Reading is a culturally invented skill, not a product of evolution, that must be acquired through instruction and practice. It requires the brain to repurpose visual object recognition mechanisms for letter and word identification, map written symbols onto spoken language representations, and coordinate multiple cognitive processes (visual, phonological, semantic, syntactic) in real time. Dyslexia — affecting 5-10% of the population — reveals what happens when specific components of this complex system fail to develop normally.
The Reading Brain
Stanislas Dehaene's neuronal recycling hypothesis proposes that the visual word form area (VWFA) in the left fusiform gyrus — originally evolved for object recognition — is repurposed through literacy training to specialize in letter and word recognition. This "letterbox" region becomes increasingly tuned to the orthographic regularities of the learned writing system, allowing rapid visual word recognition that feeds into the language processing network.
Dyslexia
Developmental dyslexia is characterized by unexpected difficulty with reading and spelling despite adequate intelligence, instruction, and motivation. The phonological deficit hypothesis — the most widely supported explanation — proposes that dyslexia stems from a core difficulty in representing and manipulating the sound structure of language (phonological processing). This deficit impairs the mapping between written letters and speech sounds (phonics) that is essential for learning to read.
Neuroimaging studies consistently show that readers with dyslexia show reduced activation in left-hemisphere posterior reading regions (temporoparietal and occipitotemporal cortex) and sometimes increased activation in right-hemisphere and frontal regions (possibly reflecting compensatory mechanisms). Structural differences include reduced gray matter in left perisylvian regions and white matter abnormalities in tracts connecting language areas. These neural differences are present before reading instruction begins, suggesting they are causes rather than consequences of reading difficulty.