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Pablo Barrionuevo, Nathaniel Nicandro, J Jason McAnany, Andrew Zele, Paul Gamlin, Dingcai Cao; Assessing Relative Rod, Cone and Melanopsin Contributions to Pupil Flicker Responses. Invest. Ophthalmol. Vis. Sci. 2013;54(15):401.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the relative rod, cone and melanopsin contributions to human pupillary light responses to flickering stimuli at mesopic light levels.
A four-primary Ganzfeld photostimulator (Diagnosys ColorDome) controlled rod and cone stimulations at four light levels (-2.7, -1.8, -0.9 and 0 log cd/m2). The pupil sizes were measured using an Eyelink II eyetracker (SR Research). Three types of sinusoidally modulated stimuli (1 Hz, 25% contrast) were used: 1) rod-isolating stimuli that modulated rod excitation with a constant cone excitation (29.3% melanopsin contrast); 2) cone-isolating stimuli that modulated cone excitation with a constant rod excitation (4.3% melanopsin contrast); 3) combined rod and cone stimuli that modulated rod and cone excitation in different phases (cone-rod phase differences: 0°, 45°, 90°, 135°, 180°, 225°, 270°, 315° or 360°; melanopsin contrast 25.0% to 33.5%). Based on the Fourier pupil amplitude and phase for each combined rod-cone stimulus, a vector sum model was used to estimate the relative contribution of rods, cones and melanopsin photoresponses.
For rod- and cone- isolating stimuli, the pupil response amplitude decreased with decreasing light level, with minimal pupil response to cone-isolating stimuli at -2.7 and -1.8 log cd/m2, which is below the melanopsin photoresponse threshold. Therefore, at -2.7 and -1.8 log cd/m2, pupil responses were primarily driven by rod inputs. At -0.9 and 0 log cd/m2, there was a pronounced pupil response to both rod- and cone-isolating stimuli. In addition, the response amplitudes and phases to the combined stimuli varied systematically with cone-rod phase differences, with a minimum response amplitude and a sharp phase change observed near the 180° cone-rod phase difference. Based on the vector sum model, the estimated cone/rod input ratios to the pupil response was 0.75 and 0.62 at 0 and -0.9 log cd/m2, respectively. In addition, the estimated melanopsin/rod strength ratios were 0.49 and 0.18 at 0 and -0.9 log cd/m2, respectively.
At mesopic illumination levels, inputs from rods, cones and melanopsin are combined in a vector sum fashion to control pupil size. The relative contribution of the rod, cone, and melanopsin photoresponse inputs depends on mean luminance level of the flickering stimuli.
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