Figure 1.1: A Neuron Cell (Drawing courtesy of Carrie Spear)
This signal transmitted by the neuron is an analog signal. Most modern computers are digital machines, and thus require a digital signal. A digital computer processes information as either on or off. This is the basis of the binary digits zero and one. The presence of an electric signal represents a value of one, whereas the absence of an electrical signal represents a value of zero. Figure 1.2 shows a digital signal.
Biological neural networks are analog. As you will see in the next section simulating analog neural networks on a digital computer can present some challenges. Neurons accept an analog signal through their dendrites, as seen in Figure 1.1. Because this signal is analog the voltage of this signal will vary. If the voltage is within a certain range, the neuron will
fire. When a neuron fires a new analog signal is transmitted from the firing neuron to other neurons. This signal is conducted over the firing neuron's axon. The regions of input and output are called synapses. Later, in Chapter 3, “Using Multilayer Neural Networks”, you will be shown that the synapses are the interface between your program and the neural network.
As you can see, a biological neuron is capable of making basic decisions. This model is what artificial neural networks are based on. You will now be show how this model is simulated using a digital computer.
Simulating a Biological Neural Network with a Computer