Quantification of the early pupillary dilation kinetic to assess rod and cone activity

Rods, cones and melanopsin contribute in various proportions, depending on the stimulus light, to the pupil light response. This study used a first derivative analysis to focus on the quantification of the dynamics of pupillary dilation that immediately follows light-induced pupilloconstriction in order to identify novel parameters that reflect rod and cone activity. In 18 healthy adults, the pupil response to a 1 s blue light stimulus ranging from - 6.0 to 2.65 log cd/m² in dark-adapted conditions and to a 1 s blue light stimulus (2.65 log cd/m²) in light-adapted conditions was recorded on a customized pupillometer. Three derivative parameters which describe the 2.75 s following the light onset were quantified: dAMP (maximal amplitude of the positive peak), dLAT (latency of the positive peak), dAUC (area under the curve of the positive peak). We found that dAMP and dAUC but not dLAT have graded responses over a range of light intensities. The maximal positive value of dAMP, representing maximal rate of change of early pupillary dilation phase, occurs at - 1.0 log cd/m² and this stimulus intensity appears useful for activating rods and cones. From - 0.5 log cd/m² to brighter intensities dAMP and dAUC progressively decrease, reaching negligible values at 2.65 log cd/m² indicative of a melanopsin-driven pupil response that masks the contribution from rods and cones to the early phase of pupillary dilation.