Early Maturation of the Linguistic Dorsal Pathway in Human Infants

Abstract

Human infants, unlike even closely related primates, exhibit a remarkable capacity for language learning. Yet how the underlying anatomical network matures remains largely unknown. The classical view is that of a largely immature brain comprising only a few islands of maturity in primary cortices. This view has favored a description of learning based on bottom-up algorithms and has tended to discard the role of frontal regions, which were assumed to be barely functional early on. Here, using an index based on the normalized T2-weighted magnetic resonance signal, we have quantified maturation within the linguistic network in fourteen 1-to 4-month-old infants. Our results show first that the ventral superior temporal sulcus (STS), and not the inferior frontal area, is the less mature perisylvian region. A significant difference of maturation in the STS favoring the right side is an early testimony of the distinctive left-right development of this structure observed during the whole life. Second, asymmetries of maturation in Broca's area were correlated with asymmetries in the posterior STS and in the parietal segment of the arcuate fasciculus, suggesting that an efficient frontotemporal dorsal pathway might provide infants with a phonological loop circuitry much earlier than expected.

Syllabic discrimination in premature human infants prior to complete formation of cortical layers

“…Gene expression in the inferior frontal regions is more similar to other temporal and parietal speech areas than to other frontal areas (37). Leroy et al (34) observed a correlation in the maturation of the inferior frontal region (area 44), the posterior temporal region, and the arcuate fasciculus during the first weeks of life. These results suggest that the superior temporal, inferior parietal, and inferior frontal regions, although distributed across three lobes, belong to a single functional unit.…”

“…Earlier research already established that the left inferior frontal region, one of the key regions of the adult linguistic network, is already active during speech perception in postterm infants (3,34). However, it was suggested that its involvement might be secondary to infant's first vocal productions (35,36).…”

Left–Right Asymmetry of Maturation Rates in Human Embryonic Neural Development

“…TFs are proteins that regulate the expression levels of their target genes through binding to specific DNA sequence motifs that are located in the vicinity of their target genes in the genome (9,11). We used binding motif data for each of 615 motifs to define sets of TF target genes.…”

“…Left-right anatomical differences of the cerebral hemispheres have been detected by ultrasound scanning as early as 18 weeks postconception (8). In infants, the right hemisphere has been reported to develop functionally and anatomically earlier than the left hemisphere in various respects, including that left hemisphere language-related brain regions develop more slowly than their contralateral counterparts (9)(10)(11). These observations indicate a broadly lateralized program of motor and cognitive development that begins in utero in our species and suggest that a general model can involve different rates of maturation on the two sides of the central nervous system (CNS).…”

Micro-, Meso- and Macro-Connectomics of the Brain

“…The picture is clearer in newborns, where leftward asymmetry of the arcuate fasciculus has been observed at birth (Dubois et al 2009;Leroy et al 2011). Thus, even if hemispheric dominance corresponds to differences in cognitive processes, the structure of the two hemispheres follows the same anatomo-functional organizational rules.…”