It’s hard for us to accept the idea that the brain stops growing, despite the large body of scientific evidence supporting this idea. The often-repeated statistic, based on years of research, is that the brain stops developing around the age of 25. More recently, an international team of neuroscientists argued in Nature that the human brain stops producing new neurons at age 13. The response from the scientific community to this most recent study has been significant, to say the least.

In their paper, published Wednesday, the researchers write that their findings “do not support the notion that robust adult neurogenesis continues in the human hippocampus.” In other words, none of the hippocampus tissue samples from adult brains they examined showed evidence of new neurons. Infants’ brains grow lots of new neurons, they report, and older children’s brains slow down a little. Meanwhile, none of their adult samples showed evidence of new neurons. And this is what other scientists don’t agree with.

“They may just not have looked carefully enough,” Jonas Frisén, Ph.D., of the Karolinska Institute in Sweden, told STAT News on Wednesday. Frisén co-authored a paper in 2015 that contradicts the findings of the Nature paper. And Frisén isn’t the only one who thinks these researchers’ conclusion may be premature.

This image shows new neurons (green) in the brains of an infant (left), a 13-year-old (middle), and a 35-year-old (right).

“There is a long history of concluding that adult neurogenesis doesn’t exist in a given species based on difficulty in identifying new neurons,” Heather Cameron, Ph.D., a principal investigator of neuroplasticity at the National Institutes of Mental Health, told The Atlantic on Thursday. “This happened in rats and then in nonhuman primates, both of which are now universally acknowledged as showing adult hippocampal neurogenesis.”

One of the major difficulties in measuring neurogenesis in the brain is that you can’t observe it in real time. The researchers, therefore, had to settle for the next best thing: brains from recently deceased patients. Unfortunately, even when directly examining brains, the best you can do is look for molecular markers that could indicate new neurons.

“You can’t just shine light onto a skull and see it,” said Salk Institute neurobiologist Fred Gage, Ph.D., who studies adult neuroplasticity, in an interview with STAT News on Wednesday. Therefore, this evidence comes down to how it’s interpreted. That’s where the disagreement lies.

Many in the scientific community are contesting the finding that the brain stops making new neurons after age 13.

Despite the uproar, the authors of the paper stood by their findings. “If neurogenesis continues in adult humans, it’s extremely rare,” Arturo Alvarez-Buylla, Ph.D., a neuroscientist at University of California, San Francisco and one of the authors on the study, told The Atlantic. “It’s not as robust as what people have said, where you could go running and pump up the number of neurons.”

In recent studies, neuroscientists have demonstrated that human adult brains can indeed produce new neurons, specifically in the hippocampus, a region associated with working memory, so Alvarez-Buylla and his team will likely need much more evidence to convince the community that their findings are correct. Plenty of research has shown that neuron development drops off as people get older, but moving the finish line back to age 13 is huge. There might not be any closure to this argument yet, but future experiments will hopefully clarify exactly at what age human brains stop producing neurons — if they ever do.