In findings that highlight the importance of mood and stress to maintaining a healthy brain, researchers and psychiatrists say that a bout of depression may raise the risks of developing dementia later in life.

People who have experienced a major depression even once in the previous 10 years in late middle age are twice as likely as those who haven't to develop problems in concentration, memory or problem-solving ability after the age of 65, according to several large, epidemiological studies.

Depression is also associated with shrinkage of the part of the brain related to memory, research shows. And while problems with concentration, decision making and memory can be common for people going through depression, studies show a significant portion of older people won't recover their mental sharpness even if their mood recovers. They may even develop new difficulties in their thinking and memory, according to a 2006 study in the American Journal of Geriatric Psychiatry.

Of course, not everyone who gets depressed ends up with dementia. And in some patients, depression may be a consequence of -- rather than a cause of -- conditions such as Alzheimer's. But many doctors say this link with cognitive impairment provides yet another reason to seek treatment for depression.

"The literature strongly suggests that depression is a risk factor for dementia," says Meryl Butters, associate professor of geriatric psychiatry at the University of Pittsburgh School of Medicine. "Depression is toxic to your brain in a bigger way than we ever realized."

Wes Blaha of Monroeville, Pa., sank into a depression two years ago. Seventy-five-years old at the time and head of the local Chamber of Commerce, Mr. Blaha also began to have trouble visualizing numbers and images in his head. This was worrisome for a man who was used to picturing racquetball shots in his head before executing them.

Mr. Blaha eventually sought treatment at the Late Life Depression clinic at the University of Pittsburgh Medical Center. With the help of an antidepressant and counseling, his mood lifted and his mental-visualization abilities improved. Today, Mr. Blaha sits on five boards of directors and says he feels "like my old self again." But his cognitive problems qualified him for a study in which he takes an Alzheimer's drug in hopes of preventing full-blown dementia.

The connection between depression and cognitive deficits has been observed for years, yet is not well understood. Generally, psychiatrists and researchers believe it is unlikely that depression in and of itself causes dementia in the absence of any other brain abnormality. The more likely explanation is that depression creates changes in the brain that exacerbate some underlying condition.

Some scientists believe that depression weakens the brain by bathing it in damaging chemicals called glucocorticoids, produced during periods of stress. Glucocorticoids, such as cortisol, may erode pathways between neurons (nerve cells in the brain that transmit messages). If this erosion occurs on top of some pre-existing brain abnormality, it may accelerate cognitive problems, according to experts.

The pre-existing signs of Alzheimer's that researchers focus on are "amyloid plaques" and "neurofibrillary tangles," which are brain deposits that signal cell death. In two separate reports published in the past year, researchers injected glucocorticoids into mice that had predispositions to Alzheimer's. The Alzheimer's-related indicators increased in those mice, compared with mice that weren't injected.

"Depression creates a tremendous burden to a brain that already had increased predisposition to dementia," says George Alexopoulos, a late-life-depression expert and psychiatry professor at Cornell University's Weill Medical College in New York.

Adds Dr. Butters: "Getting depressed is another hit on the brain. It may unmask deficits, or deficits may develop sooner."

Researchers at the Alzheimer's Disease Center at Rush University Medical Center in Chicago have also found clues to such a link. In their continuing study tracking the mental health of nearly 900 elderly people, researchers autopsied the brains of more than 200 participants who had died by 2006. They found signs of Alzheimer's in 98% of brains. But fewer than half of the study participants had shown evidence of dementia when they were still living.

It was those who had experienced more distress in the years before their deaths, including depressive symptoms, who had shown more cognitive impairment. These findings, published in the journal Psychosomatic Medicine, suggest that people with low levels of distress are better able to tolerate age-related changes in the brain, says Robert S. Wilson, psychology professor and a neuropsychologist with the Rush Alzheimer's group.

"Over a life span, chronic negative emotional states take a toll," Dr. Wilson says.

Another theory of how depression affects cognitive function involves the hippocampus -- a part of the brain related to memory. Studies show that people with more days of depression untreated by antidepressants, at any point in their lives, exhibit an average 10% reduction in volume in the hippocampus, which may result in subtle changes to their memory capabilities, according to Yvette Sheline, author of the research, which was funded by the National Institute of Mental Health and published in the American Journal of Psychiatry. There is no such effect from days of treated depression. The findings suggest that shrinkage may be prevented by antidepressant treatment, says Dr. Sheline, director of the Center for Depression and Neuroimaging at the Washington University School of Medicine in St. Louis.

In some individuals, vascular medical conditions associated with inflammation of the blood vessels, such as hypertension, could generate both dementia and depression, for instance. And mood changes may simply be the first noticeable sign of Alzheimer's disease or other dementia for some individuals.

"For some people, depression is probably the tip of a pretty bad neurological iceberg," says David Steffens, head of the Division of Geriatric Psychiatry at Duke University Medical Center in Durham, N.C.

Of particular concern are depressed elderly people with prominent short-term memory difficulties, according to preliminary work conducted by Dr. Steffens and his colleagues. "Memory problems are a particularly ominous warning sign" of future cognitive problems and dementia, Dr. Steffens says.

These findings could be especially valuable, some experts say, because if individuals at high risk for dementia could be better identified, it may be possible to prevent further decline. "If we could understand the neurobiology better, we have every reason to think it could lead directly to intervention," says Dr. Wilson of Rush University.

There is evidence that treating depression, either with medication or certain types of psychotherapy, not only benefits a patient's psychological well being, but also reduces the frequency or severity of future depression, and seems to spare the brain from injury from the load of chronic stress, say experts. Treatment is particularly important in the long term for people who get depressed young and have repeated episodes throughout their life, psychiatrists say.

Antidepressants are also known to provoke neurogenesis, or the growth of new neurons. However, it is unclear whether such neurogenesis can actually prevent dementia.

And there is an added benefit in treating depression in older individuals. "Treating depression also improves the person's ability to adhere to other medical treatments," particularly with hard-to-control conditions like hypertension and diabetes, says Charles Reynolds, professor of Geriatric Psychiatry at the University of Pittsburgh School of Medicine.

Write to Shirley S. Wang at newseditors@wsj.com