The decay in the influence of a stimulus as a function of the number of items intervening between it and reinforcement . The ordinates are the probability that a summary response that indicated the preponderant color (e.g., "mostly red") was the same as the color presented in the ith position. Each element was presented for 425 ms. Filled circles: The time between stimuli (ISI) = 75 ms; Open circles: ISI = 425 ms. The memorial part of the model was a geometric decay, with rate = 0.4 per item. In this figure a comparable exponential function (Equation 1 in Appendix 1) was used to generate continuous forgetting functions. The data are averages over 5 pigeons.