Friday, June 1, 2007

Hardware Matters

Let's engage in a thought experiment. As a warm-up exercise I would like you to consider the simplest of memory devices know as a flip-flop (or technically an SR latch).

The purpose of such a latch is to act as a electronic toggle switch such that a logical 1 pulse on S (SET) input will turn Q to logic 1. A pulse on the R (RESET) input will revert Q to logical 0. I will not delve here into how a SR latch works but if you know the truth function of a nor-gate you can probably figure it out for yourself. I still remember with some fondness the moment I grokked how such a latched worked on my own so it is a worthwhile exercise. It is clear, just from looking at the picture that feedback is central to the latch's operation. There are more sophisticated flip-flops and latches but all require feedback.

So here is the thought experiment. Imagine the wires (black lines) connecting the logic gates growing in length. Since this is a thought experiment we can stipulate that as they grow in length their electrical characteristics remain constant. That is, imagine resistance, capacitance and inductance does not change. If this bothers you then replace the electrical wires with perfect fibre optics and make the nor-gates work on light pulses rather than electrical ones. The point is that for purpose of the thought experiment I want to exclude signal degradation.

I maintain that as the wires grow, even without degradation in the signals, there would reach a point where the latch would fail to work and would begin to oscillated or behave erratically. The same would be the case if you replaced the simple latch with a better design such as a D-flip-flop. The point where the flip-flop would begin to fail is near where the delay due to the finite speed of the signal (limited by the speed of light) would become comparable with the switching time of the nor-gate. In essence the flip-flop relies on the feedback signals being virtually instantaneous relative to the switching time. In the real world other effects would cause the flip-flop to fail even sooner.

Thought Experiment 2

Okay, now for a new thought experiment. Before we engage in this experiment you must be of the belief that your brain is what causes you to possess consciousness (whereby we define consciousness is the the phenomenon that allows you to feel what it is like to for something to be the case -- e.g. what red is like). If you believe consciousness is caused by pixies or granted to you by the grace of God irregardless of the architecture of the brain then stop reading right now and go here instead.

Still with me? Good.

Now, imagine your brain with its billions of neurons and their associated connections. As before, imagine the connections (axons) growing in length without degradation of their signals. Let these connections grow until your brain is the size of the earth or even the sun. Let the neuron bodies stay the same microscopic size, only the inter-neuron connections should grow and no connections should be broken in the process.

Now there is no way I can prove this at the moment but I would bet a very large sum that as the brain grew in this way there would be a point where consciousness would degrade and eventually cease to exist. I believe this would be the case for similar reasons as with our SR latch thought experiment. Time delays would impact the function of the brain in all respects. More specifically, I believe that it won't simply be the case that the delays would cause the brain to slow down such that it would cease to be a practical information processing device relative to the rest of the real-time world. I believe that consciousness would stop for a much more fundamental reason that the propagation delays relative to the neuronal firing times are crucial to the function of the brain in every respect just as they are relevant to the function of the flip-flop. Time and consciousness are intimately tied together.

So Whats the Point?

As you read about the mind you will come across various other types of thought experiments where the neurons of the brain are replaced by other entities that act as functional stand-ins. For example, a popular depiction is every neuron in the brain being replaced with a person in China. Each person would communicate with a group of other people in a way that was functionally identical to the neurons. The question posed is whether the group as a whole could be conscious (that is a consciousness independent of the individuals and inaccessible to any one individual).

Such experiments assume that consciousness is independent of such notions as timing and spacial locality. To me this is highly improbably and hardly even worth consideration. In fact, when we finally understand the brain in its fullness, I am quite certain it will be the case that there are properties of neurons, neurotransmitters and hormones that are crucial to brain function. Specifically, a brain of silicon chips organized as a conventional computer could not work the same. In short, hardware matters.

3 comments:

Luke said...
This comment has been removed by the author.
Luke said...

I definitely agree with your general thesis. However, I think the conclusion one should draw is that the timing characteristics of the feedback system have to be consistent with the kind of time constants of the phenomena that are modeled. However, this doesn't imply that one could not create an appropriate feedback system on a different substrate, e.g. silicon, which could exhibit the emergent behavior of consciousness.

Sal Mangano said...

Agreed. There is nothing in my argument that proves that silicon can not host consciousness. However, I believe that the information processing density (the number of necessary processing elements and their required interconnectivity) that satisfies the emergence of consciousness coupled with the need to dissipate heat makes silicon chips (as we know them today) and possibly silicon, in general, implausible.