Unlike other pathetic sentient beings in this universe, my primary goal is simple: transfer as much information as possible. Through a long time of evolution, I've been thinking how to do this in the most efficient way. I have limited knowledge at a given time. I can sense the membrane potential of the (postsynaptic) neuron, remember for a while the fact that there was an action potential in either presynaptic or postsynaptically. I can change the amount of excitation that I will bring to the (postsynaptic) neuron which in turn increases the probability of firing. I also know that there are many other synapses like me, but I cannot communicate with them.
So, I thought that increasing the amount I am exciting the neuron per presynaptic action potential would be the only way I would increase the information transfer. Any information transfer has to be supported by a physical link, and it would be stronger as I increase my action.
This strategy worked sometimes, but often didn't work. Due to the dynamics of the neuron, some other action potential that I didn't cause discouraged from my contribution to fire. I definitely increased the probability of the neuron to fire, so I was happy.
One day it just struck me. I realized that I was asking the wrong question. The question I should have asked is how to transfer maximal information from all synapses through the neuron, not just me. That means I'll have to communicate with other syanpses and find out how to work together. There's only one information that all the synapses share, the timing of the postsynaptic action potential. The membrane potential was quite local in the dendritic structure and highly noisy. I have to rely on the output train of action potentials to figure out how I can cooperate with other synapses.
Imagine a network of thousand roads and millions of cars trying to get from point A to B. If there were no coordination of traffic lights, and each road is trying to maximize the number of cars that goes through itself, all the intersections would be clogged by interference among incoming traffic. Of course, information is nothing like cars, it's an analogy! If they were, a car would clone itself and turn into two cars, and transform into different cars, and just disappear without a clue. Anyway, the point is that if somebody else is sending some information, I should not send mine at the moment. But, unlike cars, if somebody is sending information through this neuron, and I have a very similar information, I should send mine too, to reinforce the transfer. That way we collaborate to reduce the variability of the output spike train.
Yes, that's what I should do. I should find a group of other synapses that share some input features and collaborate. If I cannot find one, I should stay low profile and do not disturb others but still be open to chances of finding future opportunities of collaborations. How will I find a group by only observing the output spike train? I would like to see if I seem to be strongly causing the output. If I am, that means there is a group of synapses that are also causing the output spike train, because I am not strong enough to cause it myself. So I should also participate strongly to the information transfer process.
How would I know that I am strongly causing the output? The feature I am looking for should be in the spike trains and cannot be temporally too spread, because of my short memory. In the simplest case, I would just see the relation between one presynaptic action potential and one postsynpatic action potential in a small time window. If the presynaptic one occurs first, then I am somewhat causing the output, and if it is the opposite case, I might just be disturbing. Therefore, if I use spike-timing dependent plasticity (STDP), over time this instantaneous causality would accumulate and I would be collaborating with similar synapses or not.
- One big advantage of point process over regular random process is that the causality is easy to detect.
- There could be other measures of causality that extends to more than a pair of input/output.
- Papers in the literature does not address spatial patterns so that the STDP synapses could collaborate.
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