May 6, 2010 29 Comments
I worry too much. I live too far into the future; always so acutely aware of the potential distant knock-on effects of my actions that I’m sometimes quite paralyzed. On the downside this can be a real handicap, but on the upside it means I’m intelligent, because seeing into the future is what intelligence is for. But how? And how do we differentiate between past, present and future? What do we really mean by “now”?
My main thesis for this project is that the brain is a prediction machine. In other words I think it takes so long for nerve signals to reach the brain and be analyzed by it (you may be surprised to know it takes about a tenth of a second merely for signals to reach the primary visual cortex from the retina, never mind be turned into understanding), that we’d be dead by now if it weren’t for our ability to create a simulation of the world and run it ahead of time, so that we are ready for what’s about to happen instead of always reacting to what has already happened. I’m suggesting that this simulation ability derives, at least in part, from a capacity to make small predictions based on experience, at ALL levels of the nervous system. These little fragments of “if this happens then I suspect that will happen next” are there to counter processing delays and reaction times, and give us the ability to anticipate. But they also (I suggest) provide the building blocks for other, more interesting things: a) our ability to create a contextual understanding of the world – a stable sense of what is happening; b) our ability to form plans, by assembling sequences of little predictions that will get us from a starting state to a goal state; and c) our capacity for imagination, by allowing us to link up sequences of cause and effect in an open-ended way. The capacity for imagination, in turn, is what allows us to be creative and provides the virtual world in which consciousness arises and free thought (unconstrained by external events) can occur.
I rather think some clever tricks are involved, most especially the ability to form analog models of reality, as opposed to simple chains of IF/THEN statements, and the ability to generalize from one set of experiences to similar ones that have never been experienced (even to the extent that we can use analogies and metaphors to help us reason about things we don’t directly understand). But I’d say that the root of the mechanism lies in simple statistical associations between what is happening now and what usually happens next.
So let’s look at a wiring diagram for a very simple predictive machine.
This is the simple touch-sensitive creature I talked about in Brainstorm 6. The blue neurons receive inputs, from touch-sensitive nerve endings, which occurred some milliseconds ago on its skin. The red neuron shows two touch inputs being compared (in this case the cell has become tuned to fire if the right input is present just before the left input). I think we can call the red neuron an abstraction: it takes two concrete “I am being touched” inputs and creates an abstract fact – “I am being stroked leftwards here”. This abstraction then becomes an input for higher-level abstractions and so on.
The green neuron is then a prediction cell. It is saying, “if I’m being stroked leftwards at this point, then I expect to be touched here next.” Other predictions may be more conditional, requiring two or more abstractions, but in this case one abstraction is enough. The strength of the cell’s response is a measure of how likely it is that this will happen. The more often the prediction cell is firing at the moment the leftmost touch sensor is triggered, the stronger the connection will become, and the more often that this fails to happen, the weaker it will become (neurologically I’d hypothesize that this occurs due to LTP and LTD (long-term potentiation and long-term depression) in glutamate receptors, giving it an interesting nonlinear relationship to time).
So what do we DO with this prediction? I’m guessing that one consequence is surprise. If the touch sensor fires when the prediction wasn’t present, or the prediction occurs and nothing touches that sensor, then the creature needs a little jolt of surprise (purple neuron). Surprise should draw the creature’s attention to that spot, and alert it that something unexpected is happening. It may not be terribly surprising that a particular touch sensor fails to fire, but the cumulative effect of many unfulfilled predictions tells the creature that something needs to be worried about, at some level. On the other hand, if everything’s going according to expectations then no action need be taken and the creature can even remain oblivious.
But for the rest of my hypothesis to make sense, the prediction also needs to chain with other predictions. We need this to be possible so that top-down influences (not shown on the diagram) can assemble plans and daydreams, and see far into the future. But I believe there has to be an evolutionary imperative that predates this advanced capacity, and I’d guess that this is the need to see if a trend leads ultimately to pain or pleasure (or other changes in drives). Are we being stroked in such a way that it’s going to hurt when the stimulus reaches a tender spot? Or is the moving stimulus a hint that some food is on its way towards our mouth, which we need to start opening?
Now here comes my problem (or so I thought): In the diagram I’m assuming that the prediction gets mixed with the sensory signal (the green axon leading into the blue cell) so that predictions act like sensations. This way, the organism will react as if the prediction came true, leading to another prediction, and another. Eventually one of these predictions will predict pleasure or pain.
[Technical note: Connectionists wouldn’t think this way. They’d assume that pleasure/pain are back-propagated during learning, such that this first prediction neuron already “knows” how much pleasure or pain is likely to result further down the line, since this fact is stored in its synaptic weight(s). I’m not happy with this. For one thing, thinking is never going to arise in such a system, because it’s entirely reactive. Secondly (and this is perhaps why brains DO think), the reward value for this prediction is likely to be highly conditional upon other active predictions. This isn’t obvious in such a simple model, but in a complete organism the amount of pleasure/pain that ultimately results may depend very heavily on what else is going on. It may depend on the nature of the touch, or have its meaning changed radically by the context the creature is in (is it being threatened or is something having sex with it?). It’s therefore not possible to apportion a fixed estimate of reward by back-propagating it through the network. That sort of thing works up to a point in an abstract pattern-recognition network like a three-layer perceptron, but not in a real creature. In my humble opinion, anyway!]
Oh yes, my problem: So, if a prediction acts as if it were a sensation (and this is the only way it can make use of the subsequent (red) abstraction cells in order to make further predictions) then how does the organism know the difference between what is happening and what it merely suspects will happen??? If all these predictions are chained together, the creature will feel as if everything that might happen next already is happening.
This has bugged me for the past few days. But this morning I came to a somewhat counter-intuitive conclusion, which is that it really doesn’t matter.
What does “now” actually mean? We think of it as the infinitesimal boundary between past and future; between things that are as yet unknown and our memories. But now is not infinitesimal. I realized this in the shower. I was looking at the droplets of water spraying from the shower-head and realized that I can see them. This perhaps won’t surprise you, but it did me, because I’ve become so conditioned now to the view that the world I’m aware of is actually a predictive simulation of reality, not reality itself. This HAS to be true (although now is not the time to discuss it). And yet here I was, looking at actual reality. I wasn’t inventing these water droplets and I couldn’t predict their individual occurrence. Nor was the information merely being used to synchronize my model and keep my predictions in line with how things have actually turned out – I was consciously aware of each individual water droplet.
But I was looking at water that actually came out of my shower-head over a tenth of a second ago; maybe far longer. By the time the signals had caused retinal ganglion cells to fire, zoomed down my optic nerve, chuntered through my optic chiasm and lateral geniculate nucleus, and made their tortuous and mysterious way through my cortex, right up to the level of conscious awareness, those droplets were long gone. So I was aware of the past and only believed I was aware of the present. (In fact, just to make it more complex, I think I was aware of several pasts – the moment at which I “saw” the droplets was different from the moment that I knew that I’d seen the droplets.)
Yet at the same time, I was demonstrably aware of an anticipated present, based upon equally retarded but easier to extrapolate facts. I wasn’t simply responding to things that happened a large fraction of a second ago. If a fish had jumped out of the shower-head I’d certainly have been surprised and it would have taken me a while to get to grips with events, but for the most part I was “on top of the situation” and able to react to things as they were actually happening, even though I wouldn’t find out about them until a moment later. I was even starting bodily actions in anticipation of future events. If the soap had started to slip I’d have begun moving so that I could catch it where it was about to be, not where it was when I saw it fall. But for the most part my anticipations exactly canceled out my processing delays, so that, as far as I knew, I was living in the moment.
So I was simultaneously aware of events that happened a fraction of a second ago, as if they were happening now; events that I believed were happening now, even though I wouldn’t get confirmation of them for another fraction of a second; and events that hadn’t even happened yet (positioning my hands to catch the soap in a place it hadn’t even reached). ALL of these were happening at once, according to my brain; they all seemed like “now”.
Perhaps, therefore, these little predictive circuits really do act as if they are sensations. Perhaps the initial sensation is weak, and the predictions (if they are confident) build up to create a wave of activity whose peak is over a touch neuron that won’t actually get touched until some time in the future. Beyond a certain distance, the innate uncertainty or conditionality of each prediction would prevent the wave from extending indefinitely. Perhaps this blurred “sensation” is what we’re actually aware of. Perhaps for touch there’s an optimum distance and spread. In general, the peak of the wave should lie over the piece of skin that will probably get touched X milliseconds into the future, where X is the time it takes for an actual sensation to reach awareness or trigger an appropriate response. But it means the creature’s sense of “now” is smeared. Some information exists before the event; some reaches awareness at the very moment it is (probably) actually happening; the news that it DID actually happen arrives some time later. All of this is “now.”
Or perhaps not. After all, if I imagine something happening in my mind, it happens more or less in real time, as a narrative. I don’t see the ghosts of past, present and future superimposed. This, though, may be due to the high-level selection process that is piecing together the narrative. Perhaps the building blocks can only see a certain distance into the future. Primitive building blocks, like primary sensations, only predict a few milliseconds. Highly abstract building blocks, like “we’re in a bar; someone is offering me a drink” predict much further into the future, but only in a vague way. To “act out” what actually happens, these abstractions need to assemble chains of more primitive predictions to fill in the details, and so the brain always has to wait and see what happens in its own story, before initiating the next step. I’m not at all sure about this, but I can’t see any other way to assemble a complex, arbitrarily detailed, visual and auditory narrative inside one’s head without utilizing memories of how one thing leads to another at a wide range of abstractions. These memories have to have uses beyond conscious, deliberate thought, and so must be wired into the very process of perception. And in order for them to be chained together, the predicted outcomes need to behave as if they were stimuli.
I’m going to muse on this some more yet. For instance I have a hunch that attention plays a part in how far a chain of predictions can proceed (while prediction in turn drives attention), and I haven’t even begun to think about precisely how these simulations can be taken offline for use as plans or speculations, or precisely how this set-up maps onto motor actions (in which I believe intentions are seen as a kind of prediction). But this general architecture of abstractions and predictions is beginning to look like it might form the basis for my artificial brain. Of course there’s an awful lot of twiddly bits to add, but this seems like it might be a rough starting point from which to start painting in some details, and I have to start somewhere. Preferably soon.