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


Perhaps neurogenesis doesn’t happen in adult humans?

A new study suggests that new neurons are not born very often in human adults.

Birth of New Neurons in the Human Hippocampus Ends in Childhood

“The lab’s new research, based on careful analysis of 59 samples of human hippocampus from UCSF and collaborators around the world, suggests new neurons may not be born in the adult human brain at all. The findings present a challenge to a large body of research which has proposed that boosting the birth of new neurons could help to treat brain diseases such as Alzheimer’s disease and depression. But the authors said it also opens the door to exciting new questions about how the human brain learns and adapts without a supply of new neurons, as in seen in mice and other animals.”

My labmates are all monkey neuroanatomists, and for years they have been skeptical about the neurogenesis narrative, particularly in primates. Another famous dissenter is Pasko Rakic. Read about his complaints in this Guardian article from 2012:

Does your brain produce new cells?


Why does dreaming sometimes produce Inception-style time distortions?

I answered the following question on Quora:

Last night I slept for 8.5 hours and had a dream that lasted for a month. It was full of incredible landscapes, animals, and interesting interactions with people. I woke to my alarm at 9:30, silenced it, then went back to my dream for a week before waking up at 10:30. What was happening in my brain?

Looks like no one has mentioned hippocampal replay yet!

I’m not a big fan of the movie Inception, but there is some tentative neuroscientific evidence that the time distortions experienced during dreaming have measurable neural correlates. (Experiential time distortions are of course purely subjective, so no one can tell you that you didn’t experience them. All experiences are real experiences.)

There are neurons in the hippocampus called place cells that tend to fire when an animal is in a particular location. (Incidentally, the discoverers of place cells and grid cells won the 2014 Nobel Prize in Physiology or Medicine.)

Let’s say a rat is navigating through a maze. When it reaches point A, a particular cell (or group of cells) fires. When it reaches point B, another cell fires. So there is a sequence of neuronal firing patterns that corresponds to the sequence of locations that the animal has experienced. In the picture above, each color represents the firing of one place cell. So each place cell covers a region of the maze/track.

So what does any of this have to do with dreaming? Well, when the animal is in REM (dreaming) sleep, or is quietly resting, the place cells that were recently active become reactivated. These reactivations are typically much faster than actual experience. They can also run backwards relative to prior waking experience, and can even be jumbled.

Of course, your experiences in dreams are more than a sequence of places. To extend the insights from rodent place cells into the study of actual human dreaming, we have to make a few speculative leaps. Perhaps in humans, there are ‘experience cells’ or ‘episode cells’ that encode broad categories of perception and cognition. Many neuroscientists refer to the set of cells that participate in such categorization as a cognitive map. Sleep seems to involve a free-form exploration of the cognitive map.

Hippocampal replay is widely seen as crucial for consolidating memories, and for learning. If you’ve been doing something during the day, when you sleep or rest, unconscious neural processes help you extract useful information, so that the next day your performance can improve.

Most of the data on hippocampal processing come from animals. It’s worth remembering that we can’t know if animals have the kinds of dreams that humans do. Nevertheless, the picture of sleep emerging from various lines of inquiry suggests that dreaming may be a subjective ‘side-effect’ of various sleep-related neural processes such as hippocampal replay. (Nowadays most neuroscientists would refrain from claiming that the dreaming experience per se is the purpose of dreaming.)

Many neuroscientists think that one of the purposes of sleep is to replay the past, and thereby discover new possibilities for the future. Perhaps speeding up the process is an efficient way to cycle through multiple ‘angles’ on the past, or on the wider space of possibilities. No one has any idea why this process should have subjective experiential correlates — and we are even more in the dark about why these experiences tend to be so vivid and bizarre.

Last night I slept for 8.5 hours and had a dream that lasted for a month. It was full of incredible landscapes, animals, and interesting …