The power spectrum density of the whole brain gray matter

The power spectrum density of the whole more info gray matter (Fig. 2) showed significant and distinctive developmental patterns for the two typically defined frequency bands, i.e., the low frequency band (f<0.1Hz) and the high-frequency band (f>0.1Hz). Within the low-frequency band, neonates exhibited significantly greater power between and 0.0167Hz than both 1- and 2-year-olds. However, 1- and 2-year-olds showed increased power in the frequency range of 0.0278–0.0333Hz, resulting in a clear right-ward shift in the peak-frequency with age: the PSD peaked at 0.0056Hz for neonates and 0.0278Hz for 1- and 2-year-olds (Fig. 2A, Table 1). Additionally, 2-year-olds showed decreased power in the frequency range 0.05–0.0611Hz compared to both neonates and 1-year-olds (Fig. 2A, Table 1). In the high-frequency band (>0.1Hz), the spectral power increased in both older groups compared with neonates within the frequency range of 0.1167–0.1389Hz (Fig. 2A, Table 1). The changes in the PSD of the BOLD signal after global signal regression (Fig. 3A) and after high-pass filtering of the BOLD signal (Fig. 3B) were highly consistent with the original results, showing a clear rightward shift in the peak-frequency during development. The voxel-wise spectral power at a frequency of 0.0056Hz (i.e., the peak-frequency for neonates) and 0.0278Hz (i.e., the peak-frequency for 2-year-olds) was visualized on brain surfaces for the three age groups in Fig. 2B and C. It is immediately clear that, at the whole brain level, neonates showed much higher power at the 0.0056Hz frequency than both 1- and 2-year-olds while the two older age groups demonstrated much higher power at the 0.0278Hz frequency than did neonates. Quantitative comparisons showed that the decrease of power in 0.0056Hz during the first year is largely globally uniform (Fig. 2D) but the increase of the 0.0278Hz power in the two older groups is less so (Fig. 2E). Particularly, the increase is largely focused in the posterior part of the brain, with the medial and lateral prefrontal regions largely void of significant increases (Fig. 2E). The changes in both frequencies during the second year of life were minimal. The developmental patterns of the PSD for the nine functional networks were highly consistent with that of the whole brain (Fig. 4A and B; Table 1). Peaks of all the explored networks were consistently located in 0.0056Hz for neonates and between 0.0278 and 0.0333Hz for 1- and 2-year-olds (Fig. 4A). Quantitatively, neonates exhibited significantly greater power than both 1- and 2-year-olds between and 0.0167Hz (Table 1) while the two older groups showed greater power between 0.0278 and 0.0389Hz (Fig. 4A, Table 1). For the gray matter, sensorimotor, default mode, salience and right frontoparietal networks, 2-year-olds exhibited additional decreases in the PSD for frequencies within the range of 0.0333–0.0611Hz. In addition, the visual networks showed significant decreases between 0.2 and 0.25Hz in the older groups. For most networks, older groups exhibited scattered increases between 0.1 and 0.15Hz. Consistent with Fig. 2E, the age-related increases in PSD at the 1/2-year peak-frequency (i.e., 0.0278Hz) was the strongest for the lateral visual, sensorimotor, auditory and default mode networks, followed by the medial visual and occipital poles networks while the frontal-lobe-centered bilateral frontoparietal and salience networks consistently showed the least increase for both 1- and 2-year-olds when compared with neonates (Fig. 4C). Finally, the individual Mullen Scale scores showed positive correlations with the PSD in the peak-frequency (i.e., 0.0278Hz; Fig. 5) in 1-year-olds. Specifically, the power of peak-frequency in the sensorimotor and lateral visual networks showed positive correlations with Fine Motor (r=0.365, p=0.018, bootstrapping confidence interval (c.i.): [0.058, 0.567]) and Visual Perception (r=0.339, p=0.028, c.i.: [0.085, 0.604]) scores, respectively. Although not significant after multiple comparisons correction, the confidence intervals established based on bootstrapping support their significance. The corresponding analyses in 2-year-olds showed no significant correlations.