By: Catherine Joachin
What is Dyslexia?
Dyslexia is a learning disorder stemming from individual differences in brain areas involved in language processing (Mayo Clinic, 2022). This disorder is characterized by reading impairments in the form of poor word recognition, decoding, and spelling abilities despite normal intelligence and adequate training (Banich & Comptom, 2018, pg. 470). Individuals with a family history of dyslexic difficulties are at higher risk of developing the disorder (Mayo Clinic, 2022). Dyslexia can affect several areas of learning including word comprehension, sequence learning, numeracy, and pronunciation (British Dyslexia Association, n.d.). Degrees of impairment vary, but common signs include:
Difficulty accurately identifying letters (e.g. “d” and “b”)
Difficulty understanding reading material
Trouble associating speech sounds with specific letters
Poor processing speed (i.e. slow reading and/or writing)
(Cleveland Clinic, 2020)
These challenges can lead to poor self-esteem and significant behavioral issues, especially if said difficulties are not addressed earlier in life (Mayo Clinic, 2022). Dyslexia is one of the most common learning disabilities affecting children in North America, with a bias towards the male sex (Banich & Comptom, p. 470, 2018). Although dyslexic mistakes are commonly observed in most reading learners, dyslexia involves a deficit in linking a given letter to a specific sound and manipulating sound units to correctly pronounce a word (Banich & Comptom, 2018, p. 471).
Dyslexia can be hard to detect, and many children are never diagnosed (Cleveland Clinic, 2020). There is no standard test for diagnosing dyslexia. Instead, certified psychologists and other qualified professionals carry out assessment for reading disability by evaluating their client’s phonological awareness, spelling, vocabulary, and decoding abilities, among other skills (Cleveland Clinic, 2020).
Early intervention, having a strong support network and the implementation of strategies to work with dyslexia are important factors that can play into individuals’ prognosis (National Institute of Neurological Disorders and Stroke, 2022).
Bases of Dyslexia
There are several possible explanations for the onset of developmental dyslexia. Neuroimaging research suggests that dyslexia involves a disconnection between left hemisphere regions that are critical for processing speech sounds and speech output. Dysfunction in the left fusiform gyrus especially is thought to lead to impairments such as reading difficulties (Ulfarsson et. al., 2017). Functional imaging studies argue that individuals with dyslexia show less activity than non-dyslexics in the left perisylvian area, a brain region responsible for language-related functions (Banich & Comptom, 2018, p. 471).
Genes associated with neural migration and axon growth have also been linked to dyslexia, suggesting that disrupted cell migration may cause reading impairments (Banich & Comptom, 2018, p. 472). Neuropsychological research indicates that the cognitive profile of 15q11.2 copy number variant deletion carrier is similar to that of dyslexics, implying an association between poor reading ability and genetic etiology (Ulfarsson et. al., 2017). This same genetic trait is associated with the higher risk of schizophrenia, autism, intellectual disability, attention deficit hyperactivity disorder, epilepsy, and specific learning disorders.
Other interpretations for dyslexia include a deficit in visual word representation and disrupted visuospatial attention.
Dyslexia and Epilepsy
Studies indicate that individuals with Rolandic epilepsy are more likely to develop reading disability and speech sound disorder, indicating that reading disability may be genetically linked to epilepsy (Clarke, Strug et. al., 2007). That being said, there is no evidence implying that Rolandic epilepsy itself constitutes a risk factor for the development of reading impairments.
EEG epileptiform discharges can also disrupt ability to perform certain reading tasks. Research suggests that impaired reading performance is correlated with nocturnal epileptiform activity. Nocturnal epileptiform activity refers to frequent epileptic seizures during sleep. Abnormal EEG discharges could disturb the functioning of neural networks, hence affecting reading performance (Ebus, et. al., 2011). Supporting this argument is the noticeable improvement in reading performance following the reduction of epileptiform activity through the administration of carbamazepine/oxcarbazepine, antiepileptic drugs used to control seizures (Ebus et. al., 2011).
White matter abnormalities involving the sensorimotor cortex in children with Rolandic epilepsy have also been associated with language difficulties, including reading disability, indicated by damage to temporal regions involved in reading processes (Besseling et. al., 2013).
Conclusion
Dyslexia is a reading disability linked to critical language areas of the brain. Damage to the neural connections enabling reading can lead to dyslexic symptoms and epileptiform activity, suggesting that people with dyslexia and individuals with epilepsy share similarities in terms of brain structure and chemistry.
Resources
Banich, M.T., & Comptom, R.J. (2018). Cognitive Neuroscience. Cambridge University Press, (4) 470-472.
Besseling, R.M.H., Jansen, J.F.A., Overvliet, G.M., van der Kruijs, S.J.M., Ebus, S.C.M., de Louw, A.J.A., Hofman, P.A.M, Vles, J.S.H., Aldenhamp, A.P., & Backes, W.H. (2013). Reduced Structural Connectivity Between Sensorimotor and Language Areas in Rolandic Epilepsy. Plos One, 8(12), e83568-e83568. https://doi.org/10.1371/journal.pone.0083568
British Dyslexia Association (n.d.). Dyslexia. British Dyslexia Association. Retrieved from: https://www.bdadyslexia.org.uk/
Clarke, T., Strug, L.J., Murphy, P.L., Bali, B., Carvalho, J., Foster, S., Tremont, G., Gagnon, B.R., Dorta, N., & Pal, D.K. (2007). High Risk of Reading Disability and Speech Sound Disorder in Rolandic Epilepsy Families: Case-Control Study. Epilepsia (Copenhagen), 48(12), 2258-2265. https://doi.org/10.1111/j.1528-1167.2007.01276.x
Cleveland Clinic (2020). Dyslexia. Cleveland Clinic. Retrieved from: https://my.clevelandclinic.org/health/articles/6005-dyslexia
Ebus, S.C., Overvliet, G., Arends, J.B.A., & Aldenkamp, A. (2011). Reading performance in children with Rolandic epilepsy correlates with nocturnal epileptiform activity, but not with epileptiform activity while awake. Epilepsy & Behavior, 22(3), 518-522. https://doi.org/10.1016/j.yebeh.2011.08.008
Mayo Clinic (2022). Dyslexia. Mayo Clinic. Retrieved from: https://www.mayoclinic.org/diseases-conditions/dyslexia/symptoms-causes/
National Institute of Neurological Disorders and Stroke (2022). Dyslexia. National Institute of Neurological Disorders and Stroke. Retrieved from https://www.ninds.nih.gov/health-information/disorders/dyslexia
Ulfarsson, M.O., Walters, G.B., Gustafsson, O., et. al. 15q11.2 CNV affects cognitive, structural and functional correlates of dyslexia and dyscalculia. Transl Psychiatry 7, e1109 (2017). https://doi.org/10.1038/tp.2017.77
