New findings suggest a hypothesis for a much-debated question in neuroscience : what exactly is the role of new neurons born in the adult human brain? These brain cells may help link memories of events that occurred within a week or two of each other, a paper published in Neuron reports.

"It's really novel, and I think it's quite informative," said behavioral neuroscientist Andrea Chiba of the University of California, San Diego, who was not involved in the work.

Fred Gage a neuroscientist at the Salk Institute in La Jolla, Calif., published a controversial study in 1998 identifying the formation of new neurons in the adult hippocampus, a brain region associated with memory. Til then, neuroscience dogma had held that humans are born with all of the neurons they will ever have. But the function of these newly formed cells has never been identified.

In the current study, Gage and his colleagues built a computational model to capture how neurons form, make connections, and integrate into existing brain circuitry. The team modeled the dentate gyrus, the area of the hippocampus where these cells are generated, as a network with multiple layers of different types of cells, only one of which can grow new brain cells. When a neuron fires, it sends information to other neurons in the circuit, which also fire if they receive enough stimulation. New neurons take fewer input signals from other cells to fire. Over time, each cell's firing pattern comes to reflect the inputs they receive, Gage said.

Researchers have proposed many theories to explain the role of young neurons in the dentate gyrus, but this one is the first to convincingly bring together neurophysiological properties and knowledge about the structure and anatomy of hippocampal tissue, Chiba said.

New neurons are hyper-excitable, firing at the slightest stimuli and forming connections with each other; more mature nerve cells, on the other hand, are more discriminating. This hair-trigger response allows newborn brain cells to link events and memories that happen around the same time, a phenomenon called pattern integration, Gage said. Mature cells, by contrast, help pick out differences between separate events.

After about a week or two, the young brain cells mature, join established neural circuitry and become less excitable. Yet clusters of neurons that "grew up" around the same time still retain the memories forged in their youth. So when you remember your high school prom, the neurons that were newly formed at that time fire, also bringing back memories of final exams from the week after. Memories of college and high school graduation, by contrast, will feel distinct, because the neurons that store each of those memories were born and matured at different times.

The next step, said Chiba, is to test the model's hypothesis in mice and rats. By labeling newly formed brain cells with a fluorescent marker molecule, researchers can track how neurogenesis affects the animals' ability to perform pattern integration tasks, Gage said. Diseases such as Huntington's and Alzheimer's also have a side effect of reducing the formation of new neurons, so understanding adult neurogenesis could help researchers devise new therapies, he added.


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