Work on artificial neural networks, commonly referred to as “neural networks,” has been motivated right from its inception by the recognition that the human brain computes in an entirely different way from the conventional digital computer. The brain is a highly complex, nonlinear, and parallel computer (information-processing system). It has the capability to organize its structural constituents, known as neurons, so as to perform certain computations (e.g., pattern recognition, perception, and motor control) many times faster than the fastest digital computer in existence today. Consider, for example, human vision, which is an information-processing task. It is the function of the visual system to provide a representation of the environment around us and, more important, to supply the information we need to interact with the environment.
To be specific, the brain routinely accomplishes perceptual recognition tasks (e.g., recognizing a familiar face embedded in an unfamiliar scene) in approximately 100–200 ms, whereas tasks of much lesser complexity take a great deal longer on a powerful computer.
For another example, consider the sonar of a bat. Sonar is an active echolocation system. In addition to providing information about how far away a target (e.g., a flying insect) is, bat sonar conveys information about the relative velocity of the target, the size of the target, the size of various features of the target, and the azimuth and elevation of the target. The complex neural computations needed to extract all this information from the target echo occur within a brain the size of a plum. Indeed, an echolocating bat can pursue and capture its target with a facility and success rate that would be the envy of a radar or sonar engineer.
How, then, does a human brain or the brain of a bat do it? At birth, a brain already has considerable structure and the ability to build up its own rules of behavior through what we usually refer to as “experience.” Indeed, experience is built up over time, with much of the development (i.e., hardwiring) of the human brain taking place during the first two years from birth, but the development continues well beyond that stage.
A “developing” nervous system is synonymous with a plastic brain: Plasticity permits the developing nervous system to adapt to its surrounding environment. Just asplasticity appears to be essential to the functioning of neurons as information-processing units in the human brain, so it is with neural networks made up of artificial neurons.
