The black and light gray lines provide the spectral radiance of the low and high (± 5% contrast) arms of the LMS directed flicker around this background. The dark gray line is the spectral radiance of the low-melanopic background (same as in the panel on the right). Right: example spectral modulations for LMS directed flicker. ( c) Left: spectral radiance of the low- (dark gray) and high- (red) melanopic backgrounds. Each sensitivity is normalized to a peak of unity. ( b) Spectral sensitivities of the L, M and S cones and of the melanopsin-containing ipRGCs. (1) Recurrent axon collaterals from ipRGCs can provide inhibitory signaling on cone pathways (2) ipRGCs can project to LGN cells that also receive inputs from the classical RGCs and (3) cone and melanopsin signals may interact within the ipRGCs themselves. There are several hypothesized locations at which signals from the melanopsin-containing ipRGCs (blue) might interact with signals from the cones, which are predominantly conveyed by the classical RGCs (gray). Theoretical background and experimental design.
Therefore, signals from the melanopsin-containing ipRGCs could interact with the classical RGC pathways downstream from the retina. A third potential point of interaction is present in the projections of the ipRGCs to the lateral geniculate nucleus (LGN), where they signal overall retinal irradiance in both a tonic and phasic manner 4, 15, 16, 17.
Electroretinogram (ERG) recordings in mice support this idea, as b-wave responses to cone-directed light flashes are reduced in amplitude and latency with melanopsin-directed stimulation 13, 14. This provides another possible site for an indirect effect of melanopsin on signals originating in the cones. In addition, in both the primate and rodent retina, a subset of ipRGCs send recurrent axon collaterals to the inner plexiform layer 12 where they are hypothesized to influence the sensitivity of cone inputs to the “classical” RGCs (i.e., midget, parasol, small bistratified). Such interactions could occur within the ipRGCs themselves, as these cells receive signals from the cones. In addition to this direct effect, signals from the ipRGCs may interact with those carried by the cones, producing effects upon perception by an indirect mechanism (Fig. Studies of this kind support the claim that ipRGC signals have a direct effect upon perception. When human observers are presented with stimuli that include an increase in melanopsin stimulation, participants report that the spectral change appears as increase in “brightness” 4, 5, 6, 7, 8, 9, 10, 11. Beyond these “reflexive” visual functions, several studies have examined if signals from the ipRGCs contribute to visual perception (for a comprehensive review, please see 3. The ipRGCs mediate numerous physiologic effects of light, including variation in pupil size and photoentrainment of the circadian rhythm 1, 2. Also active in daylight are the melanopsin-containing, intrinsically photosensitive retinal ganglion cells (ipRGCs). Under daylight conditions, signals originating in the cone photoreceptors pass through several classes of retinal ganglion cells to support visual perception.
Our results suggest that even large changes in melanopsin stimulation do not affect near-threshold, cone-mediated visual perception. Across the three experiments, no effect of melanopsin stimulation upon threshold flicker sensitivity was found. In Experiments 2 and 3, this test was repeated, but now for luminance flicker presented on a transient pedestal of melanopsin stimulation. In Experiment 1, we tested for a change in the threshold for detecting luminance flicker in three participants after they adapted to backgrounds with different degrees of tonic melanopsin stimulation. Here, we tested for the existence of an interaction by asking if a 350% change in melanopsin stimulation alters psychophysical sensitivity for the detection of luminance flicker. It is possible that the ipRGCs contribute to conscious visual perception, either by providing an independent signal to the geniculo-striate pathway, or by interacting with and thus modifying signals arising from “classical” retinal ganglion cells that combine and contrast cone input. These cells express the photopigment melanopsin and are known to be involved in reflexive visual functions such as pupil response and photo-entrainment of the circadian rhythm. In addition to the rod and cone photoreceptors the retina contains intrinsically photosensitive retinal ganglion cells (ipRGCs).
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