Many people’s brains deteriorate with age, becoming filled with defective proteins that cause cell death and loss of memory and cognition. But other people’s brains remain almost intact, with the same mental acuity at age 80 as at age 50.

A paper published Wednesday in the journal Nature offers a new potential explanation for this discrepancy and addresses one of the hottest debates in neuroscience: whether human brains can generate new neurons in adulthood, a phenomenon called neurogenesis.

The study found that the so-called super-aged (people aged 80 or older with the memory capacity of someone 30 years younger) had about twice as many new neurons as older adults with normal memory for their age, and 2.5 times more than people with Alzheimer’s. The research focused on an area of ​​the brain called the hippocampus, which is important for learning and memory and is believed to be the main source of new neurons.

“This paper shows biological evidence that the aging brain is plastic,” even when a person reaches age 80, said Tamar Gefen, an associate professor of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine, who contributed to the research.

To look for neurogenesis in older adults, scientists first tried to detect evidence of it in autopsied brains of young adults, ages 20 to 40, who died with normal cognition. They identified genetic markers for three key types of cells: neural stem cells, neuroblasts and immature neurons.

“It’s almost as if the neural stem cells are babies, the neuroblasts are sort of teenagers, and the immature neurons are almost adults,” said Orly Lazarov, a professor of neuroscience at the University of Illinois College of Medicine at Chicago, who led the research. The presence of all three types could suggest that stem cells are active and dividing in the brain, and that these new stem cells are maturing into adult neurons.

Next, the scientists looked for these same three types of cells in the brains of four groups of older adults: people with normal cognition, people with mild cognitive impairment, people with Alzheimer’s, and elderly people, who had donated their brains for study after they died. Each group showed evidence of all three cell types, but the numbers differed considerably between them and appeared to be related to people’s cognition at the time of death.

The super-aged people had many more immature neurons in the hippocampus, not only compared to other older adults, but also compared to young adults. These immature neurons also had unique genetic and epigenetic characteristics that, according to the researchers, gave them resilience to aging.

“Super aging occurs not only because there are more of these young cells, but because there is a type of genetic programming” that allows their preservation, Dr. Gefen said.

Dr. Bryan Strange, a professor of clinical neuroscience at the Polytechnic University of Madrid who studies a different group of older people, said neurogenesis could help explain other unique aspects of older people’s brains, including the fact that the hippocampus is usually much larger than in typical older adults.

But he noted that older people have other brain differences, such as greater volume in areas that do not undergo neurogenesis and greater connectivity between brain regions, that cannot be explained by the new findings.

The research also revealed something interesting about people with Alzheimer’s. In fact, they had more neural stem cells than other older adults, but far fewer neuroblasts and immature neurons.

“If neurogenesis is normal, stem cells are gradually lost,” said Hongjun Song, a professor of neuroscience at the Perelman School of Medicine at the University of Pennsylvania, who researches neurogenesis but was not involved in the study. One interpretation of the new finding is that, in Alzheimer’s, neurogenesis is altered and stem cells are deactivated, preventing them from advancing to the next stage of development, thus preserving the pool of stem cells.

“If that’s true, it really has opened up a new direction for the field” to potentially treat Alzheimer’s by reactivating dormant stem cells, Dr. Song said.

Not everyone is convinced by the new findings. Shawn Sorrells, an associate professor of neuroscience at the University of Pittsburgh who has also researched neurogenesis, said the scientists’ goal of mapping “how the hippocampus changes with aging and how it changes differently in people who age differently is extremely interesting and important.”

However, Dr. Sorrells worries that the study suffers from some of the same methodological shortcomings and assumptions as other neurogenesis research. He added that he would like to see the findings validated using other techniques.

Experts agree that babies and young children are capable of generating new neurons in the brain, as are several species of adult animals. However, many believe that it is still unclear whether human adults have the same capacity. There are numerous studies that provide evidence on both sides, and the results are often influenced by the methods used by the researchers.

This latest study probably won’t settle the debate, but it does give scientists new clues to investigate. For her part, Dr. Lazarov is trying to understand how the special immature neurons of the super-aged relate to the group’s superior memory, and whether it might be possible to capture some of that activity in a drug to help others stay more awake for longer.