Distinct contributions of rod, cone, and melanopsin photoreceptors to encoding irradiance.
Bottom Line: These photoreceptors define circadian responses at very dim "scotopic" light levels but also at irradiances at which pattern vision relies heavily on cones.By contrast, cone input to irradiance responses dissipates following light adaptation to the extent that these receptors make a very limited contribution to circadian and pupillary light responses under these conditions.Our data provide new insight into retinal circuitry upstream of mRGCs and optimal stimuli for eliciting irradiance responses.
Affiliation: Faculty of Life Sciences, AV Hill Building, University of Manchester, Manchester M13 9PT, UK.Show MeSH
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Mentions: To confirm this finding and to provide a more detailed picture of cone contributions to defining pupil size, we next described full irradiance response relationships at 500 and 650 nm for the dark-adapted pupillary light reflex (PLR). In Opn1mwR mice there is a substantial divergence in relative sensitivity of melanopsin, rods, and red cones between these two wavelengths (Figure 1B). We found that irradiance response curves at 500 and 650 nm were remarkably similar for Opn1mwR, but not littermate wild-type, mice (Figures 2A and 2B). In fact, when corrected for the difference in red cone sensitivity at these two wavelengths (650 nm irradiance measures ×0.13), pupil responses became indistinguishable at irradiances <1011 photons/cm2/s in Opn1mwR mice (Figure 2C). By contrast, responses were highly divergent when correction factors based upon the relative sensitivity of either rods or melanopsin were applied (Figure 2D). This suggests that under these conditions red cones define the magnitude of pupil constriction in the range of ∼108–1011 photons/cm2/s. The lower limit of this range is ∼1 log unit below the reported threshold for cone-based vision in mice (Nathan et al., 2006), indicating that cone pathways presynaptic to mRGCs are at least as sensitive as those subserving pattern vision. The upper limit is probably defined by cone saturation, which similarly occurs around 3 log units above threshold for light steps in mice (Nikonov et al., 2006).
Affiliation: Faculty of Life Sciences, AV Hill Building, University of Manchester, Manchester M13 9PT, UK.