Editing Dyslexia (section)

== Causes ==
[[File:Inferior parietal lobule - superior view animation.gif|thumb|300px| Inferior parietal lobule – superior view animation]]
Researchers have been trying to find the neurobiological basis of dyslexia since the condition was first identified in 1881.<ref name="Oswald Berkhan ref 1" /><ref name="ReidFawcett2008x">{{cite book|author1=Reid, Gavin|author2=Fawcett, Angela|author3=Manis, Frank|author4=Siegel, Linda|title=The SAGE Handbook of Dyslexia|url=https://books.google.com/books?id=937rqz4Ryc8C&pg=PA127|year=2008|publisher=SAGE Publications|isbn=978-1-84860-037-9|page=127|url-status=live|archive-url=https://web.archive.org/web/20170109200307/https://books.google.com/books?id=937rqz4Ryc8C&pg=PA127|archive-date=9 January 2017}}</ref> For example, some have tried to associate the common problem among people with dyslexia of not being able to see letters clearly to abnormal development of their visual nerve cells.<ref name="Stein2014" >{{cite journal |first1=John |last1=Stein |year=2014 |title=Dyslexia: the Role of Vision and Visual Attention |journal=Current Developmental Disorders Reports |volume=1 |issue=4 |pages=267–80 |pmid=25346883 |pmc=4203994 |doi=10.1007/s40474-014-0030-6}}</ref>

===Neuroanatomy===
[[Neuroimaging]] techniques, such as [[functional magnetic resonance imaging]] ([[fMRI]]) and [[positron emission tomography]] (PET), have shown a correlation between both functional and structural differences in the brains of children with reading difficulties.<ref name="Whitaker2010r">{{cite book|author=Whitaker, Harry A.|title=Concise Encyclopedia of Brain and Language|url=https://books.google.com/books?id=GNcDiRV2jJQC&pg=PA180|year=2010|publisher=Elsevier|isbn=978-0-08-096499-7|page=180|url-status=live|archive-url=https://web.archive.org/web/20170109173223/https://books.google.com/books?id=GNcDiRV2jJQC&pg=PA180|archive-date=9 January 2017}}</ref> Some people with dyslexia show less activation in parts of the left hemisphere of the brain involved with reading, such as the [[inferior frontal gyrus]], [[inferior parietal lobule]], and the middle and [[Brodmann area 20|ventral temporal cortex]].<ref name=Pammer2014/> Over the past decade, brain activation studies using PET to study language have produced a breakthrough in the understanding of the neural basis of language. Neural bases for the visual [[lexicon]] and for auditory verbal [[short-term memory]] components have been proposed,<ref>{{cite journal|last1=Price|first1=cathy|title=A Review and Synthesis of the first 20 years of Pet and fMRI studies of heard Speech, Spoken Language and Reading|journal=NeuroImage|date=16 August 2012|volume=62 |issue=2|pages=816–847|doi=10.1016/j.neuroimage.2012.04.062|pmid=22584224|pmc=3398395}}</ref> with some implication that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural). fMRIs of people with dyslexia indicate an interactive role of the [[cerebellum]] and cerebral cortex as well as other brain structures in reading.<ref>{{cite journal|last1=Sharifi|first1=S|title=Neuroimaging essentials in essential tremor: a systematic review.|journal=NeuroImage: Clinical|date=May 2014|pages=217–231|pmid=25068111|pmc=4110352|doi=10.1016/j.nicl.2014.05.003|volume=5}}</ref><ref>{{cite journal|last1=Brandler|first1=William|title=The genetic relationship between handedness and neurodevelopmental disorders|journal=Trends in Molecular Medicine|date=February 2014|pages=83–90|pmid=24275328|pmc=3969300|doi=10.1016/j.molmed.2013.10.008|volume=20|issue=2}}</ref>

The cerebellar theory of dyslexia proposes that impairment of cerebellum-controlled muscle movement affects the formation of words by the tongue and facial muscles, resulting in the [[fluency]] problems that some people with dyslexia experience. The cerebellum is also involved in the [[Autonomic nervous system|automatization]] of some tasks, such as reading.<ref>{{cite book|last1=Cain|first1=Kate|title=Reading development and difficulties|date=2010|publisher=TJ International|page=134|edition=1st|url=https://books.google.com/books?id=FT6RALjOr9QC&q=cerebellar+theory+of+dyslexia&pg=PA134|access-date=21 March 2015|isbn=9781405151559}}</ref> The fact that some children with dyslexia have motor task and balance impairments could be consistent with a cerebellar role in their reading difficulties. However, the cerebellar theory has not been supported by controlled research studies.<ref>{{cite book|last1=Levav|first1=Itzhak|title=Psychiatric and Behavioral Disorders in Israel: From Epidemiology to Mental health|date=2009|publisher=Green Publishing|page=52|url=https://books.google.com/books?id=W2RzffMnpg8C&q=cerebellar+theory+of+dyslexia&pg=PA52|access-date=21 March 2015|isbn=9789652294685}}</ref>

===Genetics===
Research into potential genetic causes of dyslexia has its roots in post-autopsy examination of the brains of people with dyslexia.<ref name="Stein2014" /> Observed anatomical differences in the [[language center]]s of such brains include microscopic [[cerebral cortex|cortical]] malformations known as [[wikt:ectopia|ectopias]], and more rarely, [[blood vessel|vascular]] micro-malformations, and [[microgyrus]]—a smaller than usual size for the gyrus.<ref name="Faust2012">{{cite book|author=Faust, Miriam|title=The Handbook of the Neuropsychology of Language|url=https://books.google.com/books?id=UEWVqdNFL4cC&pg=PA941|year=2012|publisher=John Wiley & Sons|isbn=978-1-4443-3040-3|pages=941–43|url-status=live|archive-url=https://web.archive.org/web/20170109200538/https://books.google.com/books?id=UEWVqdNFL4cC&pg=PA941|archive-date=9 January 2017}}</ref> The previously cited studies and others<ref>{{cite journal|last1=Benitez|first1=A|title=Neurobiology and neurogenetics of dyslexia|journal=Neurology (In Spanish)|date=November 2010|pmid=21093706|doi=10.1016/j.nrl.2009.12.010|volume=25|issue=9|pages=563–81|doi-access=}}</ref> suggest that abnormal cortical development, presumed to occur before or during the sixth month of [[fetal]] brain development, may have caused the abnormalities. Abnormal cell formations in people with dyslexia have also been reported in non-language cerebral and subcortical brain structures.<ref>{{cite journal|last1=Kere|first1=Julia|title=The molecular genetics and neurobiology of developmental dyslexia as model of a complex phenotype|journal=Biochemical and Biophysical Research Communications|date=September 2014|pages=236–43|doi=10.1016/j.bbrc.2014.07.102|pmid=25078623|volume=452|issue=2|doi-access=free}}</ref> Several genes have been associated with dyslexia, including [[DCDC2]]<ref name="Marshall2012v">{{cite book|author=Marshall, Chloë R.|title=Current Issues in Developmental Disorders|url=https://books.google.com/books?id=jHqYP39rI40C&pg=PA53|year=2012|publisher=Psychology Press|isbn=978-1-136-23067-7|pages=53–56|url-status=live|archive-url=https://web.archive.org/web/20170109103320/https://books.google.com/books?id=jHqYP39rI40C&pg=PA53|archive-date=9 January 2017}}</ref> and [[KIAA0319]]<ref>{{cite journal |vauthors=Paracchini S, Thomas A, Castro S, Lai C, Paramasivam M, Wang Y, Keating BJ, Taylor JM, Hacking DF, Scerri T, Francks C, Richardson AJ, Wade-Martins R, Stein JF, Knight JC, Copp AJ, LoTurco J, Monaco AP |title=The chromosome 6p22 haplotype associated with dyslexia reduces the expression of KIAA0319, a novel gene involved in neuronal migration|journal=Human Molecular Genetics|volume=15|issue=10|date=15 May 2006|pages=1659–1666|doi=10.1093/hmg/ddl089|pmid=16600991|doi-access=free|hdl=11858/00-001M-0000-0012-C979-F|hdl-access=free}}</ref> on [[chromosome 6]], and [[DYX1C1]] on [[chromosome 15]].<ref name="Rosen2013v">{{cite book|author=Rosen, Glenn D.|title=The Dyslexic Brain: New Pathways in Neuroscience Discovery|url=https://books.google.com/books?id=ZHBxBEekGSkC&pg=PA342|year=2013|publisher=Psychology Press|isbn=978-1-134-81550-0|page=342|url-status=live|archive-url=https://web.archive.org/web/20170109143349/https://books.google.com/books?id=ZHBxBEekGSkC&pg=PA342|archive-date=9 January 2017}}</ref>

===Gene–environment interaction===
The contribution of gene–environment interaction to reading disability, which estimates the proportion of variance associated with a person's environment and the proportion associated with their genes, has been intensely studied using [[twin studies]]. Both environmental and genetic factors appear to contribute to reading development. Studies examining the influence of environmental factors such as parental education<ref>{{cite journal |title=Parental Education Moderates Genetic Influences on Reading Disability |journal=Psychol. Sci. |volume=19 |issue=11 |pages=1124–30 |date=November 2008 |pmid=19076484 |pmc=2605635 |doi=10.1111/j.1467-9280.2008.02213.x |last1=Friend |first1=A |last2=Defries |first2=J. C. |last3=Olson |first3=R. K.}}</ref> and teaching quality<ref>{{cite journal |bibcode=2010Sci...328..512T|title=Teacher Quality Moderates the Genetic Effects on Early Reading|journal=Science|volume=328|issue=5977|pages=512–4|last1=Taylor|first1=J.|last2=Roehrig|first2=A. D.|last3=Hensler|first3=B. Soden|last4=Connor|first4=C. M.|last5=Schatschneider|first5=C.|year=2010|doi=10.1126/science.1186149|pmid=20413504|pmc=2905841}}</ref> have determined that genetics have greater influence in supportive, rather than less optimal, environments.<ref name=pmid19209992>{{cite journal |last1=Pennington |first1=Bruce F. |last2=McGrath |first2=Lauren M. |last3=Rosenberg |first3=Jenni |last4=Barnard |first4=Holly |last5=Smith |first5=Shelley D. |last6=Willcutt |first6=Erik G. |last7=Friend |first7=Angela |last8=Defries |first8=John C. |last9=Olson |first9=Richard K.  |date=January 2009 |title=Gene × Environment Interactions in Reading Disability and Attention-Deficit/Hyperactivity Disorder |journal=Developmental Psychology |volume=45 |issue=1 |pages=77–89 |doi=10.1037/a0014549 |pmid=19209992 |pmc=2743891}}</ref> However, more optimal conditions may just allow those genetic risk factors to account for more of the variance in outcome because the environmental risk factors have been minimized.<ref name=pmid19209992/>

As environment plays a large role in learning and memory, it is likely that [[epigenetic]] modifications play an important role in reading ability. Measures of [[gene expression]], [[Histone#Histone modification|histone modifications]], and [[methylation]] in the human periphery are used to study epigenetic processes; however, all of these have limitations in the extrapolation of results for application to the human brain.<ref>{{cite journal |last1=Roth |first1=Tania L. |last2=Roth |first2=Eric D. |last3=Sweatt |first3=J. David |s2cid=23229766 |date=September 2010  |title=Epigenetic regulation of genes in learning and memory |journal=Essays in Biochemistry |volume=48 |issue=1 |pages=263–74 |pmid=20822498 |doi=10.1042/bse0480263}}</ref><ref>{{cite journal |last1=Smith |first1=Shelley D. |title=Approach to epigenetic analysis in language disorders |journal=Journal of Neurodevelopmental Disorders |date=December 2011 |volume=3 |issue=4 |pages=356–364 |doi=10.1007/s11689-011-9099-y |pmid=22113455 |pmc=3261263 |issn=1866-1947}}</ref>

====Language====
The [[orthographic depth|orthographic complexity]] of a language directly affects how difficult it is to learn to read it.<ref name="BrunswickMcDougall2010">
Paulesu, Eraldo; Brunswick, Nicola and Paganelli, Federica (2010).  "Cross-cultural differences in unimpaired and dyslexic reading: Behavioral and functional anatomical observations in readers of regular and irregular orthographies. Chapter 12 in [https://books.google.com/books?id=0vJ5AgAAQBAJ&pg=PA266 Reading and Dyslexia in Different Orthographies] {{webarchive|url=https://web.archive.org/web/20170109135414/https://books.google.com/books?id=0vJ5AgAAQBAJ&pg=PA266 |date=9 January 2017 }}.  Eds. Nicola Brunswick, Siné McDougall, and Paul de Mornay Davies. Psychology Press. {{ISBN|9781135167813}}</ref>{{rp|266}} English and French have comparatively "deep" [[phonemic orthographies]] within the [[Latin alphabet]] [[writing system]], with complex structures employing spelling patterns on several levels: letter-sound correspondence, syllables, and [[morpheme]]s.<ref name="DickinsonNeuman2013">{{cite book|author=Juel, Connie|chapter=The Impact of Early School Experiences on Initial Reading|editor1=David K. Dickinson|editor2=Susan B. Neuman|title=Handbook of Early Literacy Research|chapter-url=https://books.google.com/books?id=_chXAQAAQBAJ&pg=PA421|year=2013|publisher=Guilford Publications|isbn=978-1-4625-1470-0|url-status=live|archive-url=https://web.archive.org/web/20170109162332/https://books.google.com/books?id=_chXAQAAQBAJ&pg=PA421|archive-date=9 January 2017|df=dmy-all}}</ref>{{rp|421}} Languages such as Spanish, Italian and Finnish primarily employ letter-sound correspondence—so-called "shallow" orthographies—which makes them easier to learn for people with dyslexia.<ref name="BrunswickMcDougall2010"/>{{rp|266}} [[Logograph]]ic writing systems, such as [[Chinese characters]], have extensive symbol use, and these also pose problems for dyslexic learners.<ref>{{cite journal|title = Annual Research Review: The nature and classification of reading disorders – a commentary on proposals for DSM-5|journal = Journal of Child Psychology and Psychiatry, and Allied Disciplines|date = 1 May 2012|pmc = 3492851|pmid = 22141434|pages = 593–607|volume = 53|issue = 5|doi = 10.1111/j.1469-7610.2011.02495.x|first1 = Margaret J|last1 = Snowling|first2 = Charles|last2 = Hulme}}</ref>