you've already lost 50% of your brain cells. it's very difficult for any treatment to be effective at that point. >> but what if you could spot the disease 5, 10, even 20 years before you became symptomatic? he says he may have found a way by looking at the eyes into the brain. >> normally we don't think of the eye as an extension of the brain, but during the embryonic development, the retina actually develops from the brain itself, so it's brain tissue. >> one telltale sign of alzheimer's is a build-up of sticky plaques called beta amyloids. they're inside the brain. they could start to develop years before the first symptoms, and in fact, those same plaques develop in eye tissue as well. >> the beauty of that is that it allows us to essentially have a noninvasive, repeatable, high-resolution test to be able to see these protein changes that are the hallmark of alzheimer's disease very early. >> and joining me now from los