This chapter examines techniques for evaluating the probability of finding a synaptic contact between neurons. A simple case for which one might want to estimate that probability is shown in Figure 2.0.1, where neurons from region A send their axons into region B and establish synaptic contacts there.

One of the principal reasons to evaluate the probability of such contact is to compare the evaluated probability and the experimental findings. Such comparisons can reveal the existence of rules that govern the connectivity between neurons. To illustrate this point, let us suppose that we are able to obtain an estimate of the probability of contact between a neuron from A and a neuron from B, assuming complete randomness (i.e., every neuron from A has the same probability of establishing contact with every neuron in B). We can then conduct experiments in which electrodes are thrust into A and B. With one electrode we stimulate a neuron from A, and with the other we record the response of a neuron from B. Such an experiment can help us decide if the cell we stimulate indeed affects the recorded cell synaptically. By repeating the experiment many times, we can experimentally evaluate the probability that a neuron from A will make a functioning synaptic contact on a neuron from B.

If the probability of contact that we determined experimentally agrees with the probability evaluated theoretically, then the idea of completely random connectivity can be accepted.