Neurophysiological Basis of Dyslexia and Neuroplasticity depended Rehabilitation: A comprehensive Overview.

                                                 

This article deals with  the current understanding about the Neurophysiological basis of developmental dyslexia and  Neuroplasticty depended rehabilitation of dyslexic children.Dyslexia or reading disability is characterized by  an individual experiencing  significant difficulty with speed and accuracy of word decoding, comprehension of text and spelling. Dyslexia being one of the most common developmental learning abnormality, its exact prevalence is not conclusively known. However epidemiological estimations varying greatly between 5 to 17% of the population ( Dyslexia International 2014). Developmental dyslexia can be classified as Phonological dyslexia which is characterized by difficulty in reading pseudo words ( New words that reader have never encountered) and surface dyslexia which is characterized by difficulty in reading exception words. These are words with inconsistent spelling eg-: hymn. Dyslexic children in the general schools have been poorly trained due to the inadequate knowledge of teachers and parents about the pathophysiology of dyslexia  and the means of rehabilitation to improve the quality of life and academic performance of dyslexic children.

 Neurobiology of reading 

. The ability to read letter strings require the translation of  visual codes (orthography) into pronunciations ( phonology) with meanings( semantics) emerging when the phonology  corresponds to a known word. FMRI studies have given a better understanding about the complex neurobiological mechanisms that govern the seemingly simple process of reading.These studies have demonstrated that reading activates a widely distributed neural circuits in occipito-temporal, posterior temporal,pre-central and inferior frontal gyri ( Turkeltaub P E & Eden GF 2002).  Reading engages bilaterally distributed set of brain regions. Anterior fusiform gyrus, middle temporal gyrus , angular gyrus and anterior left inferior frontal gyrus are sensitive to or engaged in semantic tasks ie. comprehending the meaning of words.Where as posterior left inferior frontal gyrus and bilateral supramarginal gyri  and angular gyrus are involved in processing the phonological tasks ( Cathy J Price & Andrea Mechelli 2005).

Brain Regions significant in Linguistic Function. 

Current research have demonstrated that acquisition of reading skills is reflected by progressively greater activation of left hemispheric occipital , frontal and temporal regions and less activation of right posterior hemispheric regions 
( Turkeltaub PE et all 2003). A neural network of reading can be strongly left lateralized by the age of 6 or 7 ( Galliard WD et all 2003).

 Pathophysiology of Dyslexia

Past 20 years of research in Neuropsychology  have consistently demonstrated that dyslexia is a deficit of language processing. Dyslexia involves deficit decoding of individual linguistic units called phonemes, which are the smallest detectable sounds in spoken words.Phonemes are the building blocks of linguistic system and is critical for developing spoken language. Phonological processing area of the brain , the superior temporal sulcus should break words into phonemic units before an individual can identify, understand, store or remember them ( Gregory Hickok 2009). In semantic dyslexia or acquired dyslexia which is not usually seen in children may have abnormal neuronal processing in the neuroantomical networks involved. It can also occur due to stroke and neurodegenerative conditions. Whereas speech requires blending the phonemes into complete words. children with difficulty in production of meaningful words may have abnormal neuronal activity or abnormal functional connectivity between the  brain regions involved in blending the phonemes.

Graphemes are the smallest unit in written language. It is a visual symbol of spoken phoneme.Studies have shown than individuals with dyslexia shows impaired phonological awareness and as a result can not adequately pair the visually processed graphemes with their associated phonemes( Paul L et all 2006). The process of associating a grapheme with its phoneme must occur fast enough to for reading fluency and the reader must also remember and retain the word read long enough to recall there meaning. Slow grapheme - phoneme processing appears to be the primary cause of dyslexic reading problem ( Erick R Crouch 2014)  

Pugh K R et all in 2000 reported that dyslexic readers who have difficulty in phonological assembly  have a disruption in the functional connection between angular gyrus and related occipital and temporal lobe sites. Their  findings also  supports the fact that neurobiological anomalies in developmental dyslexia are largely confined to phonological processing domain in the language dominant left hemisphere. FMRI studies shows dysfunction of the connection between cerebellum the agility brain and other cerebral language processing sites as possible etiology of dyslexia. Difference in cerebellar asymmetry  and gray matter volume are some of the consistent structural brain findings in dyslexic readers. It is assumed that impaired cerebro- cerebellar connectivity as the fundamental developmental abnormality leading to difference reading neuronal network ( Stoodly CJ & Stain JF 2013). More advancement functional analysis of brain may shed light into the exact mechanism that govern language functions and the exact neuropathology behind developmental and acquired dyslexia.

                           

FMRI Image 

Evidence for positive neuroplasticty in Developmental dyslexia.

Research in Neurophysiology using FMRI have demonstrated very strong evidence for positive neuroplasticity in children with developmental dyslexia. In a 2012 study published in proceedings of national academy of science by Jane.H & Steve G.Z et all. In their  research they studied the impact of classroom FM system use for 1 year on Neurophysiology and reading skills in dyslexic children. The researches found that FM system use reduced the variability of subcortical response to sound and this improvement was linked to concomitant increase in reading and phonological awareness. 

Ann M, Timothy AK & M A Just have published a novel longitudinal study in 2008 that provided strong evidence for Neuroplasticity in dyslexic children.Their research assessed the impact of intensive remedial instruction on cortical activation among children with dyslexia during semantic task. The cortical activation and improvement in semantic performance was assessed 3 times. Prior to remediation, 100 hours post remediation and 1 year post remediation in both dyslexic and Non-dyslexic children. The central finding was prior to intervention. the dyslexic had poor cortical activation in parital cortex bilaterally. Immediately after the instructions The semantic dyslexic children had signifcat improvement in cortical activation at left angular gyrus and left superior parital lobule. They also showed substantial gains in reading abilities.The activation in this regions continued to improve and finally resulted in normalization and significant improvement in semantic comprehension and reading. 

Another promising study published in 2003 in the Proceedings of  National Academy of Science by Elise Temple et all demonstrated that behavioral interventions ameliorates dysfunctional neural mechanism in children with dyslexia. The researches did FMRI analysis pre and post intervention during phonological processing. The intervention consist of auditory processing and oral language training on 20 dyslexic children between  the age of 8 &12. The FMRI analysis showed increased cortical activation in multiple brain areas as compared to Post FMRI data in the study population. Increase in cortical activation were seen in  left temporo- parital cortex and left inferior frontal gyrus. The activation were observed to be closer to that of non-dyslexic children. Increased activity was observed in right hemisphere frontal and temporal regions and anterior cingulate gyrus. The researchers reported that children with dyslexia showed a correlation between the magnitude of increased activation in left temporo-parital cortex and improvement in oral language ability. 

List of  Non-invasive behavioral interventions and rehabilitation Methods.

• Wilson Reading 
• Spell Read Phonological Auditory Training ( P.A.T)
• Corrective reading 
• Failure free reading. 
• Focused auditory stimulation developed by Dr. Hodson
• Dore Program 
• Transcranial Magnetic Stimulation- This is the best method for inducing Neuroplasticity in children with dyslexia. All though it is noninvasive, presently it is not easily accessible. 

 The details of this methods will be discussed in the next article. I hope this article may provide the readers with a comprehensive overview about the neurophysiological basis of  linguistic abilities and Neurobiological basis of dyslexia .I also hope that this article provides good overview about the scientific evidence for intensive rehabilitation and behavioral intervention induced Neuroplasticity in Developmental dyslexia. 

Siby C Chacko MPE ( Exercise Physiology)          

References        

1)http://www.dyslexiainternational.org/Educational%20Authorities/About%20dyslexia%20ea.

2) Turkeltaub P E , Eden G F, Jhones K M, Zefero T A, Meta - Analaysis of the functional Neuroanatomy of single word reading : Method and Validation , Neuroimaging  16:765-780( 2002).

3) Cathy J Price, Andrea Mechelli , Reading and reading disturbance, Curr.Opinion in Neuro. 15:231-238 ( 2005).

4) Tukeltaub P E , Gareau L, Flowers D L, Zeffiro TA , Eden G F, Development of Neural Mechanism for reading. 6(7) 767-773 ( 2003).

5) Galliard WD, Balsamo LM, Ibrahim Z, Sacs BC, Xu B, FMRI identifies regional networks for reading in young children. Neurology 60: 94-100 ( 2003).

6) Gregory Hickok, The Functional Neuroanatomy of language. Phys. lif.rev.6(3)121-143 ( 2009).

7) http://reference.medscape.com/medline/abstract/16441886.

8)http://emedicine.medscape.com/article/1835801-overview#a0104.

9) Pugh K R, Menchel W E, Shaywitz BA, Fullbright R K, Constable RT, Skudlarski P , Marchione KE et all, The angular gyrus in developmental dyslexia: Task specific difference in functional connectivity with posterior cortex.Psycho.scie. 11(1) 51-6 ( 2000).

10) Stoodly C J , Stain JF, Cerebellar function in developmental dyslexia. Brain & lang. 12(2)267-76( 2013).

11) Jane H , Steven G Z & Nina Cruz. Assistive listening devices drive Neuroplasticity in children with dyslexia. Prs. Natl. acad. sci. USA109(41) 16731-16736( 2012).

12) Ann M, Timothy A k, M A Just. Modifying The brain activation of poor readers during sentence comprehension with extended remedial instruction. Nueropsycho. 46(10) 2580-2592 ( 2008).  

13) Elise Temple, Gayle K D, John D E G,  Neural deficit in children with dyslexia ameliorated by behavioral remediation: Evidence from FMRI . Proc. Natl. Acad. Sci. 100(5)_ 2860-65 ( 2003).