On the limits of spreading activation in ACT-R: Predictions and testability

In the fan effect, reaction time (RT) increases as a function of fan size (i.e. the number of associations of a fact). Spreading activation in ACT-R provides a good account of the fan effect at low fan size (i.e., 1--4). However, little is known about the predictions of ACT-R at ecologically valid scales. We developed a general guessing mixture model (GMM) within ACT-R in which a guessing process is triggered by retrieval failures, and analyzed the predictions for fan sizes much larger than those used in laboratory experiments. Our analysis revealed the following properties of the GMM: RT increased as a function of fan size, but stays within a plausible range (< 2 seconds) as long as the retrieval threshold is not excessively low, and, in the limit, accuracy asymptotes at the value of the guessing bias parameter. We discuss practical challenges with testing the predictions at larger fan sizes.