One in nine Americans aged 65 and older has Alzheimer's disease. Therapy could come in the form of new drugs, but some experts suspect drug trials have failed so far because compounds were tested too late in the disease's progression. By the time people show signs of dementia, their brains have lost neurons. No therapy can revive dead cells, and little can be done to create new ones.

So researchers running trials now seek participants who still pass as cognitively normal but are on the verge of decline. These “preclinical” Alzheimer's patients may represent a window of opportunity for therapeutic intervention. How to identify such individuals before they have symptoms presents a challenge, however.

Today most Alzheimer's patients are diagnosed after a detailed medical workup and extensive tests that gauge mental function. Other tests, such as spinal fluid analyses and positron-emission tomography (PET), can detect signs of approaching disease and help pinpoint the preclinical window but are cumbersome or costly. “There's no cheap, fast, noninvasive test that can identify people at risk of Alzheimer's,” says Brad Dolin, chief technology officer of Neurotrack in Palo Alto, Calif., a company developing a computerized visual screening test for Alzheimer's.

Unlike other cognitive batteries, the Neurotrack test requires no language or motor skills. Participants view images on a monitor while a camera tracks their eye movements. The test draws on research by co-founder Stuart Zola of Emory University, who studies learning and memory in monkeys. When presented with a pair of images—one novel, the other familiar—primates fixate longer on the novel one. But if the hippocampus is damaged, as it is in people with Alzheimer's, the subject does not show a clear preference for the novel images.

The findings seem to hold in people. In a study published in 2013 Zola and his colleagues gave the half-hour test to 92 seniors. Scores predicted who would develop Alzheimer's three years in advance. The company has since developed a five-minute Web-based test that uses webcams and is launching a three-year study of the test with up to 3,000 seniors in Shanghai this winter. Additional studies in the U.S. will evaluate the tool alongside PET and other measures for preclinical Alzheimer's. And a number of pharmaceutical companies will include Neurotrack in clinical trials of Alzheimer's therapies in the next few years, according to Neurotrack's CEO Elli Kaplan. Experts not involved with Neurotrack think it shows promise. The test paradigm has “an excellent base of supporting literature,” says Peter Snyder of Brown University.

Blood tests, retinal scans and computerized cognitive tests are also in the running as simple screens for presymptomatic Alzheimer's. It is unclear which is most accurate, and doctors likely would use several to assess the disease's progression.