Connectome-Wide Differences in Brain Organization Associated with Reading Ability

Post by Elisa Guma

What's the science?

Reading requires the ability to quickly and accurately recognize and map certain language components (e.g., phonological, orthographic, and semantic) to be able to understand written text. However, as many as 10% of children exhibit reading disabilities, including dysfluent and inaccurate reading performance. These disabilities are thought to be related to differences in the structure and function of certain brain regions and networks. One way to map the network architecture of the brain is to create a connectome, or “wiring diagram,” based on the strength of white matter connections between different regions of the brain. This week in Developmental Cognitive Neuroscience, Lou and colleagues aim to investigate associations between the connectome structure of the brain reading performance in a group of children with varying reading ability.

How did they do it?

The authors recruited 73 native English-speaking school-age children (of whom 64 were retained for final statistical analysis) to participate in the study. The children performed four tasks assessing reading ability including:

1.     Sight Word Efficiency: children had to read familiar words as quickly and accurately as possible in 45s;

2.     Phonemic Decoding: children had to read pseudowords as quickly and accurately as possible in 45s;

3.     Reading comprehension: children had to read a sentence or paragraph and provide a missing word;

4.     Rapid Automatized naming: children had to name a letter from a set of 4 presented in a random grid as quickly as possible.  

In addition to these behavioural measures, the authors acquired magnetic resonance imaging data (MRI), including structural MRI (to visualize brain anatomy), and diffusion weighted MRI measuring white matter connections between brain regions. For each participant, the authors created a connectome, which describes the human brain as a network with nodes representing cortical regions, and edges representing white matter tracts that connect them to the matrix. To do so, they: 1) parcellated each participant’s structural MRI scan into 90 gray matter regions (based on the Automated Anatomical Labelling template) to act as nodes in their connectome and 2) extracted streamlines from white matter connections between regions based on diffusion weighted data, to be used as edges in the connectome. The strength of the connection, or edge weight, was determined by an estimate of the white matter fiber strength between regions. 

Next, hub brain regions — regions centrally embedded in the connectome — with a higher than usual level of connectivity with other regions, were identified based on the degree of connections to other nodes. Hub regions in the brain are critical for global communication and therefore are highly connected within themselves, and also more likely to connect to each other, forming what is referred to as a “rich-club”.

Three types of connections were identified: rich club connections (connections between two hub nodes), feeder connections (connections between a hub and a non-hub node), and local connections (connections between two non-hub nodes). Additionally, the authors defined a ‘reading network’ based on brain regions identified in two meta-analyses of fMRI studies investigating reading abilities in children (with typical reading abilities and reading disabilities, respectively), and a set of regions with fewer white matter fibers identified in one previous white matter network study. Finally, reading ability scores were correlated with both the rich-club and reading connectomes overall, and separately for boys and girls.

What did they find?

The authors found that the hub regions and rich-club connectome identified were consistent with previous reports, with hubs in the bilateral superior frontal lobes, precuneus, supplementary motor area, and thalamus.

They found that the strength of feeder connections was correlated with scores on the sight word efficiency and phonemic decoding tasks, measuring familiar and nonword reading ability. Similarly, the connection strength between rich-club and reading network nodes was significantly correlated with phonemic decoding. A follow-up analysis revealed that the correlation between rich club reading network nodes and feeder connection strength with reading ability (sight word efficiency and phonemic decoding) was still present in girls, but not in boys when analyzed separately.

The authors validated their findings by adjusting the way in which fiber length and strength were calculated, to ensure that variability in the way fiber strength was captured in different participants was not the reason for their findings.  

What's the impact?

This study provides exciting evidence for the relationship between the network-like architecture of the brain and reading performance. Across a number of measures, there was a significant association — possibly stronger in girls than boys — between network structure and performance on reading tasks. Interestingly, white matter fibers outside the reading network also contribute to reading performance, as feeder connections were positively correlated to the sight word efficiency and phonemic decoding scores. Future work may examine the influence of various environmental factors such as socioeconomic status on the relationship between brain networks and reading ability.

Chenglin Lou et al Rich-club structure contributes to individual variance of reading skills via feeder connections in children with reading disabilities. Developmental Cognitive Neuroscience (2021). The original scientific publication here.