A small study, published in the journal Neurology, linked damage to the retina with declining brain function.
They believe issues with the blood supply may be damaging both the eye and the brain.
Alzheimer's Research UK said the study showed the importance of a healthy heart.
The eye condition the researchers were looking at was retinopathy, which is common in patients with Type 2 diabetes or high blood pressure. Damage to the retina can eventually lead to blindness.
Scientists followed 511 women, who were 65 or older, for a decade. Some 39 were diagnosed with retinopathy.
Those with the eye condition tended to have lower scores in tests of brain function, including memory and abstract reasoning exams.
Brain scans also showed up more areas of damaged brain tissue, ischemic lesions, in those with retinopathy.
Dr Mary Haan, from the University of California, San Francisco, said: "Problems with the tiny blood vessels in the eye may be a sign that there are also problems with the blood vessels in the brain that can lead to cognitive problems.
"This could be very useful if simple eye screening could give us an early indication that people might be at risk of problems with their brain health and functioning."
There was only a small number of patients with retinopathy in the study. Much larger studies would be needed to see if the findings could be used as a clinical test for declining brain function.
While there was no suggestion of dementia in the patients, brain decline can be an early sign of the disease.
Dr Simon Ridley, head of research at Alzheimer's Research UK, said: "Accurate early detection of the cognitive decline that can be associated with dementia could unlock our ability to treat it.
"This small study offers clues for another possible route doctors could consider when monitoring for the signs of cognitive decline.
"The study adds to mounting evidence linking vascular health to cognitive decline, and underlines the importance of looking after our hearts. It will be useful to see whether the people in this study went on to develop dementia."