Memory, Interference and Depression.

Abstract

This dissertation explores the role of interference in short-term memory and how interference in memory may influence depression. The first chapter tackles whether forgetting in the short-term is due to decay with the mere passage of time, interference from other memoranda, or both. A series of studies are presented using a novel paradigm to address this problem directly. The results of these studies indicate that short-term memories are subject to very small decay effects with the mere passage of time, but that interference plays a much larger role in their degradation. We discuss the implications of these results for existing models of memory decay and interference. Chapters 2 and 3 explore rumination in Major Depressive Disorder (MDD). MDDs often ruminate about their depression, impairing their concentration and performance on daily tasks. We examined whether rumination might be due to a deficit in the ability to expel negatively valenced interfering information from short-term memory (STM) as well as the neural structures involved in this ability using fMRI. MDD participants had more difficulty than did healthy controls (HCs) in expelling negative, but not positive, words from STM. Overall, the neural networks involved in resolving interference were quite similar for both groups; but MDDs exhibited more spatial variability in activation in the left inferior frontal gyrus (a region critical for inhibiting irrelevant information) which may mediate their relative inability to inhibit or overcome interference from negative information. MDD has also been characterized by excessive default-network activation and connectivity with the subgenual cingulate. These hyper-connectivities are often interpreted as reflecting rumination, an internally generated form of interference. We examined connectivity of the default network specifically in the subgenual cingulate both on- and off-task, and examined the relationship of connectivity to rumination. A seed-based connectivity approach revealed that MDDs show more neural functional connectivity between the posterior-cingulate cortex and the subgenual-cingulate cortex than healthy individuals during rest periods, but not during task engagement. Importantly, these rest-period connectivities correlated with behavioral measures of rumination and brooding, but not reflection. Implications of this research on both theoretical psychology and daily life and practical system design are discussed.