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Parts of the brain’s outer layer, the cortex, may become thinner before people develop symptoms of Alzheimer’s disease, a new study suggests. Researchers were able to see the difference on a magnetic resonance imaging (MRI) scan. The study included 159 people. Their average age was 76. They were given MRIs as the study began. They also took tests of brain functions such as memory and problem solving. About 12% of the people showed thinning of the cortex. This was seen in areas that also become thinner in people with Alzheimer’s disease. Most people had a cortex of average thickness. About 15% had a thicker cortex than average. People in these 3 groups did not differ in their results on brain function tests. But about 3 years later, 21% of the group with the thin cortex had lower scores on brain function tests.