Building Memories: Remembering and Forgetting of Verbal Experiences as Predicted by Brain Activity

Abstract

A fundamental question about human memory is why some experiences are remembered whereas others are forgotten. Brain activation during word encoding was measured using blocked and event-related functional magnetic resonance imaging to examine how neural activation differs for subsequently remembered and subsequently forgotten experiences. Results revealed that the ability to later remember a verbal experience is predicted by the magnitude of activation in left prefrontal and temporal cortices during that experience. These findings provide direct evidence that left prefrontal and temporal regions jointly promote memory formation for verbalizable events.Memory encoding refers to the processes by which an experience is transformed into an enduring memory trace. Psychological studies have shown that the memorability of an experience is influenced greatly by the cognitive operations engaged during initial encoding of that experience, with semantic processing leading to superior memorability relative to nonsemantic processing (1). Functional neuroimaging studies have implicated left prefrontal cortex in verbal encoding: left prefrontal activation is greater during semantic relative to nonsemantic encoding (2), and left prefrontal participation decreases and memorization is impaired when semantic encoding operations are disrupted (3). These studies have all relied on blocked experimental designs, where trials from each encoding condition are presented sequentially, inseparable from each other during the functional scan. While blocked designs allow comparison between encoding conditions that yield, on average, higher or lower levels of subsequent recollection, they do not allow a direct trial-by-trial comparison between specific encoding trials that lead to subsequent remembering and those that lead to subsequent forgetting. Results from event-related potential (ERP) studies, which allow for trial-by-trial analysis, suggest that the neural signature during verbal encoding differs for subsequently remembered and subsequently forgotten experiences, with remembered experiences being associated with a greater positive-going response over frontal and parietal regions (4). However, ERP studies are characterized by limited spatial resolution. Thus, the precise functional neuroanatomic encoding differences that predict whether a particular verbal experience will be remembered or forgotten are currently unknown.A second unanswered question concerns the exact roles of medial temporal structures in memory encoding. Lesion studies in humans and other species indicate that medial temporal regions are essential for the processing of experiences such that they can be remembered at a later time (5). However, modulated medial temporal activation has been notably absent in neuroimaging studies that systematically varied the nature of cognitive operations engaged during encoding (2). Rather, parahippocampal gyrus, a subcomponent of the medial temporal memory system, has been indirectly implicated in memory encoding because parahippocampal...

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