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AdamMarblestone's profile
Adam Marblestone
Adam Marblestone
Adam Marblestone
@AdamMarblestone

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Adam Marblestone

@AdamMarblestone

Technologist, Scientist

adammarblestone.org
Joined February 2009

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    1. David Pfau‏ @pfau 25 Aug 2019
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      Replying to @AdamMarblestone @xaqlab and

      Also I think "the brain minimizes an objective function" is a vacuous statement. All dynamical systems can be reframed as solving a variational problem.

      3 replies 0 retweets 5 likes
    2. xaq‏ @xaqlab 25 Aug 2019
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      Replying to @pfau @AdamMarblestone and

      E.g. a flow field with curl is not the gradient of anything.

      3 replies 0 retweets 4 likes
    3. Adam Marblestone‏ @AdamMarblestone 25 Aug 2019
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      Replying to @xaqlab @pfau and

      Right. The claim is not just that brain’s dynamics satisfies some arbitrary Euler-Lagrange equations, but that it’s learning dynamics actually corresponds to ongoing optimization during the lifetime of the organism, i.e., brain is itself carrying out an optimization algorithm .

      1 reply 0 retweets 1 like
    4. David Pfau‏ @pfau 25 Aug 2019
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      Replying to @AdamMarblestone @xaqlab and

      Ok. What about belief propagation? The fixed points are minima of the Bethe free energy, but it doesn't look much like gradient descent.

      1 reply 0 retweets 0 likes
    5. Adam Marblestone‏ @AdamMarblestone 25 Aug 2019
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      Replying to @pfau @xaqlab and

      I think a good example. So we have BP (backprop) and BP (belief propagation). Vicarious seems to like the latter. And people know to search for this in large scale cortical experiments, e.g., many of the IARPA MICRONS proposals centered on this in 2015 or so.

      1 reply 0 retweets 0 likes
    6. David Pfau‏ @pfau 25 Aug 2019
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      Replying to @AdamMarblestone @xaqlab and

      Sure, I mean, I picked that as an example because I know it's Xaq's favorite theory of how cortex works (and Pearl himself was inspired by Rumelhart's speculation about neuroscience).

      2 replies 0 retweets 1 like
    7. David Pfau‏ @pfau 25 Aug 2019
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      Replying to @pfau @AdamMarblestone and

      But I think it also shows that there are so many possible ways that an optimization could be implemented, that it seems inconceivable that we could actually pin down conclusively how the brain is doing it (and there is surely more than one way).

      1 reply 0 retweets 2 likes
    8. Adam Marblestone‏ @AdamMarblestone 25 Aug 2019
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      Replying to @pfau @xaqlab and

      This may well be right. I certainly can’t say I’m confident it will be figured out with the level of data we have now, plus thinking. But I think BP and BP are two good broad classes of possibility that are not vacuous and tend toward very different predictions when fleshed out.

      1 reply 0 retweets 0 likes
    9. David Pfau‏ @pfau 25 Aug 2019
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      Replying to @AdamMarblestone @xaqlab and

      From what I've seen after a decade in the field, people who like deep learning and people who like Bayes can take the same data and claim it supports their favorite type of BP. I'd like to know what the different experimental predictions would be.

      1 reply 0 retweets 0 likes
    10. David Pfau‏ @pfau 25 Aug 2019
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      Replying to @pfau @AdamMarblestone and

      But more broadly - and I'd have to think more carefully about this to be sure it's true - I suspect that any fixed point iteration can be reframed as an optimization problem. Which, if true, would take "the brain does optimization" to "neural dynamics converge to a fixed point".

      1 reply 0 retweets 1 like
      Adam Marblestone‏ @AdamMarblestone 25 Aug 2019
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      Replying to @pfau @xaqlab and

      These last two are concrete and actionable. They may mean that you can only really test more specific and refined implementation-level models, as opposed to general classes. But maybe not.

      7:53 AM - 25 Aug 2019
      1 reply 0 retweets 0 likes
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        2. David Pfau‏ @pfau 25 Aug 2019
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          Replying to @AdamMarblestone @xaqlab and

          Well getting back to the tweet that set off this thread, I do hope in 100 years we have a much clearer view of things.

          1 reply 0 retweets 0 likes
        3. deen-chan‏ @sir_deenicus 25 Aug 2019
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          Replying to @pfau @AdamMarblestone and

          I don't see why it has to be backprop or belief prop. It could be something that takes on facets of both and more depending. I think this is where the evidence is already pointing. So while backprop is physically unlikely, it can work as long as subpopulations of neurons can

          2 replies 0 retweets 1 like
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