Differences in Brain Anatomy in Dyslexia
Dr Shelley Hyman
Developmental dyslexia is a disorder that can be characterized by below average reading ability despite normal intelligence and education levels. Due to the complex and interconnected processes associated with reading, brain imaging studies have examined a variety of potentially abnormal brain areas. A review conducted by Eckert (2004) examined multiple cortical regions on functionality and volume, and their associated language problems.
Voxel based studies have assessed differences between children with dyslexia and controls in grey and white matter. One study found significantly less grey matter in adults with dyslexia in areas associated with written language including the bilateral occipital lobes, left inferior and middle temporal lobe and the left and right angular gyri.
Majority of studies examining brain differences associated with impairments in oral language have concluded that there are significant differences in the parietal operculum, inferior fontal gyrus and the cerebellum.
Heschl’s gyrus is the site of the primary auditory cortex, with volume relating to size and sensitivity of auditory maps and potentially language impairment. Some individuals have a second gyrus posterior to Heschl’s, which is more common to appear in the right hemisphere. This gyrus duplication has been extensively studied in relation to dyslexia due to the association with decreased phonological processing abilities. Mixed findings have suggested that individuals with Heschl’s gyrus duplication may possess a genetically unique dyslexic subtype affecting phonology only.
The planum temporale is posterior to Heschl’s gyrus and covers a large portion of the superior temporal gyrus. It is common for the planum temporale to be larger in the left hemisphere, which may represent the left lateralisation of language organisation. It is therefore proposed that dyslexia may result from reversed or exaggerated asymmetry of the planum temporale.
Research has shown that adults with dyslexia have smaller temporale lobe volume than controls. Follow up studies have also reported less grey matter in the inferior, middle and superior temporal gyri, with the superior region showing the most significant group differences. Passive listening to words activates the superior temporal gyrus which activates the inferior parietal lobule. Numerous studies have found exaggerated leftward asymmetry in parietal-temporal regions.
The inferior frontal gyrus has connections to the superior temporal gyrus which maps phonological representations for speech output. Abnormal mapping between these two regions may explain why children with dyslexia produce atypical activation of the temporal-parietal and frontal regions during phonological tasks. Decreased grey matter in the left inferior frontal gyrus has also been reported in children with dyslexia versus controls.
The cerebellum has shared connections with the frontal gyrus, superior temporal sulcus and associated parietal regions. Deficits that have been reported due to abnormal cerebellum functioning include poor naming, reading errors, and decreased activation in phonological tasks. Decreased grey matter of the left lobe has also been reported in children with dyslexia versus controls.
Issues in connectivity of networks associated with reading include bilateral differences in the temporal-parietal white matter regions. These connections correlated with pseudo and real word reading ability. Anatomical differences of the corpus callosum may also explain some of the deficits associated with dyslexia. Individuals with dyslexia show a more rounded corpus callosum than controls, which may be due to the shortened posterior midbody.
- Eckert, M. (2004). Neuroanatomical markers for dyslexia: A review of dyslexia structural imaging studies. Neuroscientist, 10, 362-371. DOI: 10.1177/1073858404263596