All Roads Lead to Rome? Distinct Neural Circuits in Different Developmental Disorders are Related to Reading Difficulties in Children

Reading is defined as the ability to extract semantically meaningful, verbal information from written language. This is one of the most important cognitive academic abilities and has been found to be related to scores on the American College Test [1]. However, reading is not as intuitive as we may think. The neuroimaging era provides insights into the neural circuits involved in reading, and it appears that this cognitive ability relies not only on neural circuits related to language processing [2,3] but also those related to visual processing [3] and higher-level cognitive abilities, such as executive functions [4,5]. A fascinating orchestra of synchronized activation between the superior temporal gyrus and inferior frontal gyrus (i.e., language processing, see [2] the fusiform gyrus, cuneus and precuneus (i.e., visual processing, see ref. [3]), and the anterior cingulate cortex in particular and the cinguloopercular and fronto-parietal cognitive control networks in general [46] all are needed for fluent reading. Given this involvement of multiple neural circuits and cognitive abilities, it is not surprising that reading difficulties may result from a variety of impairments or an altered activation in any of the aforementioned neural circuits. Alternatively, reading difficulties can result from a lack of synchronization between the activation of these neural circuits (i.e., altered functional connectivity). Children with dyslexia [3] attention deficit hyperactivity disorder (ADHD) [7], psychiatric disorders [8] epilepsy [9] autism [10], or mental retardation [11] all suffer from reading difficulties. However, these types of reading challenges are only the tip of the iceberg, as this is what a teacher notices in the classroom. Neuroimaging data provide us with insight into the underlying causes and pathway “flosses” (the bottom of the iceberg) that are the basis for these reading challenges. Children with dyslexia show an under activation of the fusiform gyrus compared to typical readers during reading [3], but also have decreased functional connectivity between visualand executive functionsrelated regions (fusiform gyrus and anterior cingulate cortex) during reading [12] and the cingulo-opercular cognitive-control network (related to executive functions), even during rest [13]. Conversely, children with both ADHD and dyslexia, who showed much more severe reading difficulties than those with dyslexia alone, demonstrated greater activation in neural circuits related to executive functions (i.e., dorsolateral prefrontal cortex) compared to children with dyslexia during word reading (unpublished data). In the field of psychiatry, children with mood disorders demonstrated more severe reading and phonological-processing difficulties compared to those with behavioral disorders, as well as decreased white-matter diffusivity (fractional anisotropy) in white-matter tracks related to language processing (left arcuate fasciculus, which crosses the frontal and tempo-parietal language and reading regions) [14,15]. Neurologically, a case study of an epilepsy patient revealed reading difficulties that were manifested by right lateralized activation in the language-related right superior temporal lobe compared to a child with epilepsy with intact reading abilities and left lateralized activation in this region [16]. Highfunctioning children with autism and hyperlexia (i.e., an extraordinary ability to read words) showed greater activation of neural circuits related to visual processing (i.e., the fusiform gyrus) [17]. As of yet, there is no evidence of neural circuits supporting impaired reading in children with autism who have reading difficulties or for children with mental retardation. The main challenge of identifying the specific brain regions in many other neurological disorders that affect reading is in the inter-subjects variability. In Epilepsy, for example, reading difficulties can result from seizures in different brain regions related to language, visual processing or different cognitive abilities related to reading. Therefore, a functional / structural connectivity approach, which relates to associations between brain regions, related to these abilities would be more appropriate when characterizing the neural changes related to reading in these populations.