Scientists have long known that older adults with impaired vision face twice the risk of cognitive decline, the fifth-leading cause of disability among seniors. But the specific types of visual and mental keenness involved remain unclear.

A new study in the journal JAMA Network Open brings them into sharper focus.

The longitudinal study of 1,200 adults without dementia ages 60 to 94 looked at three types of vision loss and a full range of cognitive tests. Senior author Bonnielin Swenor of the Johns Hopkins Wilmer Eye Institute in Baltimore says that the work improves on previous studies, which often looked at only one measure of vision or lumped together different types of cognition.

"This study is really unique because it has robust measures of vision and a full cognitive battery. So what that means is, we had a lot of cognitive data across many cognitive domains," said Swenor. 

"So we could really unpack which domains were being impacted or affected by each type of vision impairment," Swenor said. 

Loss of depth perception and visual acuity — the kind eye charts test — correlated with greater declines in language and memory.

Loss of contrast sensitivity — the ability to see white milk in a white mug — was also linked to drops in language and memory, as well as reductions in attention and spatial relationship perception.

That suggests impaired contrast sensitivity, which occurs in a variety of eye diseases, could correlate with greater cognitive decline than visual acuity, which is more commonly measured in studies.

Ophthalmologist Heather Whitson directs the Duke University Center for the Study of Aging and Human Development. She sees the more granular results as a mug of milk half-full.

"In a way, you could say, 'Well, if I lose one, I'm also more likely to be losing the other.' But the flip side of that is, if we protect one, we may also be more likely to be protecting the other," Whitson said. 

Just what underlying factors link vision loss to cognitive declines remains the subject of research and debate. But most explanations fall into two camps: Either the same physiological problems affect both vision and cognition, as can occur in certain vascular or inflammatory disorders, or the loss of vision leads to other conditions that affect cognition, such as depression, social isolation or the loss of mind-stimulating activities like reading or doing crossword puzzles.  

Whatever the cause, experts hope findings like these will inform efforts to reduce risk of cognitive decline through early detection and management of vision loss.

"It's not like every kind of vision puts you at risk for every kind of cognitive loss. And my mind again as a clinician goes toward,  what does this new information give us that might suggest how we could intervene — how we could protect long-term cognition by better visual health," said Whitson.