Mechanisms of long-term repetition priming and skill refinement: A probabilistic pathway model

We address an omnipresent and pervasive form of human learning—skill refinement, the improvement in performance of a cognitive or motor skill with practice. A simple and well studied example of skill refinement is the psychological phenomenon of long-term repetition priming: Participants asked to identify briefly presented words are more accurate if they recently viewed the word. We simulate various phenomena of repetition priming using a probabilistic model that characterizes the time course of information transmission through processing pathways. The model suggests two distinct mechanisms of adaptation with experience, one that updates prior probabilities of pathway outputs, and one that increases the instantaneous probability of information transmission through a pathway. These two mechanisms loosely correspond to bias and sensitivity effects that have been observed in experimental studies of priming. The mechanisms are extremely sensible from a rational perspective, and can also explain phenomena of skill learning, such as the power law of practice. Although other models have been proposed of these phenomena, we argue for the probabilistic pathway model on grounds of parsimony and the elegant computational perspective it offers. Acquisition of a cognitive or motor skill occurs in several stages. First, an individual must learn the conceptual structures required for the task, including the basic knowledge necessary to perform the task. Then, over a long period of practice, the skill is refined, leading to more fluent, efficient, and robust performance. Skill refinement is an omnipresent and pervasive form of learning. Although skill refinement is sometimes deliberate, e.g., rehearsing a musical piece, it is often implicit, e.g., when typing, driving, reading, playing video games, etc. Understanding skill refinement is fundamentally about discovering the mechanisms by which one trial or performance of the skill leads to improvements on the next. Long-term repetition priming Perhaps the most direct and easily studied manifestation of skill refinement in the psychological literature is the phenomenon of long-term repetition priming. In the priming paradigm, participants engage in a series of experimental trials, and experience with a stimulus or response on one trial results in more efficient processing on subsequent trials. Efficiency is characterized in terms of faster response times, lower error rates, or both. A typical long-term perceptual priming experiment consists of a study phase in which participants are asked to read a list of words one at a time, and a test phase, during which they must respond to a series of brief, masked target words. Repetition priming occurs when a word from the study phase influences performance during the test phase. These experiments often vary the flash duration, the time between target and mask onsets, and also utilize a variety of response paradigms, including speaking the target identity aloud (naming) and forced choice identification between two alternatives (2AFC). Priming is an implicit memory phenomenon: participants are not told the study and test phases are related, and they do not try to recall study words during the test phase as a deliberate strategy for performing the task. Thus, priming is incidental to the test phase of the experiment; it comes about as a result of experience and is thus a form of skill refinement, where the “skill” here is perceptual processing of a letter string. A key question concerning repetition priming is whether priming is due to increased bias or increased sensitivity. Although the terminology is borrowed from signal detection theory, the meaning of these terms in the context of priming is somewhat different. Bias means that participants are more likely to report studied items regardless of what word is presented for identification. Sensitivity means that participants become better at perceptual discrimination of the studied items. In a word naming task, improved performance following priming could be due either to increased bias or increased sensitivity. Consequently, experimental paradigms have been designed to unconfound these possibilities. The basic result found in the long-term repetition priming literature is that priming reflects both increased bias and increased sensitivity, although the sensitivity increase is robust only for low-frequency words or novel items. Our goal is to introduce a model of performance and refinement of simple cognitive skills, such as word reading. The model has two distinct learning mechanisms contributing to skill improvement with practice. The model explains data from long-term repetition priming experiments. We model two studies isolating bias and sensitivity components to priming, and show an approximate correspondence between our two learning mechanisms and these two effects. Our model is related to models proposed previously, but its elegant and concise formulation makes it preferable on grounds of parsimony. (a) 0 0.2 0.4 0.6 0.8 1