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Jason C Park, J Jason McAnany; Influence of Stimulus Size and Luminance on Rod-, Cone-, Melanopsin-mediated Pupillary Light Reflexes.. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5764.
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© ARVO (1962-2015); The Authors (2016-present)
The human steady-state pupil size is thought to be jointly dependent on adapting field luminance and area (i.e. corneal flux density [CFD]; luminance x area). The purpose of this study was to determine if the pupillary light reflex (PLR) driven by brief stimulus presentations can also be accounted for by CFD under conditions biased toward the rod, cone, and melanopsin pathways.
Pupil size was recorded using an infrared camera from 5 visually-normal subjects. Stimuli were presented in the central visual field and consisted of short-wavelength flashes of 1-s duration presented in the dark (rod and mealnospin condition; recorded after 10-min of dark adaptation; luminance range of -4 to 2.6 log cd/m2) and against a rod-suppressing blue background (cone condition; recorded after 2-min of light adaptation; luminance range of -1 to 2.6 log cd/m2). The stimuli subtended four sizes (4°, 16°, 32° and full-field). PLR was defined as the ratio of the steady-state pupil size (baseline) to post-stimulus pupil size. Rod- and cone-mediated PLRs were measured at the time of maximum constriction following stimulus presentation, whereas the melanopsin-mediated PLR was measured at 6-8 s (median value) after stimulus offset.
The rod- and melanopsin-mediated PLRs were well accounted for by CFD, such that a lower intensity stimulus of a larger area produced the same PLR as a higher intensity stimulus of a smaller area when CFD was kept constant. The rod-mediated PLR increased as CFD increased. Melanopsin-mediated PLRs were elicited only in the higher luminance range (> 0 log cd/m2) and for larger stimulus size (> 16°), but when present, the melanopsin-mediated PLR was well accounted for by CFD. However, CFD could not account for the cone-mediated PLR, due to an approximate independence of the PLR on stimulus size, but a strong dependence of the PLR on stimulus luminance.
The rod- and melanopsin-mediated PLRs had a similar dependence on CFD, both acting as a flux integrator. This was not the case for the cone-mediated PLR, which was strongly dependent on stimulus luminance but not size. The finding that the cone-mediated PLR is not dependent on CFD, but the steady-state pupil size under photopic conditions is dependent on CFD suggests that the spatial summation characteristics differ for these two pupil responses.
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