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Wei Bi, Seiichi Tsujimura, Gordon T. Plant, John L. Barbur; The Contribution Of The Rod/Melanopsin Driven Ganglion Cells To The Dynamic Pupil Light Reflex Response. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5287.
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Recent studies (Tsujimura et al, Proc. R. Soc. B, 277, 2485, 2010) have shown that the steady-state size of the pupil during long exposure to intense stimuli and the corresponding sustained constriction in darkness following the offset of the stimulus (Gamblin et al, Vision. Res., 47, 946, 2007) are likely to involve melanopsin signals through intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells also receive spatially pooled signals from rod and cone photoreceptors and the extent to which melanopsin contributes to rapid pupil responses elicited with brief stimuli remains less clear. The purpose of this study was to examine how the dynamic pupil light reflex response changes with the level of light adaptation for stimuli that isolate luminance, colour or combined rod and melanopsin.
An integrating sphere was used to homogenise the light output of four LED sources (selected to span the visual spectrum) which were modulated temporally to produce a 2s sinusoidal envelope of luminance, colour or combined rod and melanopsin. Pupil responses were measured at three light levels: 456, 74.4 and 4.8 cd/m2. The study involved 6 normal subjects and two patients with congenital stationary night blindness (CSNB).
In normal subjects, each of the three stimuli produced brief constrictions of the pupil at stimulus onset, with larger response amplitudes at lower light levels. Consistent with previous studies, the colour modulation also produced a large response at stimulus offset. The rod / melanopsin isolation condition yields larger response latencies, with the largest difference observed at the lowest light level. The patients with CSNB yield normal responses to colour and luminance modulation, but much reduced or absent pupil constriction to rod/melanopsin modulation at each light level.
When carefully interpreted, the findings from this study suggest that pupil responses to briefly presented stimuli are mediated largely by rod and cone signals with little or no contribution from melanopsin. The results also suggest that, although more sluggish, rod signals remain unsaturated and contribute to dynamic pupil responses at much higher light levels.
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