What causes vision decline in the elderly? A team from the Vision Institute* has recently uncovered an important but hitherto little explored mechanism, involving the photoreceptors in the retina.
While it is well-known that eyesight deteriorates with age, what are the explanations for this phenomenon? Several factors are generally involved and confirmed by experience. One is the alteration of the eye’s optical qualities with, for example, clouding of the crystalline lens (in cataracts) or constricted pupils, decreasing the quantity of light that reaches the retina. Another is the efficacy of the neural process that transforms this light into perception, i.e. into images formed in the brain, which decreases during aging. "We looked at the step in-between: the capacity of the photoreceptor cells in the retina – cells known as cones – to absorb photons. This level has rarely been explored", explains Rémy Allard*. In other words, the researchers wanted to know the proportion of photons arriving physically on these cells that is converted into nerve signals.
To do so, the team asked two groups of 20 subjects, one young (mean age 26.5 years), the other older (mean age 75.9 years) but still with good visual acuity, to undergo luminance contrast perception tests. These involved discerning a simple form, namely a series of horizontal or vertical bars, against backgrounds that were plain or blurred by parasitic spots. The tests were devised so as to separate the respective impacts of the four steps of light processing: 1) the optical efficacy of the eye; 2) the absorption of the photons by the cones; 3) the "noise" – or parasite signal – emitted by the neurons of the optical nerve; and 4) the processing by the brain.
An overlooked – but essential – step
It came as no surprise that the older subjects proved to be less sensitive to contrasts than the young, under the various different lighting conditions and regardless of the characteristics of the motif they were asked to pick out (number and spacing of the bars). The researchers also found known differences, such as slightly more neural noise in older volunteers. "This is nothing new: we already knew that neurons have greater spontaneous activity with age", reiterates Allard. However, the optical performance of the eye had no significant incidence, which is hardly surprising given that the subjects were selected for their good visual acuity.
The surprise lay in the intermediate step, with the main problem located in the retina! Indeed, the cones of the older subjects absorbed four times fewer photons than those of the youngest subjects – under identical lighting conditions and without the optical qualities of the eye being implicated. How do we explain this decrease in efficacy? "The question of cone loss with aging continues to be debated, but no studies have found a loss large enough to explain such a decrease in photon absorption. So we think that they become less effective with age", maintains Allard.
Various factors could explain this loss of efficacy. Nevertheless, the Parisian team has a hypothesis: "The cones could be less well aligned due to the loss of rods – these larger cells that support them". A hypothesis that the researchers are currently testing in collaboration with Michel Paques from the Quinze-Vingts National Ophthalmology Hospital. "We have found something interesting. However, the cause of this loss of absorption needs to be found before thinking about therapeutic pathways" warns Allard.
The research was conducted within the scope of the SilverSight research chair, led by Angelo Arleo and supported by the National Research Agency (ANR), which forms part of the academic/industry partnership between the Vision Institute (Inserm, CNRS, Sorbonne Université) and Essilor International.
*unit 968 Inserm/CNRS/UPMC, Vision Institute, Aging in Vision and Action team, Paris
Source : D. Silvestre et coll, Healthy aging impairs photon absorption efficiency of cones. Invest Opthalmol Vis Sci, iovs.arvojournals.org / ISSN: 1552-5783, février 2019