Spreading activation

The prime impetus for this wide spread activity was the "energy crisis" which produced considerable uncertainty about the future availability of asphalt cement for road building purposes and as a consequence caused bid prices to soar over the past eight years. 

Fields CG, Mickelson DJ, Drake SL, McCarthy JB, Fields GB. Melanoma cell adhesion and spreading activities of a synthetic 124-residue triple-helical mini-collagen . J. Biol. Chem. 1993 268 14153-14160. 

A rather different type of production system model was developed by John Anderson (1983). His ACT architecture extended the notion of production system models in two important ways. First, Anderson hypothesized that a distinction exists between procedural and declarative knowledge, and second, he added the important mechanism of spreading activation. 

As in other production systems, the activity of the model is defined by the firing of production rules. At any single point in time, the model has a vast number of potential pattern matches to consider, one for each condition of each rule. It is essential to have some means by which the universe of possible rules is curtailed to a manageable subset. Anderson uses spreading activation for that purpose. The mechanism of spreading activation ensures that only the most pertinent portions of the declarative network will be available for processing, which in turn means that only a relatively... 

Hendler s (1991) hybrid model combines a semantic network with a neural network, as shown in Figure 12.5. In essence, this model depends upon the neural net to learn the internal representations (i.e., the essential microfeatures) that are associated with a set of input stimuli. Thus, the model develops the hidden unit layer of the network as well as the weights connecting the hidden units to the output units. After the network has settled (i.e., has learned to classify the inputs appropriately), the top two layers of units are accessed by the semantic network model by means of spreading activation. Thus, the nodes of the neural net communicate with the nodes of the semantic net. 

In a feedforward model, signals that will determine the eventual output are passed only in a forward direction from one layer of nodes to the next. Model activity initiates at the input level, and each input unit sends its signal forward to the layer immediately above it. Because activation can spread only forward and not laterally, two layers of units are needed to represent student knowledge. The first layer receives the activation signals from the input units. The second layer permits the student knowledge nodes to spread activation to each other. By having two layers of nodes, we circumvent the problem of mixing forward and lateral connections and preserve the feedforward nature of the model. 

As I pointed out earlier, the model contains two layers of nodes at the middle level. The uppermost of these is necessary to show how the nodes spread activation to each other. Every node at the lower middle level transfers its own activation upward to its counterpart in the next layer and also spreads additional activation to any other nodes to which it is connected. If a student has only isolated nodes that do not connect to each other, the activations associated with the nodes at both layers will be identical. Typically, there are connections among nodes, resulting in higher patterns of activation at the upper layer. 

Collins, A. M., Loftus, E. F. (1975). A spreading activation theory of semantic processing. Psychological Review, 82, 407-428. 

Hendler, J. A. (1988). Integrating marker-passing and problem-solving A spreading activation approach to improved choice in planning. Hillsdale, NJ Lawrence Erlbaum Associates. 

Since the discovery by Rosenberg [88] that cisplatin is an effective anticancer drug, large synthesis and evaluation programs have aimed at the creation of cisplatin derivatives that show greater and more wide-spread activity against cancer but with... 

Anderson, J. R. (1983). A spreading activation theory of mtanory. Journal of Verbal Learning and... 

The intrinsic solderability performance of Pb-free solders is being improved by two means. First, new flux formulations are available that more effectively reduce the surface tension of the solder. Second, alternative surface finishes can be specified for the component I/Os and/or circuit boards that improve wetting and spreading activity exhibited by the Pb-free alloys. 

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