Parietal lobe contributions to episodic memory retrieval

Although the parietal lobe is not traditionally thought to support declarative memory, recent event-related fMRI studies of episodic retrieval have consistently revealed a range of memory-related influences on activation in lateral posterior parietal cortex (PPC) and precuneus extending into posterior cingulate and retrosplenial cortex. This article surveys the fMRI literature on PPC activation during remembering, a literature that complements earlier electroencephalography data. We consider these recent memory-related fMRI responses within the context of classical ideas about parietal function that emphasize space-based attention and motor intention. We conclude by proposing three hypotheses concerning how parietal cortex might contribute to memory.

Introduction

Episodic memory – conscious memory for everyday events [1] – has long been known to depend on the medial temporal lobe (MTL) memory system 2, 3 and on prefrontal contributions to encoding and retrieval 4, 5. Although theoretical and experimental emphasis remains on specifying how MTL and prefrontal structures mediate the acquisition, consolidation and retrieval of episodic memories, an emerging body of functional imaging evidence suggests that a full understanding of episodic memory might also require specification of parietal contributions to retrieval 6, 7. Here we review this surprising new literature and develop hypotheses regarding how parietal cortex might support remembering.

The basic phenomenon that has sparked interest in parietal contributions to episodic retrieval is that differential parietal responses are observed when individuals correctly recognize previously encountered old items (hits) as compared with correctly identifying new unstudied items (correct rejections; CRs). The earliest observations of such ‘old/new’ effects (alternatively labeled ‘retrieval success’ effects; [8]) were obtained with electroencephalography, wherein the magnitude of event-related potentials (ERP) differed at left and medial parietal scalp electrode sites during hits and CRs (for review, see [9]). More recently, event-related functional MRI (fMRI) studies have documented greater activation during hits than during CRs in posterior parietal cortex (PPC), including inferior and superior parietal lobules, as well as medial structures that extend from precuneus into posterior cingulate and retrosplenial cortices.

Beyond simply identifying neural regions that demonstrate old/new effects (Box 1), fMRI studies have suggested a range of distinct memory-related influences on parietal activation. For example, activation in PPC modulates based on the perception that information is old (or familiar) even when that perception is in error 10, 11. PPC activation also increases when recognition is accompanied by the recollection of event details 12, 13, 14, 15, and, in forced-choice recognition designs, regions in PPC are engaged when retrieval is oriented towards recollecting episodic details as compared with detecting item familiarity 13, 16, 17. This article surveys these recent fMRI findings of PPC activation during episodic retrieval. In the course of doing so, we consider how these findings from the memory literature relate to classical attention and intention accounts of parietal function. We conclude by proposing a set of hypotheses about how parietal cortices might contribute to remembering.