No New Neurons? No Problem!

This answer was written in response to the following Quora question:

New research has found no neurogenesis in human adults, could this mean there is none or could it mean that neural stem cells are undetectable with the used techniques? What are your thoughts on this?

It’s good that you’re thinking of such things, since that is exactly what researchers themselves have to do, and what reviewers do. In order to show that the method works, there have to be adequate controls as part of the experiment.

And this is in fact the case. The paper would not have been published without controls.

In the study, the researchers successfully identified a class of newborn neurons in very young postmortem human brains, so the absence of them in older brains has nothing to do with the methodology.

Here’s a diagram from the paper:

If you have your heart set on adult neurogenesis for some mysterious reason, then you can argue that the ‘real’ new neurons (that other researchers say exist in human adults) are actually a different class of neuron entirely. I don’t know how far that reasoning will take you though.

If one is being intellectually honest, one must also now consider why the other studies might be wrong about their findings of neurogenesis in humans. This is exactly what the authors of this new paper do at the end of their paper. They suggest that the presence of growth factors in adult human brains may have nothing to do with cell division — they may instead be a byproduct of other phenomena. Two that come to mind are dendritic arborization and synapse formation.

No New Neurons? No Worries!

I am quite baffled by the disappointment people seem to feel in response to this study. If new neurons occur in mice and young babies, and not in adult humans, that has no bearing on the quality of memory and learning in human adults. A 70 year old can learn many things that mice and infants can never learn, regardless of their rate of birth of new neurons. So headlines like “Sorry, Adults, No New Neurons For Your Aging Brains” are extremely silly and misleading. In fact they may even be harmful, because they could encourage discrimination based on ageism.

If the current study is corroborated by other groups, and ‘triangulated’ using other methods, then neuroscientists may eventually come to agree that adult neurogenesis is not a feature of the human species. Consensus is not going to happen soon though, for a variety of reasons that I can’t get into here. Science is complex and messy!

But even if the “no adult neurogenesis in humans” idea is corroborated, it should not bother the average non-scientist interested in learning, memory or self-improvement, since everyone already knows that older humans are perfectly capable of acquiring new knowledge and skills. We have known this for thousands of years. Zero neuroscience is needed to establish the memory capacity of a human adult.

The proof of the pudding is in the eating, not in the molecular make-up of the pudding. 🙂

If you are not convinced of the extraordinary memory capacity of even older adults, see this essay I wrote:

3quarksdaily: Why human memory is not a bit like a computer’s

Image source: Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults