
Neurons can operate in reverse - joshrule
http://www.eurekalert.org/pub_releases/2011-02/nu-rtt021711.php
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tgflynn
What I find surprising about this type of news is why the brain would need so
much complexity.

It seems to me that a network with 10^11 neurons and 10^14 synapses should
have sufficient computational power to carry out the information processing
tasks that humans perform using only simple function neurons.

This belief is based on the following observations : \- I have personal
experience with ANN's with only thousands of nodes that are able to rival
humans at handwriting recognition. \- Current computers are far from being
powerful enough to simulate a 10^14 synapse ANN yet they seem to be rapidly
approaching human level performance on many cognitive tasks (ie. Watson).

If individual neurons are as complex as recent research results suggest I
wonder what all that computational power is being used for. Or is the human
brain just hopelessly inefficient as an information processing machine ? Maybe
it's such a recent development that evolution just hasn't had time to get
things right.

~~~
roc
Hazarding a guess... redundancy and adaptability?

Watson's not going to suffer damage to his neurons and still function, nor
lose a swath of them permanently, but eventually relearn how to talk.

Nor is it going to be able to ever independently 'learn' a new skill in
general.

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a-priori
I didn't think this was new... I remember hearing about this effect last year
and having it attributed to Oligodendrocytes, I believe.

That said, it's a very important development, because until the last few years
the glial cells have mostly been considered to be support cells (e.g.
supplying nutrients to the neurons, removing waste products and dead cells,
myelinating axons, etc.). But, now we know that they can affect the
surrounding neurons and may play a role in things like learning and memory.

~~~
ihodes
This article was from last year (don't know if it's the one you're thinking
of), but the paper just theorizes that glia may be involved in the described
mechanism.

I think we can be fairly certain that glial cells are involved in neuronal
communications, but I'd not say this paper at all proves that.

~~~
a-priori
Now that I think about it, I believe I was remembering an interview on the
Brain Science podcast with Douglas Fields[1], author of The Other Brain[2].

So sorry, but I don't have primary sources for what I said in my earlier
comment...

[1]:
[http://www.brainsciencepodcast.com/bsp/2010/5/12/exploring-g...](http://www.brainsciencepodcast.com/bsp/2010/5/12/exploring-
glial-cells-with-r-douglas-fields-bsp-69.html)

[2]: <http://theotherbrainbook.com/>

~~~
ihodes
Oh there have definitely been studies on it (a good review:
<http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2062484/>), and without a doubt
they communicate. It's just that this paper didn't really address that.

What was remarkable about this paper was that they demonstrated action
potentials (basically, the neuron's relative charge depolarizing) could start
not only in the soma, but in the axon.

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ihodes
Here's the Nature paper if anyone's interested (from Dec 2010)
<http://www.nature.com/neuro/journal/v14/n2/full/nn.2728.html>

We had known previously that the axons could send messenger proteins back to
the soma (cell body), thus modulating transmitter productions, and could have
an inhibitory or excitatory effect on the cell as a whole. We were also aware
of axo-axonic synapses, whereby axons could inhibit other axons (among some
other things).

EDIT: The above is just extremely brief background of well-known facts about
axon messaging.

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marshray
What else can they do "after all" ?

Is it simply a matter of time before we find a quantum computer in there?

~~~
ihodes
Roger Penrose (among many others) thinks so:
<http://en.wikipedia.org/wiki/Quantum_mind>

~~~
bioh42_2
We have known that enzymes make use of quantum tunneling for a while:
[http://en.wikipedia.org/wiki/Enzyme_catalysis#Quantum_tunnel...](http://en.wikipedia.org/wiki/Enzyme_catalysis#Quantum_tunneling)

And since our brain, like the rest of our bodies, is full of enzymes, I would
in no way be surprised if we find other quantum effects.

~~~
ihodes
Right. The question is not whether or not there eist quantum effects in the
brain, but whether they have any effect at all. Right now, there are only
theories: no empirical data exists to prove or disprove the theory.

~~~
marshray
Wouldn't it just be cool as heck if it turned out that this enabled "spooky
action at a distance" ESP-type stuff under conscious control by the brain?

Heck, I'd settle for a just a built-in magnetometer or something so I didn't
get lost.

(Sorry for the content-free post, I'm feeling slightly more whimsical than
usual this Friday afternoon.)

~~~
sesqu
People may have a crappy built-in magnetometer, but it seems you don't need
one - just a lot of practice. There are cultures that put such weight on the
cardinal directions that they always maintain a sense of where they are
facing.

en.wikipedia.org/wiki/Magnetoception#In_humans
[http://www.isegoria.net/2010/08/does-your-language-shape-
how...](http://www.isegoria.net/2010/08/does-your-language-shape-how-you-
think/)

~~~
Nick_C
Very interesting. I always had a very accurate sense of direction, to the
point of not needing a compass when bushwalking to a map. Yet, when I moved to
the other side of the continent, I was completely thrown.

I wondered to myself whether it was something to do with the very different
magnetic declination. No evidence, of course.

I moved back and have regained it again. _Mysterious._

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iwwr
I wonder if this will have applications to synthetic modeling of the neuron.

~~~
wladimir
Indeed, it seems that some kind of 'backpropagation' does happen in the brain,
in contrary to what was always believed. This might have impact on machine
learning research.

~~~
ollysb
It's always seemed intuitively surprising that there's no feedback mechanism
within neurons to aid learning. What's the currently favoured mechanism for
learning, neurons feeding back to previous neurons?

~~~
a-priori
There's no single mechanism in neuroscience to explain learning in general,
because the current understanding is that "learning" is a very vague term that
covers many types of adaptation, and each has its own mechanism.

I'm not familiar with any network-level mechanisms, but there are many local
(synapse- or dendrite-level) ones. The one I'm most familiar with is spike-
timing dependent plasticity (STDP) [1], which modifies the strength of a
synapse based on the millisecond-level timing of action potentials. When cell
A tends to fire just before cell B, and the two have synapses connecting them,
then cell B will increase the strength of its synapses to A. The reverse is
true too: if cell A tends to fire just after cell B, then the synapses will
decrease in strength. This is a form of Hebbian learning [2].

[1]: [http://en.wikipedia.org/wiki/Spike-timing-
dependent_plastici...](http://en.wikipedia.org/wiki/Spike-timing-
dependent_plasticity)

[2]: <http://en.wikipedia.org/wiki/Hebbian_theory>

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guscost
If the network has significant feedback, couldn't these slower "backward"
signals be understood in a similar fashion as a fast-moving propeller that
appears to reverse direction? I'm curious about how they measured this, but I
don't have thirty dollars to spend.

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DavidSTO
Info/background:

"...Maintenance of presynaptic inputs may depend on a post-synaptic factor
that is transported from the terminal back toward the soma."

-Neuron: Cell and Molecular Biology (1st edition c 1991)

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tjmaxal
Anyone else immediately jump to PTSD when they read this?

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drstrangevibes
so nature does backward propagation!

