The latest breakthroughs in Alzheimer’s research focus on the time well before patients even know they might have the neurodegenerative condition. Studies so far have found evidence that the biological processes that cause the mental decline may begin 10 to 12 years before people first notice signs of cognitive decline. But in the most recent report published Wednesday in the journal Neurology, experts say that the disease may actually begin even earlier — 18 years earlier, in fact — than they expected.

For 18 years, Kumar Rajan, associate professor of internal medicine at Rush University Medical Center, and his colleagues followed 2,125 elderly people with an average age of 73 and who did not have dementia. Every three years, the researchers gave the volunteers mental skills tests, and then compared these results over time.

When they looked at the group that went on to receive an Alzheimer’s diagnosis, they found that these people showed lower scores on their tests throughout the study period. In fact, their scores steadily declined with each test. For each unit that the scores dropped on the cognitive tests, the risk of future Alzheimer’s increased by 85%.