Memory is so fundamental to the operation of the brain and mind that students of the topic could be forgiven for feeling that their object of study is perhaps the most central in all of cognitive neuroscience. But what is memory? As I wrote in the introductory section to the memory chapters in the 2005 edition of this book, we cannot yet provide a satisfactory answer to this question, but we do know something worth knowing about what memory is not: it is not a single entity or concept. Indeed, as Endel Tulving stated in his introduction to the memory chapters in this volume's first edition, “Memory is many things, even if not everything that has been labeled memory corresponds to what cognitive neuroscientists think of as memory. Memory is a gift of nature, the ability of living organisms to retain and to utilize acquired information. The term is closely related to learning, in that memory in biological systems always entails learning (the acquisition of information) and in that learning implies retention (memory) of such information.” Tulving also characterized memory as a trick of evolution, a biological abstraction, and a convenient chapter heading for certain kinds of problems that scientists study.

Tulving's observations still make a great deal of sense. For example, the starting point for virtually any scientific analysis of memory involves a decomposition into processes of encoding, storage and consolidation, and retrieval. Furthermore, a prominent theme in cognitive neuroscience for the past two decades has been that memory can be divided into multiple forms or systems—collections of processes that operate on different kinds of information and according to different rules. Forms of memory such as working, episodic, semantic, priming, and procedural memory are all familiar to contemporary researchers.

The idea that memory is not a single thing also extends to memory's imperfections. Memory is not a simple matter of success or failure. Memory can go awry because of either forgetting or distortion, and each of these types of error can be subdivided into several distinguishable forms. Furthermore, memory interacts in important ways with a number of related processes, including emotion, cognitive control, and planning. A full understanding of memory requires us to address its varied manifestations and complexities. Happily, memory researchers have risen to the challenge by exploring and perhaps beginning to unravel the many complexities of memory.

The seven chapters in this section highlight many facets of memory at different levels of analysis. The starting point for cognitive neuroscience analyses of memory typically begins in the medial temporal lobe. Ever since the groundbreaking observations of Scoville and Milner during the 1950s concerning the severe amnesia observed in patient HM after bilateral resection of the medial temporal lobe for relief of intractable epilepsy, attempting to understand this region's role in memory and learning has constituted a kind of holy grail for memory researchers. It is therefore appropriate that the section begins with Suzuki's chapter concerning the neuroanatomy of the medial temporal lobe (chapter 45). She provides a detailed analysis that focuses in particular on similarities and differences across species in a number of key medial temporal lobe structures, including perirhinal, parahippocampal, and entorhinal cortices. Among the interesting differences observed, Suzuki notes that in monkey perirhinal cortex, unimodal input is dominated by the visual modality, whereas the rat perirhinal cortex receives a wider range of inputs from all sensory modalities. Suzuki delineates possibly important functional consequences of such differences.

In chapter 46, Shrager and Squire focus on work with human amnesic patients to examine spared and impaired functions after medial temporal lobe damage. They report a series of refined experimental studies concerning amnesic patients who have well-characterized lesions in order to address a number of topics that have been central to recent discussions of medial temporal lobe function, including working memory, habit learning, recollection versus familiarity, path integration, remote memory, and conscious awareness. Their observations help to delineate the role of the medial temporal lobe in each of the foregoing aspects of memory and learning.

Nader in chapter 47 focuses on the recently rediscovered phenomenon of reconsolidation, one of the most intensively investigated and hotly debated topics in neuroscience-based memory research during the past decade. It has long been accepted that memory is a time-dependent process, involving a consolidation phase where new memories are initially unstable or labile, and then over time become more stable and resistant to disruption. Reconsolidation refers to the observation that reactivating a seemingly consolidated memory can, under a number of conditions, return it transiently to a labile state in which it is again subject to disruption. Nader reviews recent experimental evidence, considers various alternative interpretations of the phenomenon, and attempts to link reconsolidation with approaches to memory that emphasize its constructive nature.

In chapter 48, Race, Kuhl, Badre, and Wagner examine the interface between memory and cognitive control processes, which guide thought and action in accordance with current goals. They review fMRI studies concerned with the contributions of specific regions within the frontal lobe to cognitive control over memory, focusing especially on situations in which competition between memories creates interference, and where ineffective retrieval cues yield uncertainty. Their discussion of the theoretical implications of dissociations between specific frontal subregions in several task domains illustrates the impressive specificity of neuroanatomical and functional conclusions that can be drawn on the basis of imaging studies.

Just as the interface between memory and cognitive control has brought a major topic of experimental and theoretical concerns to the surface, so has the interface between memory and emotion. Kensinger in chapter 49 reviews this increasingly impressive body of research, which shows that interactions with emotion can arise at every phase of the memory process, including encoding, consolidation, and retrieval. She considers neuroimaging data that clarify the role of the amygdala in emotional memory and that reveal conditions under which amygdala activity corresponds with increased accuracy of emotional memories. Kensinger also considers how individual differences can influence emotion-memory interactions.

The latter topic constitutes the central focus of Miller's chapter 50, especially in relation to neuroimaging studies of memory functions. Though individual differences in memory have often been overlooked in memory research, Miller argues that for neuroimaging studies especially, group analyses can be incomplete and even misleading. Miller illustrates this point with recent work from his own laboratory, and he also integrates observations concerning individual differences with some key theoretical issues in memory research.

Though the preceding six chapters cover many different facts of memory, they all approach memory as a process that is concerned with recovering information from the past. In the concluding chapter of this part (chapter 51), Schacter, Addis, and Buckner consider a recent and rapidly evolving literature that implicates memory as a key player in allowing individuals to think about and simulate possible happenings in the future. They discuss striking observations from neuroimaging and neuropsychological studies showing cognitive and neural overlap between the processes involved in remembering the past and imagining the future, and relate these observations to the idea that memory is a fundamentally constructive process, sometimes prone to errors and illusions. They consider the possibility that the flexible use of information from memory to simulate alternative future scenarios constitutes a key function of a constructive memory system.

The chapters in this section reveal expansions in both the depth and breadth of memory research, which bodes well for the future of the enterprise. We cannot know with any certainty what path memory research will follow in the upcoming years, but we can be confident that it will be exciting to find out.