Immunology as a Metaphor for Computational ... - Napier University

Chapter 6. A Self-Organising SDM — SOSDM 149was because it seemed to lend itself most obviously to an immune-based model. Forexample [Sjödin, 1996] tried to refine the basic model so that it could more efficientlydeal with non-randomly distributed data by adding an extra counter to each locationwhich counts the number of items stored at the location. A further location is addedcovering the entire space — these are then used to determine which locations shouldbe used in any read attempt from the memory. [Sjödin, 1996] shows that this methodgreatly reduces errors in the recalled strings for data that is biased when compared tothe original model, as the new model ignores many locations which are activated buteffectively contain noise.6.4 Implementation of SOSDMPseudo-code outlining the SOSDM algorithm is given in figure 6.5. Firstly, antigensare distributed to a subset of antibodies, based on the affinity of each antibody forthe antigen in a batch process. Affinity is simply the Hamming Distance between anantigen and an antibody - the closer the distance, the stronger the affinity between thetwo. This results in the counters of the subset of antibodies being updated, accordingto the strength of each antigen encounter. After all antigens have been given a chanceto encounter an antibody, the accumulated error of each antibody is calculated. Theerror is equivalent to the sum of the distances between each antibody and any antigenit recognises, weighted by the strength of the encounter. The value of the error is thenused to allow the antibodies to self-organise — antibodies gravitate towards areas ofthe space in which they recognise data, the distance and direction of the movementdetermined by the accumulated error. Each of these steps is now described in greaterdetail.6.4.1 NotationThe following notation is used to describe the manner in which SOSDM is implemented.Assume an SOSDM is defined by n antibodies (i.e hard locations in an SDM),each of which is referred to as c i . Each antibody is described by two strings, each oflength L. The first, V c¥ denotes the address or location of the antibody, the second£