Neurodevelopment of semantic systems in reading

Abstract

 The human brain is a dynamic organ that experiences a sequence of salient changes throughout the life span. The development of reading skills is continuous and gradual, and a series of significant changes in the brain networks and the acquisition of cognitive abilities are plausibly synergistic and interactive processes in the course of schooling. Previous neuroimaging studies have well-identified cortical substrates that are responsible for matured semantic processing. However, the relationship between neural correlates and age is unknown, and how neural correlates accommodate semantic processing development is unsure. To fully delineate the developmental changes in the neural substrates for semantic processing associated with age, we conducted a cross-sectional life span functional magnetic resonance imaging (fMRI) study by scanning subjects whose ages ranged from 6 years to 75 years old. Semantic judgment and font-size judgment are the two experimental tasks that were devised in this study. In study 1, we compared the differences of neural networks for semantic representations between children and adults. The main finding was that children showed greater activations than adults in the bilateral ventral occipitotemporal cortex (vOT) and left precentral cortex, whereas adults showed greater activations than children in the parietal cortices during semantic decision. Differential cortical activities are demonstrated in the visual–spatial analysis of Chinese characters in children and adults’ semantic processing. The second finding was that children showed greater activation than adults in the bilateral insula and right cingulate gyrus for semantic judgment. The second finding suggests that the specialization of brain regions is involved in filtering and denying some uncertain and inaccurate information in children. In study 2, we identified age-related neural correlates for semantic processing by correlational analysis. Age was found to be significantly correlated with activities in bilateral cortical regions for reading, especially in the left inferior temporal gyrus, left precuneus, right middle temporal gyrus, and bilateral insula cortex. Age correlated positively with the activities in the left inferior temporal gyrus, left precuneus, and right middle temporal gyrus, whereas age correlated negatively with the activities in the bilateral insula cortex. Several potential explanations for age-related effects on the activation of semantic neural systems are possible. One explanation is that semantic processes may be identical across ages. However, the entire semantic system is not maturing at the same rate. With increasing age, regression of cortical areas or automatic processing correspondingly and synergistically occurs and induces the increase and decrease of the activities in the related regions. An alternative explanation is that essential differences of semantic processes possibly exist in different age groups. Experience may modulate semantic networks. As less fluent readers, children have not yet incorporated the processing resources of some critical regions that subserve semantic processes into a strategy for performing semantic decisions. In adulthood, increased or decreased activations in the determined brain areas concomitantly occur and support the developmental strategy of semantic processing. Another possibility is the heritability of semantic systems. Neural networks were determined genetically; however, experience or maturation affects the developmental connectivity of the distributed cortical regions.