
How to do backpropagation in a brain [video] - modeless
https://www.youtube.com/watch?v=kxp7eWZa-2M&t=38m13s
======
white-flame
I'm not quite clear on this: Is this a theory of how backpropagation might be
constructed using known neural mechanisms as building blocks, or is this
actual observation of how the brain really trains neurons?

~~~
modeless
Very much a theory, which is inspired by recent successes in training
artificial neural networks. It is consistent with our observations so far but
certainly not proven by any means.

------
jcr
The title on youtube is simply "Deep Learning - Geoffrey Hinton" and the
original url (not the shortened, or time-marked) is:

[https://www.youtube.com/watch?v=kxp7eWZa-2M](https://www.youtube.com/watch?v=kxp7eWZa-2M)

From the above link, it's a recent talk:

> _Published on Jul 29, 2015_

> _Filmed at the University of Cambridge on June 25th, 2015_

~~~
modeless
I linked specifically to this part of the video because it's really a
standalone talk, as he says. "How does the brain do backpropagation?" is the
title of this talk (as he says it verbally; the slides have a slightly
different paraphrasing, "How to do backpropagation in a brain").

The earlier part of the video is on a different topic; mostly explaining the
various successes of deep learning over the past several years. Geoff doesn't
start talking until about 6 minutes in, so if you want to see that part of the
video start here:
[https://youtu.be/kxp7eWZa-2M?t=5m55s](https://youtu.be/kxp7eWZa-2M?t=5m55s)

~~~
jcr
The problem is most url shorteners like "youtu.be" are banned on HN due to
abuse, so your submission was initially "[dead]" when it was first created.
The only reason for putting the right URL and original title in my comment was
to help the mods (who have since changed both the url and title).

~~~
modeless
Ah, thank you for that. I didn't realize site-specific shorteners like
youtu.be were banned.

------
plg
1\. we don't know how the brain does complicated thing X

2\. we have discovered a way to program a computer to do complicated thing X

3\. maybe the brain works like this too

This is faulty reasoning and I see it again and again. This is not how to
proceed according to the scientific method

~~~
seanmcdirmid
No just no.

They've come up with a hypothesis by making observations and guessing. If the
hypothesis is also falsifiable, then it is definitely how the scientific
method works, since we can now run experiments to determine if the hypothesis
is false. The scientific method doesn't say much about how hypotheses are
formed, just in how they are evaluated.

Now we can argue about whether this can be falsified or not, and that is
legitimate. Bad science would be not taking that step or doing that step
badly.

~~~
plg
yes exactly. My criticism is only if point 2 is used as evidence of point 3. I
see this a lot.

If point 2 is used only as a means to generate a hypothesis, which is then
tested empirically, then absolutely this is fine

thanks for the clarification

------
stefantalpalaru
Keep in mind that artificial neural network are not meant to simulate
biological neural networks. They are just inspired by a simplified (and
obviously flawed) computational model of the brain from way back in the stone
age of neuroscience.

Finding parallels between what ANNs evolved into today and the little we know
about the brain may be interesting, but don't think, even for a moment, that
you have a model for "backpropagation in the brain".

P.S.: I like the implied criticism of the Human Brain Project at 50:10 - "You
need a computational theory (not a billion euros) to recognize what you
discovered!". We really need to work on theoretical models of computation
before we throw money and hardware at simulations. No emergent behavior is
going to jump out and save us that step.

