THE DEVELOPING BRAIN REVEALED DURING SLEEP

Abstract

Given the prevalence of sleep in early development, any satisfactory account of infant brain activity must consider what happens during sleep. Only recently, however, has it become possible to record sleep-related brain activity in newborn rodents. Using such methods in rat pups, it is now clear that sleep, more so than wake, provides a critical context for the processing of sensory input and the expression of functional connectivity throughout the sensorimotor system. In addition, sleep uniquely reveals functional activity in the developing primary motor cortex, which establishes a somatosensory map long before its role in motor control emerges. These findings will inform our understanding of the developmental processes that contribute to the nascent sense of embodiment in human infants.

Keywords: REM sleep; brain rhythms; corollary discharge; embodiment; functional connectivity; myoclonic twitching; neurodevelopment disorders; sensorimotor integration; sensory development; somatosensory.

Figure 1.

The neural causes and consequences of myoclonic twitching in sleeping infant rats. Inset: Illustration of neural activity surrounding a twitch: preceding activity indicative of motor outflow (red), following activity indicative of sensory feedback (i.e., reafference; green), and nearly simultaneous activity indicative of corollary discharge (blue). These icons are used in the main figure below, which is based largely on studies using P8 rats. Main figure: The production of a forelimb twitch begins in midbrain structures, including the red nucleus (RN) and surrounding areas in the mesodiencephalic junction (MDJ). When a forelimb twitch is produced, reafferent signals flow directly to the spinal cord, the external cuneate nucleus (ECN), and the RN. From the ECN, reafferent signals flow to the deep cerebellar nuclei and cerebellar cortex (Cb) as well as the thalamus, arriving next in primary somatosensory (S1) and primary motor (M1) cortex. From S1, reafference flows via the entorhinal cortex to the hippocampus. Coincident with spiking activity in S1 and M1, spindle bursts are detected in the local field potential. Corollary discharges emanating from neurons in and around the RN project separately to the inferior olive (IO) and lateral reticular nucleus (LRN), before projecting to the cerebellum via climbing and mossy fibers, respectively. Finally, the occurrence of twitch-triggered bursts of coherent rhythmic activity between RN and hippocampus in the theta band (θ; 4–7 Hz) and between S1 and hippocampus in the beta2 band (β2; 20–30 Hz) is illustrated. Background image is a sagittal section of an infant rat brain. See text for discussion and citations.