Part 2: Defining intelligence
Most of the following ideas come from Jeff Hawkins' book, On Intelligence. Although in the book, Hawkins seems to be confusing intelligence and consciousness in a few places, he still presents a very believable theory of what he calls "true intelligence".
It's all about the neocortex, or cortex for short. Cortex is that wrinkly outside part of the brain. First of all, the most important physiological difference between humans and chimps seems to be the size of our cortex. Sure, we also have less hair, lack a tail and aren't as good at grabbing tree branches with our feet, but the size of our cortex is what seems to be the reason why humans are smarter than chimps. Second of all, humans do not have the biggest brain of all animals, but we do have the biggest cortex, by far.
So what is the cortex made of? It is built out of billions of fatty cells called neurons. Neurons can be active or passive - they switch on and off all of the time. When a neuron is active, it sends electro-chemical signals to its neighbouring neurons across neuron-neuron connections that are called synapses. As a result of signals being sent, the synapses may get stronger or weaker, allowing for stronger or weaker signals to be sent through them. This is an extremely simplified view of a neural network in the human brain. In fact, there are other types of connections that are present between some neurons, and there are different types of neurons that perform different functions, but we will keep things simple here.
All of the neurons are arranged into six layers. That's not just a random number thrown in to make the theory look fancy; we can actually see the 6 layers through a microscope. The bottom layer has connections to our senses. Our optical nerves carry signals from the retina to the bottom layer. That is also where we receive signals from the auditory nerve and the touch, taste and smell sensors. When the bottom layer neurons receive signals, they get activated, or excited. That causes some neurons on the next layer to get excited, too. Eventually, the signals propagate to higher and higher layers. And that is how we recognize images and objects. At least that was the state of our knowledge until recently.
Here is an interesting experiment conducted by Rodrigo Quiroga and his colleagues to test an idea proposed in 1967 by a neuroscientist Jerry Lettvin. It turns out that in your brain you have a Bill Clinton neuron - a single neuron that gets excited whenever you see the face of Bill Clinton, or hear the voice of Bill Clinton, or read the words "Bill Clinton". Well, actually there are several such neurons, duplicated just to be safe. That's no joke. Such neurons that are responsible for particular concepts are called "grandmother cells" or "gnostic" units. This suggests that when we see an image of Bill Clinton, somehow the cells in the retinas of our eyes send signals down the optical nerve, and those signals get received by the bottom layer of the cortical hierarchy; then, the signals propagate to higher layers until in the top layer the Bill Clinton neuron gets excited, and we recognize the face.
That is the simple view held by many neuroscientists. In fact, things seem to be a lot more interesting. Our eyes never take just one look at a face. Instead, they always rapidly jump from one facial feature to the other - left eye, right eye, nose, left eye, mouth, right eye, etc. What our brain is getting is a sequence of snapshots of small features of a person's face. In turn, the brain is telling the eyes what to look at next. This happens on every layer - information from the censors flows up the hierarchy, and the brain's commands, or signals, flow down to the muscles.
Now here is the cool part. The signals that flow down the hierarchy are predictions that our brain makes of the future. Here is what happens when we see a face. Our eyes catch a glimpse of a nose. The brain tells the eyes, "Look down a little bit, you should see a mouth." The eyes look below and see the mouth. The brain says, "Now look up and to the left, and you should see the right cheek". Our eyes jump to the right cheek. At this point, the brain starts recognizing the face of Bill Clinton and says, "Now look at the right eye; it should look like Bill Clinton's right eye." This process continues, as the brain gets more and more certain that we are indeed looking at the face of Bill Clinton. At every moment, each neuron is receiving sensory inputs from below and predictions from above and matching the two together.
It gets better. Ask yourself, "What is learning?" When do we learn new things? The answer is: precisely when our predictions do not match what we are percieving. Suppose that you are talking to somebody you have never seen before. Your eyes are trying to recognize the person's face, but where you have been expecting to see a chin, you notice a beard instead. To your facial recognition neurons, this is unexpected, so they get excited and start sending signals to the layer above. Eventually, these signals cause you to recognize that there is a relationship between the name of the person that you are talking to and the notion of a beard. From now on, whenever the neuron representing that person in your brain gets excited, the "beard neuron" will also get excited. In fact, if you are ever asked to describe that person, you might start by saying something like, "A tall guy with a beard." I'm speculating here and hiding many details, but the general idea makes sense.
There are fascinating questions about the way in which we build new memories. An area of the brain called the hippocampus seems to play a major role in it. After damaging their hippocampus, people lose the ability to remember new events (see Memento), although their older memories seem unaffected. It is likely that the only reason we require sleep is to transfer new memories acquired during the day to more permanent memory and free up space in the hippocampus for new information. I know for a fact that when I am deprived of sleep, my short-term memory suffers; I start forgetting appointments and losing my keys.
In short, the human neocortex is an extremely powerful tool that serves two purposes - (1) translating enormous amounts of information received by our senses into a language of abstract concepts and (2) translating commands given by the brain from the languages of abstractions to sequences of nerve signals that go off to the various muscles that control the way in which we affect the world around us. That is intelligence - the fact that we are able to reason about and interact with the world in terms of high-level abstract concepts. Jeff Hawkins claims that that is all we are, and that soon we will be able to build computers that simulate the functions of the neocortex and behave intelligently - like humans do.
The book left me with the feeling that I should sit down and start programming somthing right now! However, after thinking about it a little bit, I found that there are two huge holes in Hawkins' theory. Alright, I can build a six-layer network of a million neurons, have them get excited and send signals to each other. But wait; what does a single neuron do? There is a lot of evidence that all neurons are pretty much the same in terms of what they do, but what is it... that they do? They learn to recognize patters of signals. How? Does one neuron have a very simple function or a very complicated one? That is the first problem you run into when you ambitiously open a text editor to write some brain simulating code.
The second problem is the transition between perception and prediction. Alright, I can organize a hierarchy of neurons that will pass messages up and down. On the bottom, I will connect a bunch of sensors - vision, hearing, etc. Signals will propagate up to the top layer. But what do I connect at the top? Who is it that little demon that sits there, observes the Bill Clinton, grandmother and beard neurons light up and makes a decision which muscle to move? That's consciousness! I am calling it that for lack of a better term. When you are talking to somebody, a lot of thoughts rush through your mind, but somehow out of all that mess you build a sentense and speak it. We know how from the sounds that you hear, your brain builds a complicated concept of what the speaker is saying - that's the cortex. We know how your ideas get transformed into hundreds of complicated nerve signals that move your lips and tongue to produce speech - that's the cortex, too. But what happens between the time you understand what the other person has said and the time you have decided on an appropriate reply? Who makes that decision? Is there a decision being made? This is precisely the "hard problem" of consciousness again. In a sense, the cortex is no different from the hand - it is an organ that humans have evolved to ease the task of interacting with the world. The cortex is the organ of intelligence.
Most scientists believe that these two problems are actually the same problem. The place where the little demon sits is inside the neurons. But that is where the question of intelligence ends and the question of consciousness begins, and if you thought the cortex was weird, from now on things get really hairy.