Evidence on the emergence of the brain's default network from 2-week-old to 2-year-old healthy pediatric subjects

Abstract

Several lines of evidence have implicated the existence of the brain's default network during passive or undirected mental states. Nevertheless, results on the emergence of the default network in very young pediatric subjects are lacking. Using resting functional magnetic resonance imaging in healthy pediatric subjects between 2 weeks and 2 years of age, we describe the temporal evolution of the default network in a critical, previously unstudied, period of early human brain development. Our results demonstrate that a primitive and incomplete default network is present in 2-week-olds, followed by a marked increase in the number of brain regions exhibiting connectivity, and the percent of connection at 1 year of age. By 2 years of age, the default network becomes similar to that observed in adults, including medial prefrontal cortex (MPFC), posterior cingulate cortex/retrosplenial (PCC/Rsp), inferior parietal lobule, lateral temporal cortex, and hippocampus regions. While the anatomical representations of the default network highly depend on age, the PCC/Rsp is consistently observed at in both age groups and is central to the most and strongest connections of the default network, suggesting that PCC/Rsp may serve as the main "hub" of the default network as this region does in adults. In addition, although not as remarkable as the PCC/Rsp, the MPFC also emerges as a potential secondary hub starting from 1 year of age. These findings reveal the temporal development of the default network in the critical period of early brain development and offer new insights into the emergence of brain default network.

Fig. 1.

Spatial ICA identified default network components in each age group are shown. The anatomical locations of each group are labeled in the figure. Abbreviations: MPFC: ventral/dorsal medial prefrontal cortex; PCC: posterior cingulated cortex/retrosplenial; LTC: the lateral temporal lobe, HF: the hippocampus formation; IPL: inferior parietal lobe; PHC: parahippocampal cortex; ACC: anterior cingulate cortex; InfTemporal: inferior temporal cortex; SupTemproal: superior temporal cortex; MedParitetal: medial parietal cortex; LatParietal: lateral parietal cortex; MidFrontal: middle frontal cortex.

Fig. 2.

The brain's default networks in all 4 age groups. Z score maps (Z > 1) are mapped on to the template brain surface for each individual group. For the pediatric groups, although multiple components were chosen, they were pooled together to show on the same brain surface (Z scores showed here is taken as the maximum from different components).

Fig. 3.

(A) Functional connectivity graphs for all 4 age groups. The most strongly connected regions are clustered near each other while weakly correlated regions are placed further away from each other. The width of the line between 2 nodes is proportional to the corresponding connection strength. Only significant correlations (P < 0.05) were plotted. (B) Bar plots of the degree of connection for each node in a descending order (the ratio of the number of regions connected to a specific region to the total possible connections).

Fig. 4.

(A) Mean connection strength of each node for all age groups. The bars indicate the mean connection strength averaged over the corresponding group and red asterisks represent the values of individual subjects. (B) Regression results for the connection between MPFC and PCC.

Fig. 5.

Betweenness centrality measures for individual ROIs of the 3 pediatric groups, (A) neonates; (B) 1-year-old; and (C) 2-year-old.