Understanding how academic abilities, such as reading and math, develop but also mapping the various constraints, including biological factors, that may influence its development is of great importance for both science and society. Especially since research suggests that these academic abilities are important factors in determining career success, income and even psychological well-being (e.g., Parsons & Bynner, 2005). Yet, the 2018 Program for International Student Assessment (PISA) results show that more than one in five pupils in the EU has insufficient proficiency in these key academic skills (OECD, 2019). Investigating how the brain may constrain academic achievement is not only relevant to understanding brain structure but also to providing insight into the origins of individual differences in these academic abilities.
Research has extensively studied the function of key regions for processing mathematics and reading, namely bilateral intraparietal sulcus (IPS, see Menon, 2014; Peters & De Smedt, 2018 for a review) and the Left occipito-temporal sulcus (OTS) host of the visual word form area (VWFA, Dehaene & Cohen, 2011), respectively. Much less studies have examined how individual differences in the structure of these regions are related to differences in academic performance (e.g., Evans et al., 2015; Isaacs et al., 2001; Richlan et al., n.d.; Torre & Eden, 2019). Even less studies have investigated the long-term effect of early brain development on later mathematical and reading abilities. Specifically, such study of early cerebral constraints on math and reading would allow us to further unravel the question of causes of individual differences in academic learning. Recent studies have found evidence of associations between a marker of early brain development, sulcal morphology, and academic achievement, i.e. reading (Borst et al., 2016; Cachia et al., 2017) and mathematics (Roell et al., in prep). The aim of this study was to replicate and extend these findings of an association between the sulcal morphology of the IPS and math abilities as well as the sulcal morphology of the OTS and reading.
Research has strived to delineate the brain regions functionally supporting mathematical ability. This body of literature converges to suggest that a fronto-parietal network is engaged during arithmetic in both children and adults (Arsalidou & Taylor, 2011; Peters & De Smedt, 2018; Rivera et al., 2005). Consistent across these data is the activation of the bilateral IPS during arithmetic (e.g., Arsalidou & Taylor, 2011; Peters & De Smedt, 2018). Importantly, imaging studies in children have shown that activity in the arithmetic fronto-parietal network is modulated by individual differences. Specifically, children with low arithmetic fluency have been found to show a higher activity in, particularly the Right, IPS (De Smedt et al., 2011; Demir-Lira et al., 2016; Price et al., 2013). The activation of the IPS has also been consistently associated with basic number processing, such as symbolic number comparison (Fias et al., 2003; Holloway & Ansari, 2010; Vogel et al., 2013). IPS activation for symbolic numbers has been found to be cross-culturally consistent, as the IPS number-related activity has been found to be similar in Eastern and Western populations (Eger et al., 2003; Kazui et al., 2000; Prado et al., 2011; Zhou et al., 2007). Importantly, studies have shown that over the course of development, symbolic number comparison abilities are associated with a progressive specialization of the IPS (Ansari, 2008; Holloway & Ansari, 2010). Children with developmental dyscalculia, a deficit in arithmetic and number processing (American Psychiatric Association, 2013), have been found to have impairment in the IPS when processing number magnitudes and performing calculations. That is, the IPS is not modulated in response to numerical processing demands to the same degree as typically developing children (Mussolin et al., 2010; Price et al., 2007).
Studies have also investigated which brain structures are related to individual differences in mathematical ability, with the majority of studies focusing on the role of white and grey matter. Individual differences in math ability were found to be associated with higher fractional anisotropy, a parameter related to white matter microstructure, in white matter tracts connecting frontal lobes with basal ganglia and parietal regions (Matejko et al., 2013; Navas-Sánchez et al., 2014; van Eimeren et al., 2008). Additionally, grey matter volume of the Left IPS at the end of first grade has been found to be related to math competence a year later at the end of Grade 2 (Price et al., 2016). Similarly, Evans et al. (2015) reported that grey matter volume of posterior parietal areas, including the Left IPS, predicted the growth in arithmetic across primary school. However, in a recent study, Polspoel et al. (2020) examined grey matter volume using voxel-based morphometry, as did previous studies, but also cortical complexity. They did not find a significant association between children’s arithmetic fluency and the grey matter volume or the complexity of parietal regions such as the IPS.
Reading, on the other hand, systematically activates the Left lateral Occipitotemporal sulcus (OTS) at a fixed location known as the Visual Word Form Area (VWFA, Cohen et al., 2002) relative to a reproducible mosaic of regions partially specialized for objects, faces, bodies and places (Downing et al., 2006). The specialization of the VWFA site appears progressively as children start to learn to read (Dehaene-Lambertz et al., 2018). Additionally, word-induced activation found at the site of the VWFA in good readers has been found to be cross-culturally consistent. Bolger et al.( 2005) found that the peak activation of the VWFA in Japanese kana, Japanese logographic kanji, Chinese and roman alphabet readers was all within the millimeter of each other. Furthermore, children and adults with developmental dyslexia, a specific disorder of reading acquisition (American Psychiatric Association, 2013), show an under-activation (Cao et al., 2006; Shaywitz et al., 2003) and a dysfunction in the VWFA whilst processing visual words (van der Mark et al., 2011).
Studies have also examined the brain’s structure supporting reading, again focusing largely on white and grey matter data. In their study, Myers et al. (2014) found that increases in the volume of two left temporo-parietal white matter clusters are unique predictors of reading outcomes above and beyond family history, socioeconomic status and cognitive and preliteracy measure at baseline. Similarly, Niogi & McCandliss (2006) found a strong correlation between fractional anisotropy values in a Left tempo-parietal white matter region and standardized reading scores of typically developing children. Additionally, they found that fractional anisotropy values in that region accounted for differences in reading score between typically developing children and children with dyslexia children.
Turning to the role of grey matter, Altarelli et al. (2013) examined whether cortical thickness of the ventral occipitotemporal regions differed between dyslexic and typically developing children. They found a reduction in thickness in dyslexic compared with controls in the VWFA, i.e., the Left posterior OTS. In their meta-analysis, Richlan et al. (2013) found converging evidence of reduced grey matter in the bilateral superior temporal sulcus in dyslexic participants compared to typically developing controls. They also found evidence of structural and functional abnormalities in the Left occipitotemporal region in prereaders with a family history of developmental dyslexia (Richlan et al., 2013).
One limitation of the existing body of data is that it does not furnish information on the influence of early cerebral constraints on academic achievement. Indeed, nearly all studies on the relation between neuroanatomy and academic achievement have focused on structural brain characteristics, such as grey matter density or white matter tracts (Dehaene et al., 2015; Peters et al., 2018), that are affected by brain maturation and learning (e.g., Zatorre et al., 2012). To evaluate early cerebral constraints on mathematical or reading achievement, it is important to examine neuroanatomical characteristics that are not affected by brain maturation and learning. Researchers have recently turned to the study of sulcal pattern of the brain as this qualitative feature of the cortex anatomy is determined in utero (Mangin, Jouvent, & Cachia, 2010) and is stable during development (Tissier et al., 2018). Studying brain sulcal pattern thus allows researchers to further unravel the question of causes of individual differences that are independent of learning and development.
Applying this methodology to the study of individual differences in mathematical cognition, in a recent pre-registered study (https://osf.io/w3zvc) Roell et al. (in prep) examined whether the IPS sulcal pattern explains individual differences in number processing in sample of grade 1 to grade 4 children (n = 77) and adults (n = 21). They characterized the Left and Right IPS sulcal pattern as “sectioned” vs “not sectioned” based on the presence or absence of branches completely sectioning the IPS using Zlatkina & Petrides’ (2014) classification. They found that IPS sulcal pattern explains part of the variance in both the children’s and adult’s symbolic number comparison and math fluency abilities but not in their non-symbolic number abilities. As with the data on the association between sulcal morphology and reading, it appears of importance to be able to replicate the association between the sulcal morphology of the IPS and symbolic number processing and arithmetic ability in a large sample of participants with a narrower age range.
Using the same method, Borst et al. (2016) and Cachia et al. (2017) have shown that the sulcal pattern of the OTS is associated with reading abilities. In their study, Borst et al. (2016) examined the relationship between OTS sulcal pattern and reading abilities in 8-year-old children (n = 16). They found that participants with an interrupted Left OTS had significantly better reading abilities than participants with a continuous Left OTS. Cachia et al. (2017), replicated this effect in a larger sample (n = 62) of adult participants and determined that this effect was specific to the posterior portion of the Left OTS, which hosts the VWFA. They also found that the length of the OTS posterior interruption was positively correlated with reading skills. It remains to be determined whether the effect found by Cachia et al. (2017) of a specific association between the posterior portion of the Left OTS and reading can also be replicated in a sufficiently large sample of children.
In addition to studies that either focus on reading or on mathematics, it would be of interest to examine potential overlap or specificity of the sulcal pattern effect of the IPS on mathematical abilities and OTS on reading. Indeed, mathematical and reading abilities have been found to be correlated (Grimm, 2008). Comorbidity or co-occurrence of specific learning disorders in reading (dyslexia) and in math (dyscalculia) is remarkably high (Peters & Ansari, 2019). Furthermore, functional neural overlap of arithmetic and reading has been reported (Evans et al., 2016). Children with dyslexia have been reported to show atypical brain activation patterns during arithmetic in arithmetic-related regions, such as the supramarginal gyrus (Evans et al., 2014) and there is evidence to suggest that children with dyslexia and dyscalculia show highly overlapping patterns of brain activity during the processing of number (e.g., Peters et al., 2018).
The aim of this study is threefold. For our first aim, Aim 1, we focus on the association between sulcal morphology of the IPS and individual differences in arithmetic and number processing. We wish to conceptually replicate in a larger dataset of participants in the same age range and extend our previous study (Roell et al., in prep). In continuation of Roell et al.’s study, we expect that IPS sulcal morphology explains part of the variability observed in arithmetic and symbolic number ability in typically developing children. That is, we hypothesize that children with a “sectioned” IPS will have greater symbolic number comparison and arithmetic abilities than children with a “not sectioned” IPS. In order to further our understanding of the relationship between symbolic number processing, arithmetic and the sulcal morphology of the IPS, we will extend Roell et al.’s (in prep) study by investigating whether symbolic number processing mediates the association between sulcal morphology and arithmetic. Indeed, relations between children’s mathematics achievement and their basic number processing skills have been reported in both cross-sectional and longitudinal studies (De Smedt et al., 2013; Holloway & Ansari, 2009; Schneider et al., 2017). In their longitudinal study Bartelet et al. (2014) reported that symbolic number processing was consistently a significant predictor of arithmetic achievement regardless of children’s level of arithmetic proficiency. Similarly, a significant correlation between the activation in the IPS during symbolic number task and arithmetic task has been found (Bugden et al., 2012). Moreover, Roell et al. (in prep) observed an association of sulcal morphology with both number processing and arithmetic ability. As such, it would appear interesting to examine whether the relationship between IPS sulcal morphology and arithmetic ability is mediated by symbolic number processing. We hypothesize that symbolic number processing mediates the association between IPS sulcal morphology and arithmetic ability. That is, participants with a “sectioned” IPS will have greater symbolic number comparison abilities; these greater symbolic number comparison abilities will in turn be associated with greater arithmetic abilities.
Our second aim, Aim 2, is to conceptually replicate and extend the findings of Cachia et al. (2017) and Borst et al. (2016) in a large sample of children. Specifically, we shall examine whether the effect found by Cachia et al. (2017) in adults of a specific association between the posterior portion of the Left OTS and reading can also be replicated in a large sample of children. Namely, we expect that participants with an interrupted Left OTS, in particular in its posterior portion hosting the visual word form area (VWFA), will have better reading performance than participants who have a continuous Left OTS.
Our third aim, Aim 3, is to examine whether the effect of sulcal morphology on academic abilities is specific. As discussed above, studies examining behaviour, learning disorders and functional networks point towards an overlap between reading and mathematical abilities. Additionally, the specificity of the IPS for number and arithmetic processing (Fias et al., 2007) and the VWFA for reading (Vogel et al., 2014) remains controversial. As such, in view of the existing overlap, it becomes relevant to determine whether the effect of the IPS sulcal pattern on numerical abilities is specific, as well as the effect of the OTS sulcal pattern on reading is specific. The overlap and specificity of the IPS and OTS will be examined through three different analyses. Firstly, we shall test whether the sulcal pattern of IPS predicts reading and the sulcal pattern of Left posterior OTS predicts arithmetic abilities. Secondly, we shall run the same analyses as in Aim 1 and 2 but we shall add the other academic ability as a covariate. That is, we shall examine the effect of IPS sulcal morphology on mathematical ability whilst controlling for reading ability. Similarly, we shall examine the effect of OTS sulcal morphology on reading ability whilst controlling for mathematical ability. Thirdly, we shall use a third sulcal pattern, which is much less related to reading or mathematical ability, as a control region. More specifically we will examine if the sulcal morphology of the ACC (anterior cingulate cortex) is related to math and reading abilities. Studies have found that an asymmetrical ACC sulcal pattern, that is each hemisphere had a different ACC sulcal pattern, was associated with higher inhibitory control efficiency in both children (Borst et al., 2016; Cachia et al., 2017) and adults (Fornito, 2004; Huster et al., 2009; Tissier et al., 2018). Against this background, we predict that, if the association between sulcal patterns and academic achievement is specific, this sulcal pattern of the ACC will be much less related to math abilities or reading ability. Finally, we also tested whether a relationship between the sulcal pattern of the IPS and that of the OTS could be found. That is, are participants with an “interrupted” OTS more likely to have a “sectioned” IPS and vice-versa?
To address the three aims outlined above, we will utilize structural MRI data from two existing samples collected in the same age range. These structural data were previously collected for a variety of different studies (Bellon et al., 2020; Polspoel et al., 2017) and all have the same structural image scans as well as academic achievement measures (Tempo Test Arithmetic task, symbolic number comparison task, Dutch one-minute reading test). We selected behavioural measures common to all samples that measured academic achievement. It is of note that both datasets initially focused on mathematical ability. As such, the datasets contained more common mathematical tasks which allow us to also investigate the cognitive correlates of mathematical ability (through our mediation analysis) in a more fine-grained way as compared to reading ability. The functional data has already been published (Bellon et al., 2020; Polspoel et al., 2017) and forty-seven of the structural MRI (grey matter volume and DTI) data has already been published (Polspoel et al., 2019, 2020). However, no research has been conducted on the sulcal morphology data so far.
Because sex has been shown to have a potential effect on sulcal anatomy (Duchesnay et al., 2007) and because intellectual ability (IQ) has been found to be related mathematical abilities (e.g., Passolunghi et al., 2008) and reading abilities (e.g., Tiu et al., 2003), we shall account for potential effects of both sex and IQ on the relationship between sulcal morphology and academic skills. Similarly, given that we combine data from two different samples, we shall control for the potential effect of sample.