Spreading activation is a way of reasoning over large networks or graphs. There are many variants, but in general they begin with some set of initial nodes of interstwhic are given an 'activation'. The nodes then pass on some or all of this activation to neighbouring nodes, and the process repeated so that the level of activation perculates outwards form the seed nodes throughout the graoh. Typically thise nodes closer tothe initial nodes end up witj more actvation thah those more distant and those that are connected to mutiple initial nodes get more highy activated. Some spreading activation algorithms are conservative in that the total amount of actvation remains unchanged by the algorithm, any activation passed on from a node to others must come from its own sock. Others may be non-conservative meaning that the totsl amount of actvation might increase or decrease over time.
In some variants, the seed nodes are constantly given fresh activation so that the effect is like a source of water or energy gradually spreading out. Also variats differ as to whether they stop after a fixed number of spreading cycles, or iterate until the system reaches a steady state. If there is no constant refilling the steady state is often related more to the graph structre than the inital seed activation.
In some applications, it is this graph structure which is wanted. An important example is the Google PageRank algorithm which can be viewed as a spreaidng activation over web pages and links. The simplest version of PageRank begins with a uniform initial activation and gthe spreading means that pages whuch are referenced form many other pages gradually aquire more actuvation, which is used as a measure of their importance and hence used to order search results.
In others applications, the initial activation is important. For example, in natural language processing, we might use recent references to entities to giving them initial activation and then use spreading activation over a semantic network to judge the relative likelhood of an {{ambiguous utterance}].
Used on Chap. 3: page 49; Chap. 17: page 403; Chap. 22: pages 541, 543
Spreading activation (from "Spreading Activation for Web Scale Reasoning: Promise and Problems", Akim1 et al. 2011)