Characterising sleep spindle emergence in infancy: Effects of prematurity and early locomotor intervention

Study objectives: Sleep spindles are thalamocortical oscillations evolving during early brain maturation. In adults, they support memory consolidation and are organised into rhythmic "trains" thought to support memory reactivation. How this temporal organisation emerges in infancy, and whether prematurity or early sensorimotor experience shapes it, remains unclear. This study characterised infant spindles and investigated how prematurity and early locomotor training affect their temporal and spatial organisation, providing insight into early brain maturation. Methods: Daytime high-density EEG sleep recordings were analysed in 50 infants born full-term (n=12) or very preterm (n=38, including 12 who received locomotor training) at 2 months corrected age. Spindle temporal organisation was analysed using a two-step Morlet wavelet transform to quantify slow modulations of spindle-band power (11-16Hz). Time-frequency decomposition around spindle onset was used to identify significant spindle-band scalp-wide activity, and topographical maps were statistically compared between groups. Results: All groups showed rhythmic spindle organisation, three times slower than in adults, with no group differences. Untrained preterm infants showed a more diffuse spindle-band activity than the distinct frontal and central clusters observed in full-term infants. Trained preterm infants exhibited a spatial distribution closely resembling that of full-term infants, although their frontal spindle density was lower than in both other groups. Conclusions: Spindle rhythmic organisation emerged early in infancy and remained unaffected by prematurity or locomotor training. Prematurity was associated with more diffuse spindle-band activity, suggesting reduced cortical specialisation, whereas trained preterm infants showed a spatial distribution resembling that of full-term infants, suggesting that early locomotor experience may influence thalamocortical organisation.