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D. H. McDougal, P. D. Gamlin; The Relative Contribution of Rods, Cones, and Melanopsin to the Human Pupillary Light Reflex. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3052.
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© ARVO (1962-2015); The Authors (2016-present)
It is readily accepted that the melanopsin mediated intrinsic response of intrinsically photosensitive retinal ganglion cells (ipRGCs) contributes to the pupillary light reflex (PLR) of rodents (Hattar et al. 2003), humans, and non-human primates (Gamlin et al. 2007). It remains to be determined how this photoresponse integrates with rod and cone photoresponses at the level of ipRGCs to drive non-image forming (NIF) light responses, such as the PLR. The purpose of the current study was to determine the relative contributing of these three photoresponses to the human PLR, a main NIF response.
In five human subjects with normal corrected vision, a stimulus subtending 36o was presented in Maxwellian view through a beamsplitter to the right eye, while the subject’s pupils were visualized under infra-red illumination using video cameras and ISCAN RK406 pupillometer systems. Stimuli were presented between 453 nm and 629 nm at 10 narrow-band wavelengths (8-10 nm FWHM, Thermo-Oriel). For each subject, the spectral sensitivity of half maximal pupillary constriction was generated at stimulus durations of 1, 3, 10, 18, 30, and 100 sec.
Mean spectral sensitivity plots at all duration condition show greatest sensitivity to short wavelength light and a reduced sensitivity to long wavelength light. This short wavelength sensitivity was greatest at 511 nm for the 1, 3, and 10 sec duration condition, while the peak sensitivity for the 18, 30, 100 sec duration conditions was at 471 nm. We fitted the spectral sensitivity data at each duration condition to a function combining the spectral sensitivities of rod, cone, and melanopsin in order determine the relative contribution of each of these photoresponses to the data collected. We determined that the spectral sensitivity data at the 1, 3, and 10 sec time points were well fit by a linear combination of rod and cone spectral sensitivities. At the 18, 30, and 100 sec time points, the addition of melanopsin spectral sensitivity significantly improved the composite functions’ fit to the collected data.
In general, the spectral sensitivity of half maximal pupillary constriction at the 1, 3, and 10 sec stimulus durations was rod dominated. Conversely at the 18, 30, 100 sec duration conditions, the spectral sensitivity of the response was melanopsin dominated. We conclude that melanopsin acts to overcome light adaptation of outer retina photoreceptors and maintains steady-state pupillary constriction even at mesopic irradiances. These findings have great implications in regards to the relative contribution of rods, cones, and melanopsin to other critical human NIF responses, such as circadian entrainment and neuroendocrine function.
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