Dyslexia is a specific learning difference primarily affecting reading and spelling skills. Individuals with dyslexia possess normal or even above-average intelligence, yet they encounter unexpected difficulties in acquiring literacy. Neuroimaging has deepened our understanding of how the dyslexic brain differs in its physical structure and activity patterns, revealing insights into its neurological basis.
Structural Characteristics
The brains of individuals with dyslexia often show observable anatomical differences compared to typical readers. Research indicates variations in grey matter density, which is composed largely of nerve cells responsible for processing information. Studies have found that people with dyslexia can have less grey matter in certain areas, such as the left parietotemporal region, affecting the processing of language sounds. Conversely, some studies have identified increased grey matter volume in other regions, such as the fusiform gyri.
Differences extend to white matter integrity, which forms the brain’s communication pathways. White matter is crucial for efficient information transfer between brain regions. Individuals with dyslexia often exhibit altered white matter integrity, particularly in left hemisphere occipito-temporal and temporo-parietal areas. The arcuate fasciculus, a significant white matter tract involved in language processing, has been shown to have lower connectivity in dyslexic readers.
Furthermore, specific brain regions can differ in size or organization. The planum temporale, an area in the left hemisphere associated with auditory and language processing, typically shows a larger size in the left hemisphere in most of the general population. However, in individuals with dyslexia, this typical leftward asymmetry may be reduced or even reversed. These structural variations provide a foundation for understanding how the dyslexic brain processes information.
Functional Activity Patterns
When individuals with dyslexia engage in reading and language tasks, their brains function differently. A consistent finding is under-activation in posterior reading systems, which are typically highly active in skilled readers. These areas include parts of the temporal and parietal lobes, such as the left temporoparietal cortex and the left occipitotemporal region (visual word form area). This under-activation suggests less efficient engagement of neural networks involved in mapping letters to sounds and recognizing written words.
To compensate for this reduced activity, dyslexic brains often show increased activation in other areas, particularly frontal regions. The left inferior frontal gyrus, involved in speech production and sound articulation, may exhibit over-activation. This compensatory activation indicates the brain is working harder or using alternative pathways to process reading material, making reading more effortful and less automatic. Such patterns highlight how the dyslexic brain adapts its functional strategies to navigate reading demands.
Neural Communication
Differences in how various brain regions communicate with each other are prominent in dyslexia. White matter pathways show altered organization or efficiency. The transfer of information between language and reading-related areas may not be as rapid or accurate as in typical readers.
Individuals with dyslexia often have altered white matter pathways. Key pathways like the inferior fronto-occipital fasciculus (IFOF) and inferior longitudinal fasciculus (ILF) have shown reduced integrity or connectivity in dyslexic individuals. These differences in connectivity can impact the speed and synchronicity of information processing across the brain. This disrupted communication network underscores a fundamental aspect of the dyslexic brain’s distinct wiring.
Connecting Brain Differences to Reading
The structural and functional differences, along with atypical neural communication, directly contribute to the characteristic challenges faced by individuals with dyslexia. The under-activation in posterior brain regions, coupled with altered white matter pathways, impacts phonological processing—the ability to recognize and manipulate the sounds of language. This can make decoding words, or sounding them out, a significant hurdle.
The brain’s reliance on compensatory frontal lobe activity, while enabling reading, often leads to slower, more laborious reading compared to the automaticity seen in typical readers. This effortful processing affects reading fluency, making it difficult to read smoothly and quickly. Consequently, comprehension can also be impacted, as cognitive resources are heavily diverted to the mechanics of decoding rather than understanding the text’s meaning. These neurological variations underscore that dyslexia is a neurobiological condition, providing a basis for its specific learning profile.