Light adaptation characteristics of melanopsin

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.

Functional subtypes of rodent melanopsin ganglion cells switch roles between night and day illumination

Intrinsically photosensitive retinal ganglion cells (ipRGCs), contain the photopigment melanopsin, and influence both image and non-image forming behaviors. Despite being categorized into multiple types (M1-M6), physiological variability within these types suggests our current understanding of ipRGCs is incomplete. We used multi-electrode array (MEA) recordings and unbiased cluster analysis under synaptic blockade to identify 8 functional clusters of ipRGCs, each with distinct photosensitivity and response timing. We used Cre mice to drive the expression of channelrhodopsin in SON-ipRGCs, enabling the localization of distinct ipRGCs in the dorsal retina. Additionally, we conducted a retrospective unbiased cluster analysis of ipRGC photoresponses to light stimuli across scotopic, mesopic, and photopic intensities, aimed at activating both rod and cone inputs to ipRGCs. Our results revealed shared and distinct synaptic inputs to the identified functional clusters, demonstrating that ipRGCs encode visual information with high fidelity at low light intensities, but poorly at photopic light intensities, when melanopsin activation is highest. Collectively, our findings support a framework with at least 8 functional subtypes of ipRGCs, each encoding luminance with distinct spike outputs, highlighting the inherent functional diversity and complexity of ipRGCs and suggesting a reevaluation of their contributions to retinal function and visual perception under varying light conditions.

Short-term acute bright light exposure induces a prolonged anxiogenic effect in mice via a retinal ipRGC-CeA circuit

Light modulates mood through various retina-brain pathways. We showed that mice treated with short-term acute bright light exposure displayed anxiety-related phenotypes in a prolonged manner even after the termination of the exposure. Such a postexposure anxiogenic effect depended upon melanopsin-based intrinsically photosensitive retinal ganglion cell (ipRGC) activities rather than rod/cone photoreceptor inputs. Chemogenetic manipulation of specific central nuclei demonstrated that the ipRGC-central amygdala (CeA) visual circuit played a key role in this effect. The corticosterone system was likely to be involved in this effect, as evidenced by enhanced expression of the glucocorticoid receptor (GR) protein in the CeA and the bed nucleus of the stria terminalis and by the absence of this effect in animals treated with the GR antagonist. Together, our findings reveal a non-image forming visual circuit specifically designed for "the delayed" extinction of anxiety against potential threats, thus conferring a survival advantage.

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

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

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

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

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.

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Melanopsin ipRGCs support vision independent of the rod and cone signals

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Rod pathways mediate robust visual responses in daylight

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Temporal contrast sensitivity is contingent on the melanopsin excitation level

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Visual performance is collectively regulated by melanopsin, rod and cone pathways