Within-person and between-person variability in learning and memory

Memory performance can vary at two distinct levels: within-person and between-person. Within-person variability reflects the fluctuations in performance over time in a single person; between-person variability reflects the differences in long-term averages that differentiates individuals in a group. Fundamental questions about how and why episodic memory ---- that is, memory for events ---- varies at the within-person and between-person levels still remain unanswered due to methodological shortcomings. In this thesis, I first illustrate these shortcomings using theoretical and simulation approaches. Then, I report data from repeated-measures studies that sought to understand the variability in episodic memory outcomes at both within-person and between-person levels by examining how it is related to working memory and sustained attention, cognitive functions that are thought to play important roles in episodic memory processes. In the first study, I conducted a repeated-measures experiment to answer funda-mental questions about how memory performance varies. In particular, I examined whether and how between-person (BP) and within-person (WP) variability in episodic memory relates to measures of working memory and sustained attention. First, I found that the majority of the variability in my measures of episodic memory (as well as working memory and sustained attention) can be attributed to the BP level. Second, I found that when episodic memory was measured as immediate recall, both working memory and sus-tained attention were related to episodic memory at the BP level, while only sustained at-tention but not working memory was related to episodic memory at the WP level. Third, when episodic memory was measured as delayed recall and these relationships were exam-ined separately at encoding and retrieval, I found largely consistent results to those ob-served for immediate recall for both working memory and sustained attention, but with reduced effect sizes for the delayed recall results. Moreover, both WP encoding and re-trieval findings for delayed recall were consistent with each other and also in agreement with WP findings from immediate recall. In a second study, I sought to understand variability in the effectiveness of a re-trieval-based learning intervention aimed at improving memory performance. In particu-lar, I used repeated-measures data to answer fundamental questions about the nature of the testing effect ---- that is, the differential improvement in learning from retrieval practice compared to restudy ---- and its variability. First, I found that the testing effect varies at both the WP and BP levels, and that most of the variability is at the WP level. Second, al-most all individuals show a positive mean testing effect across sessions, suggesting that re-trieval practice is a beneficial learning strategy for most individuals. However, when con-sidering fluctuations in the testing effect within individuals, the benefit of retrieval practice is not as stable and there are often sessions where retrieval practice is not beneficial for an individual (or is markedly less beneficial in some sessions relative to others). Third, I did not find compelling evidence that variability in the testing effect is related to variability in the other cognitive functions explored in this experiment: working memory and sustained attention. Together, these results provide valuable insights about the applicability of the testing effect and how the testing effect varies at WP and BP levels. Together, the results from this thesis improve our understanding of learning and memory processes. Methodologically, these studies are novel demonstrations in these areas of research showing that using repeated-measures approaches can uncover valuable in-formation about the variability in episodic memory and the testing effect. From both basic and applied science perspectives, being able to separate WP and BP contributions in the variance-covariance structures of episodic memory and the testing effect led to novel an-swers to questions that were previously unresolved due to the predominant use of cross-sectional designs. Just as there has been a long history of cross-sectional individual differ-ences studies in learning and memory, I hope that these studies will inspire future studies that use even more optimized and comprehensive repeated-measures designs. More broad-ly, it will be important to explore the generalizability of these results across multiple di-mensions. Are these findings robust to changes in particular elements of the study design such as the stimuli used? How might the variance-covariance structure of cognitive func-tions change in different experimental conditions such as in-lab vs online? Answering these types of questions will lead to a more complete understanding of both basic and ap-plied aspects of learning and memory

Advisor: Wagner Anthony David 
Gardner Justin
Poldrack Russell A.