A neural network is a biologically inspired form of computing device. Neural networks are parallel computing systems that hold the promise--according to their advocates--of being someday able to perform common cognitive tasks, such as pattern and speech recognition, that are hard for more common computational architectures yet are performed almost effortlessly by the human brain. The slight success seen by neural networks in these arenas so far is thanks to their parallel architecture, which mimics, albeit at a far lower level of complexity, certain aspects of our own brains.

In a human brain, the basic information-processing unit is a cell called the neuron. A neuron consists of a cell body where the nucleus resides. Networks of branching fibers called dendrites communicate with other neurons across synaptic gaps, which are the routes chemical signals travel. An action potential is carried from its cell body along a long...