Hofstadter, Dennett - The Mind's I

Prelude . . . Ant Fugue 194of the sodium channels is to let sodium ions into the neuron, and when their concentration ishigh enough, that then triggers the release of the neurotransmitters, whose flow from oneneuron to another constitutes the essence of neural firing.) The answer is, the sodiumchannels are voltage-sensitive, and they have just been hit by a strong enough voltage pulseto flip their state from closed to open."But what_ causes the voltage to rise originally, so that it crosses this threshold ... andsets off this sequence of events called the impulse?" they go on. The answer is, various"nodes" along the neuron's axon have simply relayed this high voltage from one station to thenext. So then the question is again transformed. This time they ask, "But what causes the veryfirst impulse to occur at the very first node? Where does that voltage shift come from? Whatprecedes the impulse?"Well, for most neurons inside the brain-" interneurons," meaning neurons that are fedinto not by sensory input but only by other neurons -the answer is, their first node's voltageshift is provoked by the total effect of the pulses of neurotransmitters coming in from otherneurons. (We could call those neurons "upstream" neurons, but that would imply, quitefalsely, that the flow of neural activity in the brain follows a line in only one direction, in themanner of a river. In fact, as a rule, neural flow patterns are far from linear and make loopsall over the place, quite unlike rivers.)Thus we seem to get into a vicious circle-a chicken-and-egg type of riddle. Question:"What triggers a neural firing?" Answer: "Other neural firings!" But the real question remainsunanswered: "Why those neurons, and not others? Why this vicious circle and not anotherneural loop in another part of the brain?" To answer this, we have to shift levels and talkabout the relationship of the brain to the ideas it encodes, which then would require us to talkabout how the brain encodes, or represents, its concepts about the world. Since we do notwish to theorize in this book on the details of such matters, we will talk about a related butsimpler concept.Imagine an intricately bifurcating and rejoining domino-chain network. Suppose thateach domino has a little time-delayed spring underneath it that stands it up again five secondsafter it has fallen. By setting up the network in various configurations, one could actuallyprogram the system of dominoes to perform calculations with numbers, exactly as one coulda full-scale computer. Various pathways would carry out various parts of the calculation, andelaborate branching loops could be set up. (Note how this image is not too different, then,from that of networks of neurons in a brain.)One could imagine a "program" trying to break the integer 641 into