People with dementia are often said to forget recent events but remember the past. Caregivers can easily imagine their loved ones' memories as a lifetime's worth of photos. The newer ones are disintegrating but the old ones are still sharp.

So it was a surprise to my brother and me when our mother, now 89, started telling stories about her past that we knew weren't true. In one I've heard a lot the last year or so, she was driving by herself in the mountains around Tucson, where she lived for a few years in the 1980s. She spied a man lying partly submerged in a roadside lake. She got out of the car carrying her trusty cellphone, which never left her side, and checked on the guy, who was dead. When the police came, she learned the man had been murdered.

But my mother, who was very late to accept cellphones, didn't have one back then. Plus, Tucson is a desert. Most important, this is a woman who could talk for an hour about visiting a thrift shop. My brother and I would know if she had found a bullet-riddled body.

Many other tales in my mother's dwindling repertoire of stories, including the one about the tall young women who came into her room in her assisted-living facility to steal dresses, are also not true. We know there's no point in arguing, but the phenomenon is intriguing. Our mother was a voracious reader of historical fiction and newspapers. On TV, she watched only CNN and the local news. Is she mixing fragments of her life with fiction and television?

Why are these false memories so firmly planted in her brain when she's forgotten so much of her true history? Can I trust other stories that sound more plausible, such as the preacher who tried to kiss her when she was a teen or the small-town boys who made sure she got home safely from work at night during World War II?

Assuming - correctly, it turned out - that we are not the only adult children who have discovered that memory loss in dementia doesn't always follow a simple playbook, I asked people who study false memories to help me understand my mother's stories. I learned that brain damage not only makes it harder for people with dementia to encode new memories but also impairs their ability to remember where snippets of information came from - experience, TV, novels, dreams - and to evaluate whether something that feels like a memory actually happened. My brother and I, for example, know our mother did not have a cellphone in Tucson. That kind of fact-checking is lost to her now.

I also learned that even people in their 60s and 70s without dementia are at higher risk for false memories than people in their 20s. And, of course, anyone who has read about the science of eyewitness testimony knows that memory is suspect at any age. Even when we're young, memories are corruptible and can change a little every time we retrieve them.

MISPLACED CONFIDENCE

However, things do worsen with age and dementia. It's hard to know when false memories become more common because researchers often study groups that have time to spare - college students and retirees - bypassing the busy working adults in the middle. In general, our memory and some types of thinking ability peak in our late 20s, while accumulated knowledge and wisdom acquired through experiences survive into late life. Research is clear that people over 65 more often have false memories, and many don't know it.

"One thing that happens is that we can paradoxically become more confident in our memories despite the fact that they become less accurate," said Aleea Devitt, a post-doctoral research fellow at Harvard University who studies imagination and false memories in normal aging.

Nancy Dennis, a cognitive neuroscientist who is an associate psychology professor at Penn State and also studies false memory in normal older adults, said that older people have two sorts of memory problems, and that they are equally common. One is the kind you're likely to recognize. You forget whether you need milk when you go to the grocery store. The other is false memories; you are sure that orange juice was on the grocery list when it wasn't.

If you show both older and younger people a drawing of a cluttered bathroom that happens to be missing a sink, the elders are more likely to say later that they are sure it had a sink.

"We are very much aware of forgetting, because we know it happens," said Daniel Schachter, who researches memory at Harvard. "We are much less aware of memory distortion, where memory is there, but wrong."

Older people, Dennis said, also have more trouble with memories that should link together. You might remember a face or a name, but you can't put the two together. You know a coworker is divorcing, but not who told you that.

WHEN THE GIST IS THE PROBLEM

Dennis said older people are more likely than the young to rely on the "gist" of a situation. They remember the broad parameters - there was a picture of a bathroom and most bathrooms have sinks - but are not as good on detail, including separating similar memories.

In much of life, this is fine. I can keep a grocery list. In real life, I have never needed to remember everything that was in a bathroom.

"Gist is really useful," said David Balota, a psychologist at Washington University in St. Louis who studies false memories in dementia. It's often a shortcut that people with much experience can use to focus on big ideas or emotions that matter more. "Older adults have 50 years additional experience compared to young college students."

The bad news is that Dennis thinks this reliance on gist is often a response to brain deterioration. The hippocampus, a key player in storing new memories and an early target of Alzheimer's disease, starts shrinking in many people as they age.

Andrew Budson, a neurologist who is chief of cognitive and behavioral neurology at Veterans Affairs Boston Healthcare System, questions whether this is part of "normal" aging and said it may be a sign of the earliest stages of dementia. Half of those who live to 85 have dementia, and the brain damage starts many years earlier. He prefers the term "average" aging.

Dennis' work, which uses brain imaging, provides evidence that older people begin using other parts of the brain to compensate for the diminished hippocampus. The bypass doesn't work quite as well and, unfortunately, the other parts eventually decline, too.

Older people have more, often similar, memories to sort through, Dennis said. By the time you're 65, there are so many more birthdays, songs, former coworkers, vacations, in your head than in your daughter's or grandchild's. Now think about how many times an older man has taken his medicine, walked the dog, or turned off the stove. "We have so many overlapping representations," Dennis said.

'CANNOT TRUST ANYTHING'

Dementia means extra brain damage from the disease process, often exacerbated by small strokes.

Budson began studying false memories early in his career after he noticed that stroke patients responded better to occupational therapy for memory than dementia patients, even when the stroke patients seemed more impaired. A big reason was that dementia patients were so sure they had taken their medication or turned off the stove that reminders were useless. "It was the false memories that were the major problem in terms of what made them so functionally impaired," he said.

With dementia, the associations between related memories deteriorate and the fact-checking function that the frontal lobe usually plays is faulty, as well, he said.

This does, indeed, call all of my mother's stories into question. "We have a rule in our (dementia) clinics that I always teach all the trainees," Budson said. "If there's no family member present, we basically cannot trust almost anything the patient says."

Balota agreed that the sort of memory confabulation my mother is experiencing may stem from frontal-lobe problems. It occurs in young people with frontal-lobe damage, as well.

Once a false narrative takes hold, it's hard to make it go away. With every retelling of the story, my mother is reinforcing it. I might be able to modify it by suggesting, repeatedly, that she found a doll instead of a dead man, but experts said it's probably easier to try to change the subject.

Several researchers, including Dennis, are trying to figure out how to help older people with and without dementia remember more accurately. There's some hope there, but it's a work in progress.

The big lesson I learned from talking to the experts is that I need to question my own memory, not just my mother's. We all should.

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