REM Illumination Memory Consolidation

TIM WALTER OFFERS A CONCISE, DIGESTIBLE SYNTHESIS OF DECADES OF WORK ON SLEEP AND MEMORY, MAKING REM ILLUMINATION A MUST-READ FOR those in the field and those interested in it. Experts will pick up tidbits of information they did not know or had not put together, such as the fact that two big events that seeded the field of sleep and memory occurred the same year, 1953, when patient H. M. underwent his historic memory-altering bilateral hippocampal resection, and Aserinsky and Kleitman discovered REM sleep. For 40 years a handful of researchers advanced sleep and memory until the field began to grow in earnest. Walter proffers a view on which most learning and memory researchers agree: that many different lines of evidence, from human clinical cases to controlled animal experiments, strongly indicate that the REM sleep state or its component traits are involved in memory consolidation; and that NREM sleep, especially the spindles of stage 2 and slow waves of stage 4, is also involved. Walter cites most of the relevant work, knitting together as coherent a story as can be had at this time. It is clear that an enthusiast wrote this book although Walter well justifies his enthusiasm and acknowledges some dissenting arguments, such as the fact that depressed persons taking REM-inhibiting antidepressants do not show profound learning or memory deficits. Walter seems particularly fascinated with the importance of emotion in dreaming and memory processing, bringing to light research that begins to point the way. Although the relationship between emotion, dreams, and learning is even less well studied to date than the general field of sleep and learning, research in this area is beginning to grow, for example, in the human labs of Walker at UC Berkeley and Born in Germany. In an impressive manner, Walter knits together the systems and behavioral work of Smith, Hennevin, and Leconte; the cellular activity studies of McNaughton, Wilson, Buzsaki, and Eichenbaum; and the human brain and behavioral investigations of Stickgold, Walker, Born, and Maquet. He also discusses the exciting developmental work of Shafferey and Roffwarg, although he omits the important contribution of Marcos Frank which demonstrates that sleep deprivation impairs plasticity in the developing visual system1. Walter does not forget, however, the intriguing contributions made recently by Margoliash (and now others) on the importance of sleep for the development of song in birds. He also nicely relates this basic research to sleep disorders. This is important because if sleep is essential to synaptic restructuring after learning, it should be especially critical during experience-dependent brain development, making it crucial to treat early childhood sleep disorders promptly. REM Illumination is especially useful to readers from a variety of backgrounds since Walter makes the physiological data he presents easy to synthesize. The fine theoretical contributions of Krueger and Obal, Lee Kavanau, and a very influential early review by McGrath and Cohen (1978) are included; their discussions and reviews have helped broaden our definition of sleep in the service of synaptic plasticity from the level of the cortical column to the behaving human. The book does not go quite far enough into the physiology behind the function of sleep for learning for my tastes. He may treat physiology lightly because it could, as he puts it, “bore or perplex the average reader.” But mechanism is at the heart of understanding, and fleshing out these details could resolve the current debates concerning the roles of various facets of sleep for the function of learning. For example, Jonathan Winson published a book2 in 1985 which presents many of the concepts of REM sleep and theta involvement in memory consolidation now found in most of the research publications concerning REM sleep and learning. Threads of Winson's ideas provide hidden substance to the fabric of Walter's book. Indeed, Winson's research with Pavlides opened the gates to measuring physiological processes at work during sleep on the single cell level. Some genetic and electrophysiology/behavior studies published since 2000 that are not mentioned in Walter's book point to an essential role for sleep in bidirectional synaptic plasticity which is key to efficient learning. Such studies include the discovery by Ribeiro and Pavlides3 that genes for synaptic strengthening are selectively activated in REM sleep after learning in those very structures involved in the consolidation of the memory. Findings from many different labs show that REM sleep deprivation induces deficits in establishing the building block for memories: long-term potentiation (LTP)e.g.,4. Datta published a series5 demonstrating the importance of P-waves (a.k.a. PGO waves, expressed during REM and the transition to REM) for consolidation. On the other side of bidirectional plasticity, Cirelli and Tononi found under non-learning conditions that genes for synaptic weakening are heightened during sleep relative to wakefulness6. And we have found that reactivation occurs after learning during REM sleep which allows both selective strengthening and weakening in separate networks of synapses, depending on the consolidation state of the specific memory7. These publications point to a unique role of sleep in synaptic pruning as well as strengthening to remodel networks during memory consolidation. This level of fine mechanistic detail points to a critical role for sleep that is not specifically addressed in Walter's book. Criticisms aside, I found this book surprisingly entertaining and informative. Walter makes it easy for those interested in the subject to find the original works for themselves since he conveniently places all citations at the end of each chapter. Thus, it is a handy synthesis and reference work for professionals—physicians, psychologists, and scientists alike, and accessible to “almost anyone with reasonable intelligence.” I admire this effort and will keep REM Illumination in my bookshelf with passages underlined. I will also assign it, along with Winson's book, as reading for my lab so that our work is properly informed by ideas in the context of history.