Memory and thinking skills naturally slow with age. Scientists are now looking inside living brains to tell if depression might speed aging. The scientists report that some of the signs they have found are worrisome.

　　Depression has long been linked to some cognitive problems. Depression late in life even may increase the risk of developing Alzheimer's disease. Yet how depression might harm the ability to remember things and think clearly is not yet known.

　　One possibility: Brain cells communicate by sending messages across connections called synapses. Generally, good cognition is linked to more and stronger synapses. With a weakening of cognitive ability, those connections slowly shrink and die. But until recently, scientists could count synapses only in brain tissue collected after the person dies.

　　Yale University scientists used a new method to study the brains of living people. They discovered that patients with depression had a lower density of synapses than healthy people of the same age.

　　The lower the density, the more severe the signs of depression. Yale University neuroscientist Irina Esterlis says this is especially true of problems with loss of interest in activities patients once enjoyed. She spoke at a meeting of the American Association for the Advancement of Science.

　　Esterlis was not studying just older adults, but people of all ages, including those too young for any cognitive changes to be noticeable. She was working from a theory that early damage can build up.

　　"We think depression might be accelerating the normal aging," she said.

　　Her studies are small. To prove if depression really does increase the risk of cognitive problems as we age would require more investigation. Jovier Evans is a scientist with the U.S. National Institute of Mental Health. He proposed a study of synaptic density on larger numbers of people as they get older, to see if and how it changes over time in those with and without depression.

　　Esterlis has announced plans for a larger study to do that. Volunteers would be injected with a radioactive substance that links up to a protein in the vesicles, or storage containers, used by synapses. Then each volunteer would be given an imaging test, known as a PET scan. During the test, areas with synapses light up, enabling researchers to see how many are in different parts of the brain.

　　Esterlis said there are no medications that target synapse damage.

　　Doctor Mary Sano directs the Mount Sinai Alzheimer's Disease Research Center in New York. She was not involved in the new research.

　　Sano warned that normal cognitive aging is a complex process that involves other health problems, such as heart disease. It might be that depression does not worsen synaptic weakening. It could just makes the problem more noticeable, she said.

　　With depression "at any age, there's a hit on the brain. At an older age the hit may be more visible because there may already be some loss," she explained.

　　I'm Ashley Thompson.

　　Lauran Neergaard reported this story for the Associated Press. George Grow adapted her report for VOA Learning English. Caty Weaver was the editor.