Post by Shireen Parimoo
What's the science?
Reading and math involve similar cognitive processes like working memory and verbalization, and math and reading-related disabilities tend to co-occur. Previous research examining reading has identified white matter tracts in the brain that are related to performance on reading tasks, such as the arcuate fasciculus and the inferior longitudinal fasciculus. However, less is known about the white matter tracts associated with math. It is also unclear whether there is an overlap in the structural properties of white matter tracts associated with reading and math ability. This week in Nature Communications, Grotheer and colleagues used multimodal magnetic resonance imaging (MRI) techniques to identify the shared and distinct structural correlates of the cognitive processes involved in reading and addition.
How did they do it?
Twenty adults completed a reading, adding, and a control color task while undergoing functional MRI scanning. The task stimuli were morphs that could be perceived as a number or a letter (e.g. the stimulus for the letter “S” could be perceived as the number “5”), which allowed the visual input to remain constant across the different tasks. A series of four stimuli were presented consecutively. In the reading task, participants had to indicate the word spelled out by the letters; in the addition task, participants had to add up the numbers; in the color task, participants had to indicate the color of the stimuli at the end of the stimulus sequence. The fMRI data was used to determine which brain regions were activated during the reading task, during the addition task, or during both tasks. These brain regions of interest were identified for each participant and co-located white matter tracts were then analyzed.
Diffusion MRI and quantitative MRI scans were performed to investigate the connectivity and microstructure of white matter, respectively. First, the authors used constrained spherical deconvolution (a method used to model the orientation of white matter fibers) on the diffusion MRI data to create a structural connectome, or a map of the white matter pathways connecting brain regions. They then applied an automated algorithm to identify the major white matter pathways (also called fascicles) in the brain, including the arcuate fasciculus (AF), the posterior arcuate fasciculus (pAF) and the superior longitudinal fasciculus (SLF). The white matter fascicles were intersected with the functionally-defined ROIs to localize the tracts associated with reading and math. This allowed the researchers to examine which white matter tracts support the connectivity within and between the reading and math networks. Finally, quantitative MRI was used to estimate the myelination of the white matter tracts connecting regions of the reading and math networks. In general, greater myelination is associated with more efficient neuronal transmission in the brain.
What did they find?
Reading and math activated largely separate but neighbouring brain regions. For instance, the occipitotemporal sulcus, the superior temporal sulcus, and the inferior frontal gyrus were active during the reading task, whereas the addition task activated the inferior temporal gyrus, the inferior post-central sulcus, and the intraparietal sulcus. Both tasks also activated distinct subregions within the supramarginal gyrus. Across the brain, reading- and math-specific regions were connected to their respective network by the SLF, the AF, and the posterior AF. The SLF and AF connected the prefrontal regions of the math and reading networks, such as the inferior frontal gyrus (reading) and the post-central sulcus (addition), to temporal and parietal regions of each network. Further, the posterior AF connected the temporal regions active during the two tasks, such as the occipitotemporal sulcus (reading) and the inferior temporal gyrus (adding) to the parietal regions of each network. Thus, the same white matter fascicles support the math and reading networks, even though the brain regions themselves are largely distinct. Crucially, though, analogous to distinct lanes in a highway, math and reading-related white matter tracts were found to run in parallel, segregated sub-bundles within the shared fascicles. The specific sub-bundles, or branches, of the SLF and AF involved in reading were located more inferiorly in the brain than those involved in addition. Moreover, the branches of the SLF and AF involved in reading were more heavily myelinated than those associated with the addition network, suggesting greater efficiency of neuronal transmission within the reading network.
What's the impact?
This study is the first to establish that spatially distinct branches of the same white matter fascicles are associated with the reading and math networks in the brain. These findings suggest that the ability to read and perform mathematical operations might develop independently, despite shared cognitive processes and a high rate of comorbidity of their associated learning disorders. This has important implications for future research exploring the relationship between white matter properties and math- and reading-related abilities.
Grotheer et al. Separate lanes for adding and reading in the white matter highways of the human brain. Nature Communications (2019). Access the original scientific publication here.