SLEEP AND CORTICAL MATURATION: SLOW AND FAST SLEEP SPINDLES IN THE FIRST 4 YEARS OF LIFE

Introduction: From birth up to adulthood, important maturational changes in brain morphology and function involve different cortical areas with different timing, mainly following a posterior-anterior trajectory (Shaw et al., 2008). According to the notion that sleep plays a key role in cortical plasticity, regional changes of cortical activity during sleep seem to mirror the evolution of local cortical maturation. Indeed, a postero-anterior shift of SWA activity maxima has been demonstrated from childhood until late adolescence (Kurth et al., 2010). Moreover, we recently showed a similar postero-anterior maturational pattern for the upper alpha activity (~11 Hz) during NREMacross the first four years of life (Novelli et al., 2016). We hypothesized that this finding could correspond to the emergence of slow sleep spindles after the first year of life. Accordingly, we re-analysed the pool of data, specifically assessing the maturational trajectory of slow and fast sleep spindles. Materials and methods: The sleep spindles were automatically detected from the NREM sleep EEG recordings (12 cortical sites) of 39 healthy, fullterm, infants and children aged between 0 and 48 months. The participants were divided in 4 groups according to their age (G1: 0e3 mo.; G2: 4e12 mo.; G3: 13e24 mo.; G4: 25e48 mo.). The density of slow sleep spindles (11e13 Hz) and fast spindles (13e15 Hz) for each scalp location was compared between age groups by one-way ANOVAs. A correlation analysis with age was performed, separately for the two type of spindles. Results: The analysis of fast and slow spindles reveals different maturational trajectories. The density of slow spindles on frontal areas progressively increases across age groups and it is significantly different between >2 years children and the younger ages, while the density of fast spindles peaks at age 4e12 months over centro-frontal sites, with a significant reduction at older ages. Only the density of slow spindles shows extensive significant and positive correlation with age, with a clear posterior-anterior gradient. Conclusions: The results confirm that the frontal pattern of the upper alpha activity reported in Novelli et al. (2016) mostly corresponds to the slow spindles. Moreover, our analyses point to different age trajectories for fast and slow spindles, with slow spindles progressively increase across age groups, while fast spindles decrease. Therefore, slow spindles follow a genuine maturation process, while this does not seem confirmed for fast spindles. Since it has been hypothesized that slow oscillations and sleep spindles in infants serve to promote the formation of the thalamocortical network (Jenni et al., 2004), we suggest that the fast spindles detected at the age 4e12 months represent an immature antecedent of spindles, when the thalamocortical circuit is still in development. The emergence of frontal slow spindles could represent the first turning point of the thalamocortical network maturation in infancy, as well as the establishment of classical parietal fast spindles in adolescence are thought to represent its fulfillment.