
Adult Neurogenesis – A Pointed Review - mantesso
http://slatestarcodex.com/2018/04/04/adult-neurogenesis-a-pointed-review/
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subroutine
Last week a paper came out saying no adult neurogenesis. But I see today this
paper came out:

[http://www.cell.com/cell-stem-
cell/fulltext/S1934-5909(18)30...](http://www.cell.com/cell-stem-
cell/fulltext/S1934-5909\(18\)30121-8)

It usually flips every month.

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ckemere
This is a critical comment that should be upvoted widely. The reason that this
is such a difficult challenge is that the rate of adult hippocampal
neurogenesis, even in rodents where everyone believes it happens, is on the
order of between 1 neuron per minute and 1 neuron per hour. In a brain of 100
billion neurons, that's hard to detect.

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jessriedel
The original SlateStarCodex post addresses this. He argues that a microscopic
amount of neurogenesis that is too small to be responsible for the effects
everyone cares about (depression, brain plasticity, etc.) should be rounded to
zero.

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SubiculumCode
It is entirely possible that the base-rate of neurogenesis in adult humans is
very low, but in response to new stimuli (a new exercise regimen, for example)
responds by generating neurons..Perhaps I am missing the information in one of
the supplemental pages, but I do not see info on history of exercise or anti-
depressant use in the Nature paper.

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savanaly
Clearly humans care a lot about neurogenesis not just because understanding
whether it happens in humans would be a stepping stone to building a robust
model of how cognition happens, but also because intuitively it has to do with
whether we can enhance our own mental abilities in our lifetime-- one of the
most fervent desires of many, I'm sure.

We've already had to write off things like growth mindset as a way to increase
our fluid intelligence, it seems. It is dismaying that, based on this study,
we'll have to write off things like exercise as well which we now realize only
work in rats. What's left that could plausibly help? Perhaps just: building
better work and study habits and simply spending more time on mental tasks. At
least we're pretty sure those aren't illusory.

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oregontechninja
I recommend practicing memory palace techniques if you really want a brain
work out. Storing sets of information, retrieving them, and replacing them is
intensely exhausting. You know you're getting a brain work out. I try and
memorize one deck of cards every day at a minimum. After a bit of practice,
you'll have laser focus. Not so sure about whether this promotes neurogenesis
though. Does anybody have any resources on this?

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krrrh
I don’t, but please tell us more about the spillover effects based on your
subjective experience. This sounds like something I would try but I’d like to
hear more.

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cobbzilla
Fun article. Best line: "if we want to believe something, it will accrete a
protective layer of positive studies whether it’s true or not."

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kragen
I've wondered for a long time if stem cell therapy could, for example, cure my
amblyopia — though it hardly seems worth the cancer risk. This very surprising
news is a data point in favor of that kind of thing.

It's also a kind of data point that we really need to improve science a lot in
order to be able to advance as a civilization.

(Amblyopia is due to anomalous neural development caused by, more or less,
early childhood early sensory deprivation from one eye. The result is a
lifelong lack of visual competence in that eye — it may focus perfectly, as
mine does, but recognition is poor, apparently because perception of Gabors at
certain spatial frequencies is very poor. I say "more or less" because the eye
is still providing sensory input, but because it's pointing in the wrong
direction, that input isn't useful.)

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berdon
This is more speculative than anything: have you tried eye-patching the good
eye to force your brain into being trained to use the bad one? Specifically
for a long period of time (months).

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kragen
Yes, I did that 12 hours a day for five years in my early childhood, starting
at 2 years old. Before that, I couldn't see out of my left eye at all.

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berdon
I wonder if you would have better results if you did it 100% of the time. I
wonder if training was hindered simply because other neural pathways would
still "win" after the 12 hours each day. But all of this speculation is
centered around neural growth and formation - I have no idea what could have
happened because of lack of use during development (ie. atrophy, etc).

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cdcox
While this comes to the right conclusion I think it mistraces a lot of the
issues. For example: a fair amount of the depression and neurogenesis studies
(and indeed neurogenesis manipulation studies) suffered from the same issue:
it's really hard to make a population of cells die without generally
poisioning the remaining cells. Most of the initial studies relied on things
like radiation, ie radiate a rat and it starts acting depressed. There is now
increasing evidence that radiation kills off a bunch of synapses and causes
some wicked disregualtion across the system so that turned out to be wrong.
Conversely BDNF enhances neurogenesis but it also enhances synaptic learning
so even positive manipulations had this issue but in reverse. Newer timed
genetic manipulation studies were more precise but the depression-neurogenesis
link has been slowly losing steam in light of these studies.

But the BIG issue boils down to this: there was never strong evidence for the
importance of neurogenesis. The dentate gyrus is a tiny region of the brain
with neurons that act in unusual ways. In humans it's an even tinier region.
While it is probably required, at least as a pass through, to learn new
memories, it's surprising how much weight people were placing on neurogenesis
in this tiny tiny structure. Given, networks can make small zones have big
effects, but the weight of evidence should have always been on people pushing
the area to prove the dentate was this massively important structure. This has
not been conclusively done (studies have linked it to some specific subtypes
of learning but those have limited it as much as they have found an important
role for it). This is a structural issue in science broadly, there is an
incentive to push forward with the next big finding but no incentive to go
back and confirm the gulf of assumptions that a literature is resting on.

The good news is, human brains could always change, synaptic plasticity is
present throughout life and has been shown by many good studies to occur
throughout adulthood. We don't need new cells in a tiny part of the brain to
learn new things, networks rewire and while that rewiring isn't something as
dramatic as new cells, small effects can lead to big outcomes in dynamic
systems. People obviously make new habits, learn new skills, and make new
memories throughout life. The onus should have always been on science to show
why it occurs, not to lend credibility that it occurs.

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garyrob
A recent study[0] seems to provide meaningful evidence that while adult
neurogenesis occurs in rats, rabbits and some other species, as mentioned in
the research referred to by the OP, it does not actually occur in humans. I'm
not claiming to know the truth on this, but I thought this research should be
part of the discussion.

[0] [https://www.theatlantic.com/science/archive/2018/03/do-
adult...](https://www.theatlantic.com/science/archive/2018/03/do-adult-brains-
make-new-neurons-a-contentious-new-study-says-no/555026/)

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garyrob
Oh wow, thank you so much for filling me in. I didn't read the whole thing
because it was quoting a lot of animal studies, which I knew were questionable
in light of the adult human research, and I was aware of no reason to expect
the turnaround in part 2! :) You were very kind about it. Sorry for wasting
everyone's time!

