Picture yourself driving through your neighborhood. Your eyes skim over the familiar landscape as you prepare to turn onto your street. A sudden flash of movement appears in the corner of your vision as a pedestrian steps off the curb to enter the crosswalk. Your foot immediately moves to the brake. Instinctively, your brain has processed a stream of visual input and used it to make a critical, split-second decision.
Do you ever wonder how that happens?
Vision is the result of your eyes and brain working together to create meaning from visual information. It starts when your eyes receive light and turn it into nerve signals, which travel through the optic nerves to the brain.
Visual processing is how the brain makes sense of what you see. It helps you understand and navigate the world around you, learn and form memories. The speed of your visual processing determines how quickly you can make decisions based on that information.
However, there’s a lot we still don’t understand about visual processing and the relationship between vision and cognition, the process of thinking, learning and understanding. Studies suggest there is a link between vision loss and cognitive decline — but why?
“What we've known for a number of years now is that older adults with poor vision have a greater chance of going on to have new-onset dementia and their cognitive function decreases at a faster rate than those who have good vision,” explained Josh Ehrlich, MD, MPH, a professor of ophthalmology at the University of Michigan Health and a public health researcher.
Researchers are working to learn why that is the case. They hope the solution is the key to answering another question: Could protecting vision and eye health be one way to help support brain health and reduce dementia risk?
What does recent research tell us about vision and dementia?
A follow-up study was conducted based on a randomized, controlled trial originally done in the late 1990s. The updated research program found that a type of cognitive training that involves speed of visual processing could be associated with a delayed diagnosis of Alzheimer’s disease and related dementias.
It’s important to note that this type of training, known as computerized cognitive speed training, is distinct from vision therapy, a program of eye exercises used by eye care professionals to address specific visual function disorders. The training studied here is a cognitive exercise focused on the speed of processing visual information.
The trial, called the Advanced Cognitive Training for Independent and Vital Elderly (ACTIVE) study, began more than 20 years ago. The participants were recruited between 1998 and 1999. Researchers note that whether these findings apply to today's population, with its different screen habits, digital environments and lifestyle factors, has not yet been established.
The researchers set out to learn if cognitive training, or brain training, could help older adults remain independent as they aged. It assigned participants aged 65 and older to one of three training groups or a fourth control group.
One of the three training groups was given memory training, the second received training to improve reasoning and problem solving, and the third received computerized cognitive speed training, which was designed to improve how quickly and accurately they could process visual information on a screen.
During the computerized cognitive speed training, participants were shown a landscape and given a brief glance of a car and a road sign before they both disappeared. The participants then had to recall the type of car they saw and the location of the road sign on the screen. As the training progressed, the images were surrounded by distracting visual information and only shown for milliseconds before they vanished.
Each of the three groups completed approximately 10 training sessions over five to six weeks. A fourth control group did no training whatsoever. To determine whether having booster sessions at a later time would benefit the participants, about half of the group was randomly assigned to do four additional training sessions one year and then three years later.
The ACTIVE trial’s follow-up study was based on a review of Medicare claims in the two decades since the initial training. It found that participants who completed the initial speed training and booster sessions showed a statistically significant 25% reduction in the risk of diagnosis with Alzheimer’s disease and related dementias (ADRD) over the follow-up period.
Participants who completed the initial training without the booster sessions did not show a statistically significant reduction in ADRD diagnoses. This suggests that the booster sessions, completed at the one- and three-year marks, may have played an important role in the observed benefit, though researchers have not yet determined why.
What are the main takeaways from the ACTIVE study?
The study’s lead author, Marilyn Albert, PhD, a professor of neurology and director of the division of cognitive neuroscience at the Johns Hopkins University School of Medicine and director of the Johns Hopkins Alzheimer’s Disease Research Center was surprised by the research findings. The study showed that providing speed of visual processing training was associated with a reduced likelihood of an ADRD diagnosis.
"If you asked me when they started the study whether or not the major outcome was going to be speed of processing, I would have said no with respect to dementia,” she said. “But over the years, it became clear that the speed of processing group was doing better than the others. And now we see that it has this big impact on lowering risk for dementia.”
While these findings are encouraging, researchers caution that the mechanism behind this association remains unknown, and further studies are needed before any clinical recommendations can be made. Dr. Albert said future studies could use magnetic resonance imaging (MRI) and blood biomarkers to better understand how the brain responds to the training.
“It's very gratifying to see that there might be things that we've thought all along you could do, but now there's some evidence that they really make a difference,” she said. “All of us who work in dementia know that it costs $400 billion a year to take care of people with dementia (in the U.S.). So if you can reduce that by 25%, that's a huge savings and it's a huge reduction in burden for everybody.”
How is impaired vision linked to cognitive decline?
There are still many questions about why vision loss and dementia risk appear to be associated. According to Dr. Ehrlich, researchers have identified several hypotheses that need further investigation:
- Poor vision may increase cognitive load, a term that describes the mental effort it takes to perform daily tasks. This puts stress on the brain that might raise the risk of developing dementia.
- A neurodegenerative condition or microvascular disease that affects blood flow to both the eyes and brain potentially could cause both cognitive decline and vision loss to happen in tandem.
- Poor vision may make a person more isolated and limit their physical activity, both of which are risk factors for dementia.
- Finally, vision loss may directly affect the structure of the brain.
The eye itself may also offer a direct window into the brain. The retina, the light-sensing layer at the back of the eye, develops from the same tissue as the brain and central nervous system. Some researchers believe that changes in the retina may match up to changes taking place in the brain. This could help explain why vision and cognition appear closely linked.
READ MORE: The possible link between the eyes and Alzheimer’s disease
What can I do now to lower my risk of dementia?
Research on cataract surgery offers one of the strongest links to dementia. In a study of more than 3,000 older adults, those who had cataracts removed were nearly 30% less likely to later be diagnosed with dementia. This benefit held for more than a decade and did not appear after glaucoma surgery. While the study can't directly prove cataract surgery prevents dementia, it is recommended to schedule a comprehensive eye exam with your eye doctor to help understand appropriate cataract treatment options.
Findings like these can connect a growing body of evidence between eye, vision and brain health. In 2024, The Lancet Commissions published a report that added vision loss to a list of factors associated with increased dementia risk for the first time. The list also included conditions like depression, hypertension and hearing loss.
Vision impairment often goes uncorrected or untreated around the world, particularly in low- and middle-income countries, even though treatment is effective and cost-efficient for an estimated 90% of people. In light of those facts, the report said, "A clear opportunity for dementia prevention exists with treatment of visual loss."
Researchers emphasize, however, that this reflects an association between untreated vision loss and dementia risk, not that treating vision loss in itself will prevent dementia.
Experts say more trials are needed to establish the causes of dementia and identify the most effective ways to reduce risk.
For example, Dr. Ehrlich is a co-investigator of a study in India that is giving eyeglasses to older adults with poor distance or near vision. The trial will assess whether the glasses are associated with a delay in or reduction of cognitive decline. While research is ongoing, he encourages people to protect their vision health and not ignore signs of vision loss as they age.
“It's a misnomer that vision loss is a normal part of aging. It's not,” says Dr. Ehrlich. “You should not be experiencing visual decline even as you get older. And if you are, it means that something is wrong.”
It's worth noting that some age-related changes in vision, such as difficulty focusing on close objects (presbyopia), are common and physiologically normal. Dr. Ehrlich's comment refers specifically to vision loss beyond typical aging.
Getting a comprehensive eye exam is the first step to learn how to prevent, correct or slow the process of vision loss.
Dr. Albert is hopeful that, after further study, visual speed of processing training may prove to be another tool to help lower dementia risk.
Until then, her recommendation for people already living with impaired vision is to stay active and not become isolated. This will help them avoid other dementia risk factors, like depression and physical inactivity.
“I do think that if people are visually impaired,” she said, “it's important for them to say, ‘I'm not going to limit the way I live. I’m going to try to be interactive.’”
