Abstract:

Children who are assessed to be more likely to develop dyslexia before school are called children at risk of developmental dyslexia. At-risk children have not received reading teaching, and the neurological abnormalities related to reading ability before learning to read will not be the result of reading experience. Therefore, investigating the neurological abnormalities of at-risk children is helpful to find the early neural markers of dyslexia, which is great important for early prediction and intervention.

Research based on horizontal comparison shows that the brain function and structure of at-risk children are abnormal. In view of brain function, the electrophysiological mechanism of language and non-language processing in children at risk is abnormal. Perceived speech and non-speech induced mismatch response waves are smaller in amplitude and longer in latency. Children at risk have abnormalities in brain areas for verbal and nonverbal processing. Under-activation of regions such as bilateral temporoparietal, bilateral inferior frontal, and bilateral middle frontal gyrus during phonological tasks. Under-activation of bilateral temporoparietal and occipitotemporal regions during a word processing task. Under-activation of auditory processing areas such as the left hemisphere prefrontal and left temporal lobes during the perception of non-verbal stimuli.

In view of the brain structure, the gray matter in the temporal parietal region and occipital-temporal region of at-risk children is small. And its sulcal pattern also has atypical manifestations. At the same time, at-risk children did not show the trend of left temporal plane. On the white matter connection, the FA values of the left arcuate tract and the left inferior frontal occipital tract are smaller than those of typical developing children.

Children at risk may not necessarily develop dyslexia, and their brain abnormalities may only be the effect of risk factors. Only by longitudinal tracking to determine whether at-risk children will develop dyslexia can we find the neural changes related to reading development and reveal the early neural markers of dyslexia. Longitudinal research shows that MMR induced by speech processing, abnormal function of left temporal parietal region, visual word-shaped region and abnormal structure of left arcuate tract can distinguish whether at-risk children develop dyslexia, which is an early biological marker of dyslexia.

Exploring neurological abnormalities of preschool-age children at risk for developmental dyslexia (DD) is useful for identifying early neural markers of dyslexia. The body of research is important for early prediction and intervention of dyslexia. Cross-sectional studies among children at risk for DD show abnormalities in brain function and structure: Mismatch Responses (MMR) induced by speech and non-speech auditory perception in children at risk for DD have smaller amplitude and longer latency; there are functional and structural abnormalities in the ventral and dorsal pathways of reading. Compared with preschoolers, the research in school-age children shows neural changes associated with reading development, revealing early neural markers of dyslexia. Longitudinal studies show that anomalies in the MMR during speech processing, dysfunctions in the left temporoparietal, occipitotemporal and Visual Word Form Area, and structural abnormality in the left arcuate fasciculus could serve as early markers of DD. In addition, longitudinal studies of the brains of children at risk for DD are rare, and small samples may reduce the reliability of the results. This warrants longitudinal studies with larger samples in the future. Lastly, more future studies should focus on the neural basis of Chinese children at risk for dyslexia to uncover the uniqueness and universality of cognitive neural risk factors for Chinese dyslexia.

Although some conclusions have been drawn from the current longitudinal research, the longitudinal research on the brains of at-risk children is relatively rare. And the limited sample size will amplify the influence of individual differences and lead to the decline of the reliability of the results. In the future, a longitudinal study based on a large sample is needed to examine the neural development track of reading and verify the current conclusions. A feasible solution is to combine data from different laboratories to improve the statistical power and reliability of the results. Secondly, a large number of researchers have paid attention to the group of children at risk, but the current research mainly focuses on alphabetic language, lacking evidence from children at risk of Chinese dyslexia. In the future, we can also compare the patterns of neurological abnormalities between children at risk of Chinese and alphabetic language. And exploring the particularity and universality of cognitive neurological risk factors for developmental dyslexia in Chinese. Children's early language skills are highly plastic. If neural indicators can be used for early screening of dyslexia, early prediction and intervention can be made to help at-risk children improve their reading ability and reduce the occurrence of overt dyslexia.

Key words: developmental dyslexia, children at risk for developmental dyslexia, abnormalities in the brain, brain structure and function