight that is personalised to the individuals' chronotype could be effective for improving sleep in early to moderate PD, and further studies evaluating controlled daylight interventions are now required utilising this standardised approach, including in advanced PD. Funding: The Michael J Fox Foundation for Parkinson's Research, Shake IT Up Australia, National Health and Medical Research Council, and Australian Research Council.

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.

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

To evaluate the nature of interactions between visual pathways transmitting the slower melanopsin and faster rod and cone signals, we implement a temporal phase summation paradigm in human observers using photoreceptor-directed stimuli. We show that melanopsin stimulation interacts with and alters both rod-mediated and cone-mediated vision regardless of whether it is perceptually visible or not. Melanopsin-rod interactions result in either inhibitory or facilitatory summation depending on the temporal frequency and photoreceptor pathway contrast sensitivity. Moreover, by isolating rod vision, we reveal a bipartite intensity response property of the rod pathway in photopic lighting that extends its operational range at lower frequencies to beyond its classic saturation limits but at the expense of attenuating sensitivity at higher frequencies. In comparison, melanopsin-cone interactions always lead to facilitation. These interactions can be described by linear or probability summations and potentially involve multiple intraretinal and visual cortical pathways to set human visual contrast sensitivity.

The role of melanopsin photoreception on visual attention linked pupil responses.

A decision during a visual task is marked by a task-evoked pupil dilation (TEPD) that is linked to the global cortical arousal state. Melanopsin expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) form the afferent pathway for this pupil response. Melanopsin activation also influences mood and arousal and increases activity in decision-making brain areas that receive direct ipRGC projections. Here, an optical photostimulation method controlled the excitations of all five photoreceptor classes in the human eye to isolate melanopsin-mediated photoreception. We hypothesised that the TEPD can be driven by directing active visual covert attention through the ipRGC pathway. When observers are completely certain of the stimulus presence, melanopsin-directed stimulation produces a TEPD of similar amplitude to a cone-directed stimulation, with their combination producing larger amplitudes. This dilation is satisfactorily modelled by linear addition with a higher melanopsin weighting in ipRGCs. Visual reaction times were longest in response to melanopsin-directed lights. Next, we asked whether the afferent photoreceptor input and decision certainty, controlled by priming the observer's a priori expectation, interact to drive the TEPD. Signal detection analysis showed that by fixing the predecision certainty (bias), the phasic arousal and TEPD amplitude vary with observer criterion (c') and sensitivity (d') but not with preferential activation of melanopsin. The signature feature of the melanopsin response during attention was a biphasic TEPD. We conclude that active covert attention can be modulated by visual information mediated via ipRGCs, but that phasic arousal responses marked using the TEPD are not increased by higher levels of melanopsin activation.

Design and validation of a chart-based measure of the limits of spatial contrast sensitivity.

PURPOSE: Current chart-based tests of spatial contrast sensitivity (SCS) with fixed or narrow frequency ranges (≤18 cycles/°) cannot characterise the limits of spatial contrast vision. Here we present the design and validation of a chart-based measure of the spatial contrast envelope. METHODS: Following the principles of the standard visual acuity (Bailey-Lovie) and contrast sensitivity (Pelli-Robson) charts, a combined spatial-contrast and visual acuity chart was designed using a language-independent triangular symbol for a four-alternative forced-choice procedure plus chart rotation. Symbol frequencies ranged between 0.38 and 60 cycles/° spaced along 10 radial axes (0.55%-100% contrast). The chart was validated with reference to the Bailey-Lovie and Pelli-Robson charts; its reliability and sensitivity to changes in illumination, simulated cataract and blur was evaluated in healthy adults. RESULTS: The photopic SCS function could be measured in 5.5 ± 0.5 min; thresholding around the spatial contrast resolution limit reduced completion times to ~2 min. There was good agreement with high-contrast visual acuity (difference = 0.08 ± 0.02 logMAR) and contrast-sensitivity at 1.5 cycles/° (0.13 ± 0.06 logCS). Test-retest reliability was excellent at all spatial frequencies (ICC = 0.99). Mesopic illumination or simulated cataract caused a generalised SCS loss; myopic blur reduced high-frequency sensitivity. Spatial contrast sensitivity was independent of radial axis orientation (cardinal or oblique). CONCLUSIONS: The chart provides a time-efficient, reliable and inexpensive measure of SCS with applications in research and clinic for detecting subtle deficits in early stages of ocular and neurological conditions that often manifest at higher frequencies. It is sensitive to vision changes occurring in dim lighting and with simulated cataract and blur. The chart is available open-access for self-printing; contrast variation in print can be controlled through user calibration and/or establishing normative SCS functions using the theoretical values.

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.

Light adaptation characteristics of melanopsin.

Following photopigment bleaching, the rhodopsin and cone-opsins show a characteristic exponential regeneration in the dark with a photocycle dependent on the retinal pigment epithelium. Melanopsin pigment regeneration in animal models requires different pathways to rods and cones. To quantify melanopsin-mediated light adaptation in humans, we first estimated its photopigment regeneration kinetics through the photo-bleach recovery of the intrinsic melanopsin pupil light response (PLR). An intense broadband light (~120,000 Td) bleached 43% of melanopsin compared to 86% of the cone-opsins. Recovery from a 43% bleach was 3.4X slower for the melanopsin than cone-opsin. Post-bleach melanopsin regeneration followed an exponential growth with a 2.5 min time-constant (τ) that required 11.2 min for complete recovery; the half-bleaching level (Ip) was ~ 4.47 log melanopic Td (16.10 log melanopsin effective photons.cm-2.s-1; 8.25 log photoisomerisations.photoreceptor-1.s-1). The effect on the cone-directed PLR of the level of the melanopsin excitation during continuous light adaptation was then determined. We observed that cone-directed pupil constriction amplitudes increased by ~ 10% when adapting lights had a higher melanopic excitation but the same mean photometric luminance. Our findings suggest that melanopsin light adaptation enhances cone signalling along the non-visual retina-brain axis. Parameters τ and Ip will allow estimation of the level of melanopsin bleaching in any light units; the data have implications for quantifying the relative contributions of putative melanopsin pathways to regulate the post-bleach photopigment regeneration and adaptation.

Supplemental light exposure improves sleep architecture in people with type 2 diabetes.

AIMS: People with type 2 diabetes (T2D) suffer from sleep disorders, with the mechanism not clearly understood. In T2D, the light transducing retinal photoreceptors that regulate sleep behaviours are dysfunctional; hence, we determine here whether supplemental light exposure ameliorates sleep quality and daytime sleepiness in T2D. METHODS: Supplemental light (10,000 Lux, polychromatic) was self-administered for 30 min every morning for 14 days by ten participants with T2D with no diabetic retinopathy (DR). The effectiveness of supplemental light was assessed by comparing subjective sleep questionnaire (PSQI and ESS) scores and salivary dim light melatonin onset (DLMO) before and after the light exposure as well as with a self-maintained sleep diary during the light exposure. RESULTS: Compared to the baseline, supplemental light significantly improved the excessive daytime sleepiness score (p = 0.004) and phase-advanced the DLMO on average by ~ 23 min. Sleep diary analyses showed that afternoon nap duration significantly shortened over the first week of supplemental light exposure (p = 0.019). Afternoon naps and midnight awakening were significantly longer in diabetic participants with thinner perifoveal retina. CONCLUSIONS: In this case series, we provide initial evidence that supplemental bright light improves daytime sleepiness in T2D with no DR, with the critical period of light exposure showing a beneficial effect after one week. We infer that supplemental light augments photoreceptor signalling in T2D and therefore optimises circadian photoentrainment leading to improved sleep. Our findings inform the development of tailored light therapy protocols in future clinical trials for improving sleep architecture in diabetes.

Threshold vision under full-field stimulation: Revisiting the minimum number of quanta necessary to evoke a visual sensation.

At the absolute threshold of vision, Hecht, Shlaer and Pirenne estimate that 5-14 photons are absorbed within a retinal area containing ~500 rods. Other estimates of scotopic threshold vision based on stimuli with different durations and focal areas range up to ~100,000 photons. Given that rod density varies with retinal eccentricity and the magnitude of the intrinsic noise increases with increasing stimulus area and duration, here we determine whether the scotopic threshold estimates with focal stimuli can be extended to full-field stimulation and whether summation explains inter-study differences. We show that full-field threshold vision (~1018 mm2, 10 ms duration) is more sensitive than at absolute threshold, requiring the absorption of ~1000 photons across ~91.96 million rods. A summation model is presented integrating our and published data and using a nominal exposure duration, criterion frequency of seeing, rod density, and retinal area that largely explains the inter-study differences and allows estimation of rods per photon ratio for any stimulus size and duration. The highest signal to noise ratio is defined by a peak rod convergence estimated at 53:4:1:2 (rods:rod bipolar cells:AII amacrine cells:retinal ganglion cells), in line with macaque anatomical estimates that show AII amacrine cells form the bottleneck in the rod pathway to set the scotopic visual limit. Our model estimations that the rods per photon ratio under full-field stimulation is ~3000X higher than at absolute threshold are in accordance with visual summation effects and provide an alternative approach for understanding the limits of scotopic vision.

Melanopsin hypersensitivity dominates interictal photophobia in migraine.

PURPOSE: To define the melanopsin and cone luminance retinogeniculate pathway contributions to photophobia in healthy controls and migraineurs. METHODS: Healthy controls and migraineurs were categorized according to the International Classification of Headache Disorders criteria. Photophobia was measured under full-field illumination using electromyography in response to narrowband lights spanning the melanopsin and cone luminance action spectra. Migraineurs were tested during their interictal headache-free period. Melanopsin-mediated post-illumination pupil responses quantified intrinsically photosensitive Retinal Ganglion Cell (ipRGC) function. RESULTS: A model combining the melanopsin and cone luminance action spectra best described photophobia thresholds in controls and migraineurs; melanopsin contributions were ∼1.5× greater than cone luminance. In the illumination range causing photophobia, migraineurs had lower photophobia thresholds (∼0.55 log units; p < 0.001) and higher post-illumination pupil response amplitudes (p = 0.03) than controls. CONCLUSION: Photophobia is driven by melanopsin and cone luminance inputs to the cortex via the retino-thalamocortical pathway. In migraineurs, lower photophobia thresholds reflect hypersensitivity of ipRGC and cone luminance pathways, with the larger and prolonged post-illumination pupil response amplitude indicative of a supranormal melanopsin response. Our findings inform artificial lighting strategies incorporating luminaires with low melanopsin excitation and photopic luminance to limit the lighting conditions leading to photophobia.

Melanopsin Cell Dysfunction is Involved in Sleep Disruption in Parkinson's Disease.

BACKGROUND: Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) signal the environmental light to mediate circadian photoentrainment and sleep-wake cycles. There is high prevalence of circadian and sleep disruption in people with Parkinson's disease, however the underlying mechanisms of these symptoms are not clear. OBJECTIVE: Based on recent evidence of anatomical and functional loss of melanopsin ganglion cells in Parkinson's disease, we evaluate the link between melanopsin function, circadian, and sleep behavior. METHODS: The pupil light reflex and melanopsin-mediated post-illumination pupil response were measured using chromatic pupillometry in 30 optimally medicated people with Parkinson's disease and 29 age-matched healthy controls. Circadian health was determined using dim light melatonin onset, sleep questionnaires, and actigraphy. Ophthalmic examination quantified eye health and optical coherence tomography measured retinal thickness. RESULTS: The melanopsin-mediated post-illumination pupil response amplitudes were significantly reduced in Parkinson's disease (p < 0.0001) and correlated with poor sleep quality (r2 = 33; p < 0.001) and nerve fiber layer thinning (r2 = 0.40; p < 0.001). People with Parkinson's disease had significantly poorer sleep quality with higher subjective sleep scores (p < 0.05) and earlier melatonin onset (p = 0.01). Pupil light (outer retinal) response metrics, daily light exposure and outer retinal thickness were similar between the groups (p > 0.05). CONCLUSION: Our evidence-based data identify a mechanism through which inner retinal ipRGC dysfunction contributes to sleep disruption in Parkinson's disease in the presence of normal outer retinal (rod-cone photoreceptor) function. Our findings provide a rationale for designing new treatment approaches in Parkinson's disease through melanopsin photoreceptor-targeted light therapies for improving sleep-wake cycles.

Rhodopsin and melanopsin contributions to human brightness estimation.

We examined the contributions of rhodopsin and melanopsin to human brightness estimation under dim lighting. Absolute brightness magnitudes were estimated for full-field, rhodopsin-, or melanopsin-equated narrowband lights (${\lambda _{\rm max}}:\;{462}$λmax:462, 499, 525 nm). Our data show that in scotopic illumination ($ - {5.1}$-5.1 to $ - {3.9}\;{\log}\;\unicode{x00B5} {\rm Watts}\cdot{\rm cm}^{ - 2}$-3.9logµWatts⋅cm-2), the perceptual brightness estimates of rhodopic irradiance-equated conditions are independent of their corresponding melanopic irradiance, whereas brightness estimates with melanopic irradiance-equated conditions increase with increasing rhodopic irradiance. In mesopic illumination ($ - {3.4}$-3.4 to $ - {1.9}\;{\log}\;\unicode{x00B5} {\rm Watts}\cdot{\rm cm}^{ - 2}$-1.9logµWatts⋅cm-2), the brightness estimates with both lighting conditions increase with increasing rhodopic or melanopic irradiances. Rhodopsin activation therefore entirely signals scotopic brightness perception and plateaus in mesopic illumination where intrinsic melanopsin contributions become first evident. We infer that all photoreceptor signals are transmitted to higher visual centers for representing scene brightness in scotopic and mesopic illumination through both conventional and melanopsin ganglion cell pathways.

The accuracy of artificial and natural light measurements by actigraphs.

Actigraphs are the reference standard for measuring light exposure in human non-laboratory experiments due to their portability and long battery lives. However, actigraphs typically have a limited illuminance operating range not representative of real-world conditions, and for many actigraphs, the accuracy of their light measurement has not been verified independently. We assessed the illuminances recorded by Activinsights GENEActiv Original and Philips Actiwatch 2 actigraphs in comparison to a calibrated, laboratory-standard photometer, under both artificial light-emitting diode (LED) and natural sunlight illuminations that might be encountered by a person under real-world conditions. We show that in response to ~20,000 lux white LED light, the GENEActiv and Actiwatch 2 underestimate illuminance by recording 50% and 25% of the true value, respectively. Under ~30,000 lux sunlight, the GENEActiv readily saturates whereas the Actiwatch 2 reports ~46% of the true illuminance. These underestimations are highly linear and we provide correction factors to estimate the illuminance levels of the ambient environment measured by the actigraphs. We also evaluate the application of neutral density filters for extending the operating range of both devices in natural sunlight illuminations (as high as 30,000 lux during our measurements) and demonstrate that this may be a viable approach for increasing the operating range of the Actiwatch 2 but not the GENEActiv. We conclude that both actigraphs provide good performance in monitoring the temporal patterning of light, whereas the absolute illuminance values require correction to accurately evaluate the effects of light intensity on human health and behaviours.

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.

The melanopsin-directed white noise electroretinogram (wnERG).

The white noise electroretinogram (wnERG) provides a measure of the impulse response function under conditions of retinal equilibrium; it is yet to be determined how the electrical response generated by melanopsin ganglion cell photoreception is expressed in the impulse response. To this end, we recorded the human wnERG to continuous temporal white noise (TWN) stimuli that were melanopsin-directed (rod and cone silent) or cone-directed (rod and melanopsin silent). The impulse response of the electroretinogram was derived by cross-correlating the TWN stimulus with the wnERG response. We observed that the LMS-cone directed wnERG contained the expected N1 wave (24.1 ±â€¯2.4 ms; mean ±â€¯SEM) and P1 wave (49.7 ±â€¯1.8 ms). Melanopsin-directed stimuli produced a unique wnERG with a slower negative deflection (Nm) at 62.9 ±â€¯3.3 ms followed by a positive deflection (Pm) at 126.3 ±â€¯5.1 ms. Additional experiments indicated this melanopsin-directed wnERG response was not due to cone intrusion. The Nm and NmPm amplitudes increased with illuminance (32,000-80,000 Td; no rod intrusion) and melanopsin contrast (10-36% Michelson contrast). As there are known pathways connecting melanopsin cells to the outer retina, we then measured the wnERG to combined melanopsin and cone-directed stimuli to quantify melanopsin interactions with cone signalling. With the combined stimuli, the N1P1 amplitudes were suppressed by ~59%, which may be a result of a destructive interference between the positive (P1) and negative (Nm) waves generated by the cone and melanopsin pathways. We conclude that the human wnERG to melanopsin-directed stimuli may reflect the combined response of intra-retinal melanopsin pathways, independent of rod and cone photoreception.

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.

Melanopsin driven enhancement of cone-mediated visual processing.

The precise control of visual sensitivity to variations in external lighting is critical for optimising human visual function in a changing environment. In photopic illumination, the cone photoreceptors and their post-receptoral pathways have a primary role in regulating these adjustments; it is not fully understood how a small population of melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs) can interact with the three cone photoreceptor classes to modulate human visual function. Here we investigated interactions between these inner and outer retinal photoreceptor classes in participants with trichromatic colour vision under conditions that independently controlled the excitations of ipRGCs, cones and rods in the retina. In the peripheral retina, we show that interaction between melanopsin- and cone-directed signals affect conscious, image-forming vision. The interaction patterns are inconsistent with any potential effects arising from artefacts due to open-field and penumbral-cone contrasts, or rod intrusion. For cone signals mediated via each of the three primary visual pathways, melanopsin activation enhances contrast sensitivity. The contrast response functions indicate this enhancement is a more generalised facilitation effect that onsets at ∼9% melanopsin contrast. The implication for human vision is that the contrast sensitivity of cone-mediated visual processes can be modulated by the level of melanopsin excitation in the stimulus light.