Abstract
Purpose :
The retina senses changes in light intensity that are proportional to the ratio of the difference between the stimulus and background compared to the background (Weber contrast) rather than the total intensity. We hypothesize that human central and peripheral rod and cone photoreceptors sense constant Weber contrast over a wide range of background light intensities and thus compare light adaptation and underlying mechanisms in the peripheral and macular regions in this study.
Methods :
We procured eyes from seven organ donors within 1 hour following the donor’s withdrawal from life support. Eyes were transported in HEPES-buffered AMES and bubbled with 100% O2. We recorded photoreceptor light responses using ex vivo ERG in the presence of DL-AP4 and barium in 5 mm diameter retina samples from the periphery or macular region. We determined light sensitivity (S) under background light ranging from ~100 to 106 photons (530 nm) mm-2 s-1 (N=3). Step flash paradigm (N=3) and Pepperberg analyses (N=3) were used to assess potential modulation of the active rhodopsin (R*) and phosphodiesterase (PDE*) lifetimes by background light.
Results :
Sensitivity of macular and peripheral photoreceptors as a function of background light could be described by two distinct Weber-Fechner functions: the first up until ~1,000 photons mm-2 s-1, and second between ~1,000 – 106 photons mm-2 s-1. Macular photoreceptors appeared to maintain larger S when normalized to the dark-adapted sensitivity. The rate-limiting (slower) rod phototransduction deactivation reaction was not significantly modulated by the background either in the periphery or macula while the faster deactivation reaction could shorten >50% in the peripheral rods and changed linearly by 18 ± 2% per 2-fold change in background light between ~1,000 – 5,000 photons mm-2 s-1.
Conclusions :
Human macular rods and cones sense constant Weber contrast over distinct ranges of background light. For human rods, this light-induced gain modulation does not happen by modulation of the rate-limiting phototransduction deactivation reaction either in the periphery or in the macula. However, the faster rod phototransduction deactivation reaction is greatly accelerated by background light in the peripheral human rods.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.