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    • Similarly we were surprised that

    2018-10-25

    Similarly, we were surprised that the amygdala did not emerge in the age-related iFC analyses as a region of interest. VS activity would be modulated by the amygdala, itself under the control of cortical inputs (Ernst and Fudge, 2009). This finding may be particularly important for emotion regulation and impact on behavioral responses during development (Hulvershorn et al., 2014). While the primary VS as well as DS iFC maps do extend into portions of the amygdala, the VS-age clusters that covered the insula did not approach or encompass the amygdala region. Likewise, no age findings were evident in extracted ROI data for the amygdala (post hoc analysis; data not shown). Collectively, the present findings can also be viewed through the lens of the canonical resting state networks (Damoiseaux et al., 2006; Fox et al., 2005; Laird et al., 2011; Smith et al., 2009), particularly the salience network (Menon and Uddin, 2010) and the default mode network (DMN; Raichle et al., 2001), of which insula and pCC are major hubs, respectively. From this perspective, adolescents are expected to show a stronger VS connection to the salience network, but weaker DS connection to the DMN network. These network dynamics need to be tested more closely using longitudinal designs and corroborated with behavioral measures.
    Conflict of interest statement
    Acknowledgement
    Introduction One of the major achievements of human fetal AP20187 development is the establishment of functional neural networks. Detailed understanding of normal processes of brain circuit formation is essential for discovering consequences of injury/insult, periods of vulnerability, and for early detection of clinically significant neural anomalies. The bulk of what is known about human brain connectional ontogeny comes from histological and magnetic resonance imaging (MRI) studies performed on post-mortem fetal brain specimens. These histological and MRI studies have shown that transient physical structures serve as scaffolding for axonal proliferation and cell migration (Kostovic and Jovanov-Milosevic, 2008) and that, in harmony with brain gyrification, long-range association pathways begin to be established during the second half of gestation (Kostović and Jovanov-Milosevic, 2006; Takahashi et al., 2012). Until recently, quantification of neuroconnectivity during the fetal period has proven elusive for human in vivo brain research. However, recent advances in diffusion and resting state MRI have provided new means for non-invasive study of neural circuit formation in the perinatal period (Mailath-Pokorny et al., 2012). Tractography analyses of fetal brain diffusion tensor imaging data has demonstrated emergence of white matter pathways in the transient intermediate zone, splenium, and genu, consistent with in vitro findings (Kasprian et al., 2008). Resting-state functional MRI (rs-fMRI) applied to prematurely born human infants has shown that, as the preterm neonate grows, neural circuit functional activity becomes increasingly synchronized (Smyser et al., 2010) and that intrinsic connectivity networks (ICNs) are largely formed by 42 weeks postmenstrual age (Doria et al., 2010). However, preterm neonates are not ideal substitutes for learning about typical fetal brain development, because the underlying reason for preterm delivery and environmental factors cannot be disentangled from neurological observations in these cases. Important questions remain about large-scale neural organization, and establishment of ICNs prior to birth. Recent investigations have established the utility of rs-fMRI for quantifying neural network development in utero. The first fetal rs-fMRI study was performed in pregnant women referred to MRI for suspected neurological anomalies discovered during fetal sonography. The investigators qualitatively compared functional connectivity (FC) networks in 16 fetuses that had healthy MRI results and minimal movement. They concluded that in the majority of cases temporal lobe networks were lateralized, whereas frontal and occipital networks were bilateral (Schöpf et al., 2012). Subsequently, two human fetal FC studies have been performed, assessing fetuses recruited as part of research studies, rather than as part of clinical follow-up. Statistical group-level comparisons of FC in 25 healthy fetuses demonstrated that cross-hemispheric connectivity increases with advancing postconceptual age, and that midline cross-hemispheric connectivity precedes lateral connectivity (Thomason et al., 2013). Graph theoretical analyses in 33 fetuses revealed that synchrony of activity across neural networks is increased in older, compared to younger, fetuses. In addition, connectivity between the posterior cingulate cortex (PCC) and other brain areas becomes more negative with increasing postconceptual age (Thomason et al., 2014). These studies provided the first indication that human fetuses possess basic forms of neural connectivity networks and that rs-fMRI may become instrumental for determining principals of human FC in utero.