Once development was ended, the founts of growth and regeneration ... dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated.Then, in the 1980s, evidence began to emerge that contradicted this theory. It started with mice, which everyone now agrees continue to create new neurons in adulthood. Then there were hints of new brain cells in adult humans, first in the hypothalamus and then in the rest of the brain.
Starting around 2000, a whole wave studies found more evidence for adult neurogenesis, and began to tie it to all sorts of other ideas. One study suggested that stress blocked neurogenesis, and that this caused depression. Others found that living an enriched life stimulated neurogenesis. And then the cascade: sex stimulates neurogenesis, running stimulates neurogenesis, listening to music stimulates neurogenesis.
These studies were not accepted by all neurologists; some found the new work unconvincing. Thus a scientific fight broke out, and the authors of the new, pro-neurogenesis studies were not shy about mocking their opponents:
A milestone is marked in our understanding of the brain with the recent acceptance, contrary to early dogma, that the adult nervous system can generate new neurons. One could wonder how this dogma originally came about, particularly because all organisms have some cells that continue to divide, adding to the size of the organism and repairing damage.So the idea that adult human brains produce no new cells was a "dogma," and we all know there's nothing worse than that. It is the job of all right-thinking people to overthrow dogma wherever we find it, and replace it with proper science that promotes mental growth in maturity, running, sex, and all other Good Things.
This week a new salvo came out from the no adult neurogenesis side, and it seems to be a very powerful attack. Ed Yong has a good summary:
In a new study, and one of the biggest yet, a team led by Arturo Alvarez-Buylla at the University of California at San Francisco completely failed to find any trace of young neurons in dozens of hippocampus samples, collected from adult humans. “If neurogenesis continues in adult humans, it’s extremely rare,” says Alvarez-Buylla. “It’s not as robust as what people have said, where you could go running and pump up the number of neurons.”Two of Alvarez-Buylla's colleagues did the key work:
Shawn Sorrells and Mercedes Paredes analyzed the brains of 17 adult humans who had died and donated their bodies to research. The duo searched for telltale molecules that are specifically made in young neurons, or in the stem cells that produce such neurons. To their surprise, they found nothing. “Even in our best-preserved samples, we didn’t see any evidence of neurogenesis,” Paredes says.They did find plenty of new neurons in the brains of children, so their method was perfectly capable of picking them up.
Obviously this is not the last word; the argument will go on. But I think the latest negative findings make this a good cautionary tale about science in our time, and about the popular reception of science. We love theories that attack old dogma; we also love theories that promote adult learning and self-empowerment ("Eleven proven ways to generate more brain cells!"). The new ideas about neurogenesis fit into our hopes like a key in a well-oiled lock. I, for one, accepted them without ever looking into the matter in detail, or checking out the writings of opponents.
We should be more careful. We should be more respectful of science that has been accepted for decades, and less quick to celebrate novelty. We should be suspicious of findings that flatter us and of results that fulfill our hopes. Especially when the connection between the supposed science and the things value was so weak to begin with; why did we ever associate the number of brain cells with all good things, when we know perfectly well that is the connections between cells that matter?