Memory Processing in Relation Chapter to Sleep Philippe Peigneux and Carlyle Smith 29 Abstract reexpression of behavior-specific neural patterns during post- training sleep, and still others have probed the effects of Although each of us empirically recognizes the utmost impor- within-sleep stimulation on sleep patterns and overnight mem- tance of sleep for the quality of our everyday life, its functions ories. Besides-demonstrating a role of sleep in memory con- have long remained shrouded in mystery. Beyond its putative solidation processes, these studies have also indicated that physiologic functions, there is now growing evidence that sleep stages are functionally different, in that they may sub- sleep plays a prominent role in brain plasticity and in memory- serve distinct memory processes. Available evidence reveals consolidation processes. According to this proposal, memory complex interactions between sleep and memory systems, in traces formed during a learning episode are not immediately agreement with the fact that memory is a complex construct stored in their definitive form. Rather, they are initially kept of specialized memory subdomains, and that sleep is com- in a labile, fragile state, during which they can be easily dis- posed of distinct stages characterized by specific physiologic rupted. Over time and especially during sleep, they subse- mechanisms. Despite advances that have refined our under- quently undergo a series of transformations during which standing of the relationships between sleep and cognitive pro- they will be consolidated and fully integrated into long-term cesses, the underlying mechanisms still remain to be fully memory. In this chapter, we present experimental data that elucidated. Further steps are now required to understand how provide support for the hypothesis that sleep exerts a promot- sleep disorders and pathologies accompanied by sleep distur- ing effect on plastic processes of memory consolidation. Some bances affect cognitive functions, and especially learning and studies have assessed the effects of posttraining sleep depriva- memory consolidation in humans, eventually leading to reme- tion on memory consolidation and on the reorganization of dial interventions. the neural substrates of long-term memories. Others have investigated the effects of learning on posttraining sleep and In 1867, Hervey de Saint Denys, fascinated by his dreams Before approaching the subject, however, we will briefly since the age of 14, published Les Rêves et les Moyens de les introduce the concepts of memory consolidation and Diriger.1 In his book, he reported a series of ingenious memory systems in humans. Previous chapters have shown experiments showing that experienced events are incorpo- that sleep is a multidimensional state of vigilance, com- rated into our dreams, in which they can be combined to posed of rapid eye movement (REM) and non-REM create original associations between “memory images” of (NREM) episodes that present distinctive features and rely the past. Hence, he strongly opposed the idea that sleep on specific neuroanatomic substrates.4 From both cogni- may be a sudden drop in a state of cognitive “non-being” tive and neurophysiologic perspectives, memory is not a in which our resting brain is disconnected. On the con- unitary phenomenon. Rather, memory should be seen as a trary, he claimed that “a sleep without dreams cannot exist, generic concept for information storage, encompassing just as a wake state without mentation does not exist.” a series of specific subdomains.5 Consequently, the interac- Besides dreaming activity, however, addressed in Section tion between multidimensional states of sleep and distinc- 7 of this book, we know that the sleeping brain houses a tive memory systems makes it logical that not all sleep wide set of cognitive processes, including the ongoing manipulations will have the same impact on performance, treatment of elaborated external stimuli, the revival of depending on the various parameters embedded in the experiences, and last, but not least, the consolidation of memory task.2,6,7 We will examine the growing number of new information in memory. Interestingly, recognition behavioral studies that have enlightened our understand- that persistence of mental activity in the sleeper may be an ing of the role played by sleep episodes in memory integral part of the physiologic processes that subtend consolidation. The rapid evolution of neurophysiologic memory consolidation arose only in the last quarter of the techniques and analytical approaches allows scrutiny of the 20th century. The hypothesis had to wait till now to receive complex relations between cognitive processes and their widespread acknowledgement, as illustrated by the fact underlying neural substrates. Part of this chapter will focus that for the first time a chapter devoted to sleep in relation on the neurophysiologic mechanisms acting in sleep to to memory has been introduced in this fifth edition of support, or at least favor, memory consolidation processes. Principles and Practices in Sleep Medicine. Molecular biology of memory consolidation is beyond the This chapter introduces the issues surrounding the role scope of this chapter and is reviewed elsewhere.4 The rela- that sleep may play in memory consolidation, focusing on tionship between consolidation of learning and sleep is a human data. Reviews about relationships between sleep crucial issue because memory is at the root of most of our and memory in animals are addressed can be found else- daily behaviors, such as simple skill acquisition (e.g., type- where.2-4 We will here outline key findings and milestones writing), sophisticated operational procedures (e.g., using in probing the sleep-for-memory hypothesis, as well as computer-based systems), and keeping track of personal ongoing debates and thoughtful questions that remain. events and relationships. L 335 Kryger_6453_Ch 29_main.indd 335 8/24/2010 7:23:50 PM 336 PART I / Section 4 • Physiology in Sleep MEMORY SYSTEMS AND MEMORY Distinctive features of nondeclarative memories are that they are not easily accessible to verbal description and can CONSOLIDATION be acquired and reexpressed implicitly. Thus, our behav- We are able to learn, store, and remember various types ioral performance can be affected by the new memory even of information in different ways and for variable periods if we are not consciously aware that new information has of time, from conscious acquisition strategies to incidental been encoded or is being retrieved. Memory abilities detection of environmental events. Cerebral damage can aggregated under the nondeclarative label also gather dif- selectively alter some of these processes while leaving ferent forms: skills, habits, priming, and conditioning. others undisturbed. These simple observations have led to Although grouped under the nondeclarative label, these the proposal that memory is not a unitary phenomenon various processes are subtended by distinct neuroanatomic but rather a complex construct of more or less specialized substrates in both humans and animals, further suggesting memory subdomains.5 First, a seminal distinction that their relative independence.5 dates back to William James is made between short- and Besides the transverse division between long-term long-term memory stores. The former is dedicated to the memory subsystems proposed by these influential models, temporary storage of volatile information for up to seconds, there is a dynamic longitudinal process. Newly acquired whereas the latter, the main focus in this chapter, houses information is not immediately stored at the time of potentially lasting information that is deemed consolidated learning in its final form, if such a stable state exists. and less susceptible to disruption. Long-term memories in Rather, memories undergo a series of transformations humans may further belong to multiple systems, primarily over hours, days, or even years, during which time they delineated between declarative and nondeclarative memo- are gradually incorporated into preexisting sets of mne- ries (Fig. 29-1). monic representations,8-10 or are subjected to forgetting.11 Distinguishing features of declarative memory are that This is the concept of memory consolidation, which can information is easily accessible to verbal description and be defined as the time-dependent process that converts that encoding or retrieval is usually carried out explicitly— labile memory traces into more permanent or enhanced that is, the subject is aware that the stored information forms.12 These transformations are made possible by our exists and is being accessed. Declarative memory is further brain plasticity—that is, the capacity of the brain to modify composed of semantic and episodic memory components. its structure and function over time, within certain bound- Semantic memory is the receptacle for our general knowl- aries.13 Eventually, time-dependent processes of consolida- edge about the world, regardless of the spatiotemporal tion and the ensuing robust memory trace will enduringly context of knowledge acquisition (e.g., we know that Paris adjust the behavioral responses to the recent environ- is the capital of France, and that fuel is combustible, but mental changes, thereby enlarging the organism’s behav- we are probably unable to recollect how and when we ioral repertoire.14 learned these facts). Episodic memory, on the other hand, Scientific evidence suggests that sleep and the associated refers