Temporal generalization and diffusion in forgetting.

Two processes may contribute to the decrement in discriminability with increasing temporal distance between the occasioning event and later choice. One is the length of the interval and the other is generalization decrement. In the model described by White and Wixted [J. Exp. Anal. Behav. 71 (1999) 91-113], choice was predicted by the relative payoff for correct delayed matching responses, conditional on the current value of the stimulus sampled from Thurstone-like probability distributions of the effect of the sample stimuli. In the model, discriminability decreased with increasing temporal distance because the variance of the distributions increased with time. However, White and Wixted did not specify the function relating variance to temporal distance. If a diffusion process is assumed, and if diffusion increases exponentially with time, the resulting forgetting function is a negative exponential. An additional process involves exponential generalization of remembering from one time to other times. Alternative diffusion functions result in hyperbolic or power forgetting functions. The combination of two exponential processes yields forgetting functions that are double exponential in form and which appear consistent with a wide range of data.