Quick Answer
Dyslexia primarily affects the left side of the brain, specifically the areas involved in language processing like the temporal lobe and occipital lobe. The left hemisphere is usually dominant for language and reading skills, so damage or abnormalities in these left brain regions can lead to difficulties with reading, spelling, writing, and speech.
What Parts of the Brain are Impacted by Dyslexia?
Dyslexia is a learning disability that affects reading and language processing abilities. Research using brain imaging techniques like fMRI and PET scans have shown that dyslexia is associated with differences in structure and function in several areas of the brain, particularly in the left hemisphere.
Left Temporal Lobe
The left temporal lobe contains Wernicke’s area which is critical for language comprehension. Studies show the left temporal lobe is often smaller and less active in people with dyslexia. This is thought to contribute to problems decoding language sounds and linking them to letter symbols and words.
Left Parietal Lobe
The left parietal lobe, specifically the angular gyrus, integrates incoming visual information with language processing. Studies indicate this region functions abnormally in people with dyslexia, impacting their ability to map letters and words onto their sounds and meanings.
Left Occipital Lobe
The left occipital lobe contains the visual word form area which specializes in processing written words. Research finds this region is underactivated and disorganized in dyslexia, hindering fast and accurate visual word recognition.
Corpus Callosum
The corpus callosum connects the brain’s hemispheres and helps transfer information between them. Studies show it tends to be smaller in people with dyslexia, limiting communication between language areas in the left hemisphere.
Cerebellum
The cerebellum plays a role in motor control and cognition. Imaging studies reveal cerebellar dysfunction in many people with dyslexia, which may impact their articulation and automaticity with language-related skills.
Right vs Left Brain Differences in Dyslexia
While dyslexia mainly stems from abnormalities in left-brain language networks, there are also some differences observed in the right brain hemisphere. However, these right brain differences are thought to be compensatory effects rather than a primary cause of dyslexia.
Right Parietal Hyperactivation
Some studies have found increased activation in the right parietal lobe during reading and language tasks in people with dyslexia. This is hypothesized to reflect compensation for weaknesses in the corresponding left brain region.
Right Occipital Hyperactivation
Overactivation of the right occipital lobe has also been documented in some cases of dyslexia. Again, this is believed to reflect the brain recruiting alternate pathways to try to boost visual word processing in response to deficits in left occipital areas.
Right Frontal Recruitment
Additional activation of right frontal lobe regions involved in attention and working memory is observed in some people with dyslexia. This may help them engage more cognitive resources to complete reading and language tasks.
Structural Brain Differences
In addition to functional differences, studies using MRI scans reveal several structural brain anomalies associated with dyslexia:
- Smaller gray matter volume in left hemisphere language regions
- Reduced white matter connectivity between left hemisphere language centers
- Atypical asymmetry between hemispheres, with the right side more dominant
- Altered development of the cerebellum
These structural differences are present from early childhood and contribute to the difficulties with fluent reading and spelling characteristic of dyslexia.
Genetic and Neurochemical Factors
Genetics play a major role in dyslexia, with inherited abnormalities in brain development and neurochemistry underlying the condition in many cases. Some examples include:
- Genes affecting neuronal migration in the fetal brain
- Genes influencing axon growth and connectivity
- Abnormalities in neurotransmitters like dopamine and serotonin
Ongoing research is working to further identify the specific genetic variants and neurochemical mechanisms involved in dyslexia to improve early detection and intervention.
Conclusion
In summary, dyslexia stems primarily from dysfunction of left-brain language and reading networks, especially temporal, parietal and occipital lobe regions. Compensatory hyperactivation in right brain areas also occurs. In addition to functional differences, people with dyslexia show structural brain anomalies like reduced gray matter volume and white matter connectivity between language regions. These brain variations arise early in development due to genetic and neurochemical factors that disrupt neuronal growth and communication pathways critical for fluent reading abilities.