Computers and Symbols versus Nets and Neurons

1 These notes are currently under review for publication by UCL Press Limited in the UK. Duplication of this draft is permitted by individuals for personal use only. A n y other form of duplication or reproduction requires prior written permission of the author. This statement m ust be easily visible on the rst page of any reproduced copies. I would be happy to receive a n y comments you might h a ve on this draftt send them to me via electronic mail at [email protected]. I am particularly interested in hearing about things that you found diicult to learn or that weren't adequately explained, but I am also interested in hearing about inaccuracies, typos, or any other constructive criticism you might h a ve. 1 1 Neural net: A preliminary deenition To set the scene it is useful to give a d e n i t i o n o f w h a t w e m e a n b y `Neural Net'. However, it is the object of the course to make clear the terms used in this deenition and to expand considerably on its content. A Neural Netwo r k i s a n i n terconnected assembly of simple processing elements, units or nodes, whose functionality is loosely based on the animal neuron. The processing ability of the network is stored in the inter-unit connection strengths, or weights, obtained by a process of adaptation to, or learning from, a set of training patterns. In order to see how v ery diierent this is from the processing done by conventional computers it is worth examining the underlying principles that lie at the heart of all such machines. 2 The von Neumann machine and the symbolic paradigm The operation of all conventional computers may be modelled in the following way memory central processing unit instructions and data data von-Neumann machine The computer repeatedly performs the following cycle of events 1. fetch an instruction from memory. 2. fetch a n y data required by the instruction from memory. 3. execute the instruction (process the data). 4. store results in memory. 5. go back t o s t e p 1). 2 What problems can this method solve easily? It is possible to formalise many problems in terms of an algorithm, that is as a well deened procedure or recipe which will guarantee the answer. For example, the …