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Study points toward lifelong neuron formation in the human brain’s hippocampus, with implications for memory and disease.
If the memory center of the human brain can grow new cells, it might help people recover from depression and post-traumatic stress disorder (PTSD), delay the onset of Alzheimer’s, deepen our understanding of epilepsy and offer new insights into memory and learning. If not, well then, it’s just one other way people are different from rodents and birds.
For decades, scientists have debated whether the birth of new neurons—called neurogenesis—was possible in an area of the brain that is responsible for learning, memory and mood regulation. Now, a new study tips the "balance" back towards--"yes". New technologies that can locate cells in the living brain and measure the cells’ individual activity, may eventually put to rest any lingering questions.
Neurogenesis in the hippocampus matters, Gage says, because evidence in animals shows that it is essential for pattern separation, “allowing an animal to distinguish between two events that are closely associated with each other.” In people, Gage says, the inability to distinguish between two similar events could explain why patients with PTSD keep reliving the same experiences, even though their circumstances have changed. Also, many deficits seen in the early stages of cognitive decline are similar to those seen in animals whose neurogenesis has been halted, he says.
In healthy animals, neurogenesis promotes resilience in stressful situations, Gage says. Mood disorders, including depression, have also been linked to neurogenesis.
Rusty Gage, president of the Salk Institute for Biological Studies and a neuroscientist and professor there, says he was impressed by the researchers’ attention to detail. “Methodologically, it sets the bar for future studies.”
Llorens-Martin said she began carefully collecting and preserving brain samples in 2010, when she realized that many brains stored in brain banks were not adequately preserved for this kind of research. In their study, she and her colleagues examined the brains of people who died with their memories intact, and those who died at different stages of Alzheimer’s disease. She found that the brains of people with Alzheimer’s showed few if any signs of new neurons in the hippocampus—with less signal the further along the people were in the course of the disease. This suggests that the loss of new neurons—if it could be detected in the living brain—would be an early indicator of the onset of the f Alzheimer’s, and that promoting new neuronal growth could delay or prevent the disease that now affects more than 5.5 million Americans.
In mice and rats, researchers can stimulate the growth of new neurons by getting the rodents to exercise more or by providing them with environments that are more cognitively or socially stimulating, Llorens-Martin says. “This could not be applied to advanced stages of Alzheimer’s disease. But if we could act at earlier stages where mobility is not yet compromised,” she says, “who knows, maybe we could slow down or prevent some of the loss of plasticity [in the brain].”
As people age, they tend to engage less in cognitive-demanding activities, particularly in the case of retirement. This reduced activity may further undermine the learning and memory capacities of the elderly. The idea that age-related cognitive decline may be slowed, arrested, or even reversed through appropriately designed training or activities is supported by some research. Studies have shown that the frequency of cognitive activities in the elderly is associated with lower risk of cognitive disorders such as dementia. For example, elderly participants who were diagnosed with dementia were assessed on the overall frequency of cognitive activities: reading, writing, crossword puzzles, games, group discussions, or playing music (Hall and others 2009). Increasing frequency of cognitive activities predicted a delay in the onset of accelerated memory decline. In a longitudinal study, the relative contribution of specific activities was investigated (Verghese and others 2003). Participants aged over 75 years were followed for 5 years. Those participants who frequently played a musical instrument were less likely to have developed dementia compared to those who rarely played a musical instrument. This protective effect of playing music was stronger than those of other cognitive activities such as reading, writing, or doing crossword puzzles.