Twilight of the eye

With age, people start to see things differently. Literally.

For most people, the vision system — the eyes and their connections to the brain — gradually deteriorates with the passing years. Risks of serious eye diseases such as glaucoma and cataracts increase with age. And even people lucky enough to be spared vision-related diseases still experience a slow decline in eyesight.

That thought can be terrifying. Blindness ranks in the top tier of maladies that people worry about. In a survey described in 2016 in JAMA Ophthalmology, people reported that losing vision would be worse than losing memory, speech, hearing or a limb. “Americans fear it, for good reason,” says ophthalmology researcher Cynthia Owsley of the University of Alabama at Birmingham. “We’re visual animals. It’s one of our primary senses. It’s how we interact. It’s how we go about our daily lives.”

Eyesight doesn’t diminish for everyone equally. But there are several physical problems that commonly occur in aging eyes. With time, lenses grow cloudy, light-collecting cells suffer and the brain’s ability to handle incoming visual signals can slow. “There are many changes in the eye, structurally, as we get older,” Owsley says. “And most of them have implications for vision.”

Such age-related changes can meddle with sight and affect daily life in multiple ways, as Owsley wrote in 2016 in the Annual Review of Vision Science. Hidden among other objects in a dim drawer, keys become invisible. Night driving becomes more treacherous. Visual cues fade, leaving people more vulnerable to falls.

But scientists have made progress in identifying aging’s influences on vision. By better understanding the problems, researchers may also find ways to slow the decline.

Losing the edge

The eyes truly do dim with age. For one thing, the pupils — the round, black holes that let light stream into the eyeball — get smaller as people get older. Because of this process, called pupillary miosis, the pupils remain small even in dark places, resulting in less light entering an older eye.

Another visual dimming involves the lens — the clear, springy disk that focuses light rays onto a thin layer of tissue called the retina at the back of the eye. Older lenses are denser than younger lenses, a thickness that can curb the amount of light that reaches the retina.

Combined, these ocular changes can interfere with people’s abilities to spot contrasts, a measure of vision called spatial contrast sensitivity. When people are nearsighted or farsighted, scenes appear blurry. In contrast, when people begin losing contrast sensitivity, scenes appear washed out and indistinct. “Think of it as a dirty windshield,” Owsley says. “You can see out of the vehicle but things aren’t as crisp in terms of edges and borders as they would be if you had a clean windshield.”

A decline in spatial contrast sensitivity is a “common and almost inevitable part of aging,” Owsley writes in her review. And these grimy windshields can have dangerous consequences, leaving people at higher risk of being in car crashes than people with normal contrast sensitivity, studies show.

Car crashes and hip fractures are both more likely in people who see the world through a fog. And reading can slow, particularly when the letters are very small or very large. (People’s contrast sensitivity has a sweet spot, worsening when targets are big or small.)

Night eyes

Situated in the retina, specialized cells called photoreceptors detect light and send visual signals zipping along the optic nerve to the brain. These photoreceptors are one of two types: rods or cones. Cones do the heavy lifting in bright conditions (such as clear daylight, a light level referred to as photopic). But when rooms are very dark (a light level known as scotopic), rods take over. Rods are both more numerous and more sensitive to light than cones. Rods are also more sensitive to aging.

Rods depend on support tissues that can go on the fritz with age, essentially starving the rods of necessary nutrients. That means that older people begin to lose their rod photoreceptors. Beginning in a person’s mid-20s, rod density starts to decline. By the time someone hits their 60s, rods are dramatically sparser, studies on donated retinas show. And when rods start to malfunction and even die, scotopic vision can suffer.

Older people also tend to be worse at transitioning from light environments to dark ones. A 70-year-old who walks from a sunny sidewalk into a dark theater will take up to 10 minutes longer to adjust to the darkness than a 20-year-old. This delay might be caused by a dearth of nutrients delivered to rods, Owsley and others suspect. Older adults who received high doses of retinol (a version of vitamin A) adapted more quickly to dark environments, one study showed.

Other low-light situations are troublesome for older people: Driving at night, reading menus in dimly lit restaurants and getting around in the dark are all challenging even for the elderly with relatively good vision.

Brain delay

Changes to the physiology of the eye can cause vision problems, but so can changes to the brain. One of the least studied aspects of vision trouble has to do with the brain’s role in parsing visual signals.

“One-third of the brain serves your eyes,” says Emily Chew, an ophthalmologist at the National Eye Institute in Bethesda, Maryland. “You see with your brain.”

Measured by a variety of tasks, visual processing speed refers to how nimble the brain is at making decisions about what the eyes see. A particularly keen test of visual processing speed involves a target in the center of a group of arrows on a computer screen. At the periphery, a secondary target lurks. Delays in visual processing speed can be measured by how long it takes the viewer to spot and assess the secondary target.

That test reveals a possible key cause of the brain’s sluggish pace — attention. Older people have trouble shifting their attention in purposeful ways — looking away from a centrally located, but useless, object, for instance.

Another potential explanation for this slowdown is differences between young and old people in the speed/accuracy trade-off. In some lab tests, older people are more likely than youngsters to value accuracy and are more willing to take the time they need to make sure they get the right answer.

An estimated 25 to 30 percent of older people suffer from slower visual processing speed — with consequences ranging from mildly annoying to deadly. People with slower processing speeds take longer to do everyday tasks such as finding an item on a shelf. Performance in driving simulators also worsens when visual processing speed slows — no surprise given the split-second decisions that drivers must make, and no doubt a factor in the increased likelihood of car collisions for older drivers.

Other evidence suggests that this type of visual impairment can circumscribe people’s lives, making them more likely to stay at home, less likely to move around and less actively involved in household activities. It’s not clear why their radius shrinks, though one possibility is that they may notice their visual skills slipping, Owsley says.

Destined to squint?

Although the news seems grim, poor eyesight isn’t destiny, Chew points out. “Aging is a continuum,” she says. And while it’s true that vision usually worsens as you get older, hidden in that average are people who retain good eyesight well into old age. And while some aspects of visual decline, such as worsening spatial contrast sensitivity, are nearly inevitable for everyone, others may be avoided.

Researchers are also investigating ways to slow — or even reverse — vision decline. Some scientists are studying whether cognitive training programs can increase visual processing speed in elderly people. Early studies have turned up positive results, but the research is far from settled, with opponents arguing that the gains made in the lab don’t extend to meaningful improvements in people’s lives.

A better understanding of how age changes the basic biology of vision may point to better ways to intervene. In the last few decades, “we’ve made great leaps and bounds in understanding many of these phenomena of visual aging,” Owsley says. What’s still needed is turning that knowledge into practical ways to preserve sight over a lifetime.