Reading in dyslexia across literacy development: A longitudinal study of effective connectivity

Abstract

Dyslexia is a literacy disorder affecting the efficient acquisition of reading and writing skills. The disorder is neurobiological in origin. Due to its developmental nature, longitudinal studies of dyslexia are of essence. They are, however, relatively scarce. The present study took a longitudinal approach to cortical connectivity of brain imaging data in reading tasks in children with dyslexia and children with typical reading development. The participants were followed with repeated measurements through Pre-literacy (6 years old), Emergent Literacy (8 years old) and Literacy (12 years old) stages, using Dynamic Causal Modelling (DCM) when analysing functional magnetic resonance imaging (fMRI) data. Even though there are a few longitudinal studies on effective connectivity in typical reading, to our knowledge, no studies have previously investigated these issues in relation to dyslexia. We set up a model of a brain reading network involving five cortical regions (inferior frontal gyrus, precentral gyrus, superior temporal gyrus, inferior parietal lobule, and occipito-temporal cortex). Using DCM, connectivity measures were calculated for each connection in the model. These measures were further analysed using factorial ANOVA. The results showed that the difference between groups centred on connections going to and from the inferior frontal gyrus (two connections) and the occipito-temporal cortex (three connections). For all five connections, the typical group showed stable or decreasing connectivity measures. The dyslexia group, on the other hand, showed a marked up-regulation (occipito-temporal connections) or down-regulation (inferior frontal gyrus connections) from 6 years to 8 years, followed by normalization from 8 years to 12 years. We interpret this as a delay in the dyslexia group in developing into the Pre-literacy and Emergent literacy stages. This delay could possibly be detrimental to literacy development. By age 12, there was no statistically significant difference in connectivity between the groups, but differences in literacy skills were still present, and were in fact larger than when measured at younger ages.

Longitudinal changes in reading network connectivity related to skill improvement

“…This idea is supported by the general decreases in connectivity found in typical children in the Morken et al 2017 study. The current study builds on previous literature by taking a unique approach to examine the longitudinal changes of the reading network in typically developing children.…”

“…inferior frontal, temporo-parietal, and occipito-temporal cortex) compared to their typically reading peers (Pugh et al, 2001; Richlan, Kronbichler, & Wimmer, 2009, 2011; Shaywitz et al, 2002). Moreover, research using functional connectivity methods has also found that individuals with dyslexia differ in how well these regions work in tandem with one another (Cao, Bitan, & Booth, 2008; Finn et al, 2014; Horwitz, Rumsey, & Donohue, 1998; Koyama et al, 2013; Morken, Helland, Hugdahl, & Specht, 2017; Quaglino et al, 2008; van der Mark et al, 2011). Together, research suggests dyslexia involves more than an underactivation of key brain regions in typical left hemisphere reading networks, but also reduced functional connectivity between them.…”

“…One study has examined changes in reading network connectivity longitudinally in Norwegian children with and without dyslexia (Morken et al, 2017). In this study, typically developing children were found to show primarily decreasing or stabilizing connectivity over time, while children with dyslexia showed aberrant patterns of development that may have reflected overcompensation or normalization relative to their baseline connectivity levels.…”

“…In this study, typically developing children were found to show primarily decreasing or stabilizing connectivity over time, while children with dyslexia showed aberrant patterns of development that may have reflected overcompensation or normalization relative to their baseline connectivity levels. However, the study by Morken et al (2017) does not examine skill within typically developing children, and does not examine the relationship between changes in connectivity to changes in reading skill. Further, Norwegian is a semi-transparent language, while English is opaque.…”

Mechanisms of interactive specialization and emergence of functional brain circuits supporting cognitive development in children

“…There have only been six longitudinal developmental fMRI studies of task-related activation, [16][17][18][19][20] with only one study examining changes in brain connectivity. 21 Furthermore, extant studies of growth trajectories using longitudinal designs have focused on regional changes in brain response. 20,22 Critically, no previous studies, either in the perceptual or the cognitive domains, have examined how specialized functional circuits emerge in children over time, and no studies have probed longitudinal changes in task-related functional circuits, which is necessary for identifying mechanisms underlying human neurocognitive development.…”