Mark Blumberg : Developing the sensorimotor system in our sleep

How are the “rudimentary” movements of fetuses and infants transformed into the coordinated, flexible, and adaptive movements of adults? Some believe that adult behaviors are built from “motor primitives,” endowed units of behavior, hardwired in the spinal cord, that arise fully formed without the need of experience. In contrast, numerous developmental psychologists and roboticists have emphasized how infantsdiscover how their bodies are built, how they change in size and shape, and how they move. Not surprisingly, this process of discovery has been presumed to occur only when infants are awake, reflecting the common wisdom that sleep is a period of sensory isolation and behavioral stillness. In fact, over the course of a night we move a lot. Some movements occur during brief awakenings when we roll over or throw off the covers. But during active (or REM) sleep, a very different sort of behavior occurs: Every skeletal muscle in our body twitches, causing jerky movements of the arms and legs, fingers and toes, eyes, and (in rodents) whiskers. These movements are particularly prominent during the perinatal period when active sleep predominates. Although considered for centuries to by-products of dreams (think dogs “chasing rabbits”), research in infant rodents over the past decade has revolutionized our conception of what causes these limb movements, how sensory feedback from twitching muscles is a primary driver of neural activity in the brainstem, cerebellum, hippocampus, and cerebral cortex, and how these “by-products” may actually participate in the self-organization of the sensorimotor system. This new view of twitching has important implications for our understanding of typical and atypical development, for recovery of function after injury or disease, and for detecting neurodevelopmental disorders earlier than is currently possible using standard clinical assessments.