Abstract
Rod and cone photoreceptors support vision across large light intensity ranges. Rods, active under dim illumination, are thought to saturate at higher (photopic) irradiances. The extent of rod saturation is not well defined; some studies report rod activity well into the photopic range. Using electrophysiological recordings from retina and dorsal lateral geniculate nucleus of cone-deficient and visually intact mice, we describe stimulus and physiological factors that influence photopic rod-driven responses. We find that rod contrast sensitivity is initially strongly reduced at high irradiances, but progressively recovers to allow responses to moderate contrast stimuli. Surprisingly, rods recover faster at higher light levels. A model of rod phototransduction suggests that phototransduction gain adjustments and bleaching adaptation underlie rod recovery. Consistently, exogenous chromophore reduces rod responses at bright background. Thus, bleaching adaptation renders mouse rods responsive to modest contrast at any irradiance. Paradoxically, raising irradiance across the photopic range increases the robustness of rod responses.
Highlights
Rod and cone photoreceptors support vision across large light intensity ranges
While rod contrast sensitivity is impaired upon stepping to backgrounds of 104 R* rod–1 s–1 and brighter, contrast sensitivity recovers over time under all backgrounds
We repeatedly presented a set of 50 ms light flashes of four different contrasts (Fig. 1a), while increasing irradiance at 10-fold increments every 30 min. This yielded seven light levels spanning a range from 1 R* rod–1 s–1 (2 × 108 rodeffective photons cm–2 s–1) to 106 R* rod–1 s–1 (2 × 1014 rodeffective photons cm–2 s–1, Fig. 1a)
Summary
Active under dim illumination, are thought to saturate at higher (photopic) irradiances. We find that rod contrast sensitivity is initially strongly reduced at high irradiances, but progressively recovers to allow responses to moderate contrast stimuli. Rods recover faster at higher light levels. Bleaching adaptation renders mouse rods responsive to modest contrast at any irradiance. Raising irradiance across the photopic range increases the robustness of rod responses. Contrast sensitivity recovers faster at higher irradiance, such that, contrary to conventional wisdom, raising the background light intensity within the “photopic” range does not reduce, but increase rod contribution to visual responses
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