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Neural Categories

24 Post author: Eliezer_Yudkowsky 10 February 2008 12:33AM

Followup toDisguised Queries

In Disguised Queries, I talked about a classification task of "bleggs" and "rubes".  The typical blegg is blue, egg-shaped, furred, flexible, opaque, glows in the dark, and contains vanadium.  The typical rube is red, cube-shaped, smooth, hard, translucent, unglowing, and contains palladium.  For the sake of simplicity, let us forget the characteristics of flexibility/hardness and opaqueness/translucency.  This leaves five dimensions in thingspace:  Color, shape, texture, luminance, and interior.

Suppose I want to create an Artificial Neural Network (ANN) to predict unobserved blegg characteristics from observed blegg characteristics.  And suppose I'm fairly naive about ANNs:  I've read excited popular science books about how neural networks are distributed, emergent, and parallel just like the human brain!! but I can't derive the differential equations for gradient descent in a non-recurrent multilayer network with sigmoid units (which is actually a lot easier than it sounds).

Then I might design a neural network that looks something like this:

Blegg1_3

Network 1 is for classifying bleggs and rubes.  But since "blegg" is an unfamiliar and synthetic concept, I've also included a similar Network 1b for distinguishing humans from Space Monsters, with input from Aristotle ("All men are mortal") and Plato's Academy ("A featherless biped with broad nails").

A neural network needs a learning rule.  The obvious idea is that when two nodes are often active at the same time, we should strengthen the connection between them—this is one of the first rules ever proposed for training a neural network, known as Hebb's Rule.

Thus, if you often saw things that were both blue and furred—thus simultaneously activating the "color" node in the + state and the "texture" node in the + state—the connection would strengthen between color and texture, so that + colors activated + textures, and vice versa.  If you saw things that were blue and egg-shaped and vanadium-containing, that would strengthen positive mutual connections between color and shape and interior.

Let's say you've already seen plenty of bleggs and rubes come off the conveyor belt.  But now you see something that's furred, egg-shaped, and—gasp!—reddish purple (which we'll model as a "color" activation level of -2/3).  You haven't yet tested the luminance, or the interior.  What to predict, what to predict?

What happens then is that the activation levels in Network 1 bounce around a bit.  Positive activation flows luminance from shape, negative activation flows to interior from color, negative activation flows from interior to luminance...  Of course all these messages are passed in parallel!! and asynchronously!! just like the human brain...

Finally Network 1 settles into a stable state, which has high positive activation for "luminance" and "interior".  The network may be said to "expect" (though it has not yet seen) that the object will glow in the dark, and that it contains vanadium.

And lo, Network 1 exhibits this behavior even though there's no explicit node that says whether the object is a blegg or not.  The judgment is implicit in the whole network!!  Bleggness is an attractor!! which arises as the result of emergent behavior!! from the distributed!! learning rule.

Now in real life, this kind of network design—however faddish it may sound—runs into all sorts of problems.  Recurrent networks don't always settle right away:  They can oscillate, or exhibit chaotic behavior, or just take a very long time to settle down.  This is a Bad Thing when you see something big and yellow and striped, and you have to wait five minutes for your distributed neural network to settle into the "tiger" attractor.  Asynchronous and parallel it may be, but it's not real-time.

And there are other problems, like double-counting the evidence when messages bounce back and forth:  If you suspect that an object glows in the dark, your suspicion will activate belief that the object contains vanadium, which in turn will activate belief that the object glows in the dark.

Plus if you try to scale up the Network 1 design, it requires O(N2) connections, where N is the total number of observables.

So what might be a more realistic neural network design?

Blegg2
In this network, a wave of activation converges on the central node from any clamped (observed) nodes, and then surges back out again to any unclamped (unobserved) nodes.  Which means we can compute the answer in one step, rather than waiting for the network to settle—an important requirement in biology when the neurons only run at 20Hz.  And the network architecture scales as O(N), rather than O(N2).

Admittedly, there are some things you can notice more easily with the first network architecture than the second.  Network 1 has a direct connection between every two nodes.  So if red objects never glow in the dark, but red furred objects usually have the other blegg characteristics like egg-shape and vanadium, Network 1 can easily represent this: it just takes a very strong direct negative connection from color to luminance, but more powerful positive connections from texture to all other nodes except luminance.

Nor is this a "special exception" to the general rule that bleggs glow—remember, in Network 1, there is no unit that represents blegg-ness; blegg-ness emerges as an attractor in the distributed network.

So yes, those N2 connections were buying us something.  But not very much.  Network 1 is not more useful on most real-world problems, where you rarely find an animal stuck halfway between being a cat and a dog.

(There are also facts that you can't easily represent in Network 1 or Network 2.  Let's say sea-blue color and spheroid shape, when found together, always indicate the presence of palladium; but when found individually, without the other, they are each very strong evidence for vanadium.  This is hard to represent, in either architecture, without extra nodes.  Both Network 1 and Network 2 embody implicit assumptions about what kind of environmental structure is likely to exist; the ability to read this off is what separates the adults from the babes, in machine learning.)

Make no mistake:  Neither Network 1, nor Network 2, are biologically realistic.  But it still seems like a fair guess that however the brain really works, it is in some sense closer to Network 2 than Network 1.  Fast, cheap, scalable, works well to distinguish dogs and cats: natural selection goes for that sort of thing like water running down a fitness landscape.

It seems like an ordinary enough task to classify objects as either bleggs or rubes, tossing them into the appropriate bin.  But would you notice if sea-blue objects never glowed in the dark?

Maybe, if someone presented you with twenty objects that were alike only in being sea-blue, and then switched off the light, and none of the objects glowed.  If you got hit over the head with it, in other words.  Perhaps by presenting you with all these sea-blue objects in a group, your brain forms a new subcategory, and can detect the "doesn't glow" characteristic within that subcategory.  But you probably wouldn't notice if the sea-blue objects were scattered among a hundred other bleggs and rubes.  It wouldn't be easy or intuitive to notice, the way that distinguishing cats and dogs is easy and intuitive.

Or:  "Socrates is human, all humans are mortal, therefore Socrates is mortal."  How did Aristotle know that Socrates was human?  Well, Socrates had no feathers, and broad nails, and walked upright, and spoke Greek, and, well, was generally shaped like a human and acted like one.  So the brain decides, once and for all, that Socrates is human; and from there, infers that Socrates is mortal like all other humans thus yet observed.  It doesn't seem easy or intuitive to ask how much wearing clothes, as opposed to using language, is associated with mortality.  Just, "things that wear clothes and use language are human" and "humans are mortal".

Are there biases associated with trying to classify things into categories once and for all?  Of course there are.  See e.g. Cultish Countercultishness.

To be continued...

 

Part of the sequence A Human's Guide to Words

Next post: "How An Algorithm Feels From Inside"

Previous post: "Disguised Queries"

Comments (11)

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Comment author: Pedant 10 February 2008 07:37:40AM 0 points [-]

"blegg-ness emerges as an attractor in the distributed network." Is this a useful application of the concept of emergence?

Comment author: MinibearRex 02 April 2011 12:13:59AM 7 points [-]

Emergence isn't, on its own, bad. It's just generally unhelpful. That doesn't mean it's wrong; my consciousness is in fact emergent from neurons firing. It just doesn't tell me anything extra. If instead, you said something like, "I've got a detailed model of the brain, that simulates the neurons, and through (insert technical explanation here) my model shows consciousness emerging as a property made up of smaller elements", nobody would complain. Emergence is only a bad explanation if it's the only explanation given.

Comment author: [deleted] 08 October 2013 01:56:16PM *  0 points [-]

Completely agreed. I think the word emergence is only bad because it's used badly.

"A emerges from B" could reformulated as "There is a causal relationship between B and A" and as such it's also obvious that the description of the causal relationship is missing.

I think the misuse is also about (But notice the difference) people struggling with attempts of understanding a hierarchy of information in the sense that you have two overlapping models... One, say M1, reduced - in the sense of reductionism - to smaller components and the other, say M2, which contains a more general abstract system. So the struggle which I'm referring to is the inability to effectively grasp the relationship between the two models M1 and M2, and then insteading of admitting you either can't explain it or you don't understand it, you just say "M2 is emergent from M1. " Which can be correct, in the sense that "if M1 is true then it follows that M2 is true" - "Though I can't quite explain why"

I think Yudkowsky's pointing out the futility of emergence as a substite for magic is spot on in many cases.

Comment author: blacktrance 13 December 2013 06:16:44PM 2 points [-]

Emergence isn't helpful by itself, but it is helpful in context. For example, if you say that consciousness is emergent from neurons firing, it doesn't describe how it does it, but it does exclude other positions - "Consciousness doesn't exist", "Consciousness is a basic feature of the world and doesn't emerge from anything, it's just there", and "Consciousness emerges from something other than neurons".

Comment author: tcpkac 10 February 2008 08:53:02AM 1 point [-]

The primary categorisation is "Threat / Not a threat", and the main categorisation bias is "Better safe than sorry". You'll find that many of your specific categorisation biases are particular examples of that. Examples are : nervousness about your Great Thing being a cult, Asch experiment situations where you have to join the group or stick out from it. Diagram 1b has 'Threat' written all over it.....

Comment author: Andy_McKenzie 11 February 2008 07:22:07PM 1 point [-]

I like this post. By the way, another argument that people always get into over the definition of a word is sports. Is nascar a sport? Is figure skating?

Also, in learning theory, how to define a reflex is of big debate. Is jealousy a reflex? Can you think of any reason to care? Seriously, I'm wondering.

Comment author: Leafy 12 October 2010 12:01:10PM 1 point [-]

Two questions that occur following reading this:

1) Using the Blegg/Rube example would it be reasonable to suggest that the reaction to a purple egg would be different had it occured 20 years of working on the machine with no anomalies, than if it was the first off of the conveyor belt ... or the fith etc? What would be a threshold between casual acceptance and dumbfounded confusion?

2) The concept of neural pathways strengthening with usage and heightening connections through multiple observations leads to the question: At birth are our neural pathways all of equal "strength" and if not have we established yet what pre-existing configurations we are born with? (Is there an ultimate human "priori" with which we all start off or are there genetic differences, and if so are there any general constants?) Apart from an unlucky few am I right in saying that at birth there is a ready-made connection between sharp teeth/claws/aggresive noises and fear regardless of the occurence of previous observations or not?

Comment author: Psy-Kosh 26 February 2011 05:11:15PM *  3 points [-]

I've been thinking lots about thingspace cluster stuff the past few days, and I remembered a thought I had a while back:

In actual reality, network 2 is not just easier to deal with, more efficient, etc, but would actually be more likely to reflect the actual underlying reality.

I mean, consider the pattern of correlations, the thingspace clusters represented in network 1. Generally such correlations don't just pop up out of nowhere, right? Correlation is not the same thing as causation, but it does at least suggest that some sort of causation link is present (most likely common cause in a case like this, right?)

In this sort of case, I'd think "There's probably some sort of actual physical process that produces bleggs, and some other process that produces rubes. That is, some process that produces things that have a high probability of being blue, egg shaped, etc etc etc... and similar for red cube shaped etc etc... Possibly distinct geological processes?"

So in that sense, the naive bayes network, with the implicit "but is it ACTUALLY a blegg?" style question would seem to reflect something legitimate, not just a name for the cluster. Specifically, such a question would translate to something like "which of, say, two distinct physical processes produced this?"

Comment author: bacon 20 January 2012 03:27:15PM *  0 points [-]

"it requires O(N²) connections" <- better use Ω(N²), i.e. big Omega instead of big O

Comment author: chaosmosis 03 May 2012 03:55:19AM 0 points [-]

This sequence reminds me of Deleuze and Guatarri so much that it's totally crazy.

The metaphors and explanations are like cousins of each other.

Comment author: odlogan 10 September 2012 11:51:37AM 1 point [-]

Is there somewhere where I could learn how to interpret these models of neural networks? I think this article may have been written when it was safer to assume that readers knew these things.