Dyslexia is a specific learning disability, neurobiological in origin, that affects how the brain processes language. It is characterized by significant difficulties with accurate or fluent word recognition, poor spelling, and decoding abilities. These challenges typically result from a deficit in the phonological component of language, which is often unexpected given the individual’s cognitive abilities and instruction quality. The modern understanding of this condition is the result of a historical journey that began with medical observations in the late 19th century.
The Initial Discovery and Description
The earliest medical documentation occurred in the late 19th century, long before the term “dyslexia” was in common use. In 1877, German physician Adolph Kussmaul identified and described a condition he termed Wortblindheit, or “word blindness.” Kussmaul observed patients who could not read printed words, even though their vision, intellect, and speaking abilities were unimpaired. His work differentiated this reading difficulty from aphasia, which typically results from a stroke or brain injury.
The focus later shifted from adults with acquired reading loss to children with developmental difficulties. This was advanced by the 1896 case published by British physician W. Pringle Morgan. Morgan described 14-year-old Percy F., who was “bright and intelligent” but struggled severely with learning to read. This detailed account of developmental reading difficulty is frequently cited as the first thorough medical description of what is now known as developmental dyslexia.
Coining the Term and Early Theories
The formal naming of the condition followed Kussmaul’s description by a decade. In 1887, German ophthalmologist Rudolf Berlin coined the term “dyslexia,” deriving it from the Greek roots dys- (difficulty) and lexis (words). Berlin introduced this term to align the diagnosis with other medical nomenclature, such as alexia, which describes complete reading loss.
The prevailing scientific theory viewed the condition as “congenital word blindness.” Physicians hypothesized that the difficulty stemmed from a visual processing deficit or a physical difference in the brain, often localized to the left angular gyrus. This visual-based model assumed the difficulty was rooted in an inability to correctly perceive the written word.
Shifting Focus: From Visual Deficit to Language Disorder
The understanding of dyslexia began to move away from the visual deficit model in the 1920s and 1930s, largely due to American neurologist Samuel T. Orton. Orton observed that many children exhibited letter and word reversals, leading him to propose a theory of incomplete cerebral organization. He coined the term strephosymbolia, meaning “twisted symbols,” believing the issue related to a failure to establish dominance in one brain hemisphere.
A major conceptual shift occurred in the mid-20th century, establishing the modern consensus that the core issue is not visual but phonological. Research identified that the primary difficulty lies in phonological awareness—the ability to recognize and manipulate the basic sounds of spoken language. This linguistic-based model provided a more accurate explanation for struggles with decoding and spelling.
This neurobiological understanding led to the inclusion of standardized diagnostic criteria in modern manuals. Current diagnostic systems, such as the DSM-5 and the World Health Organization’s ICD-11, classify the condition as a Specific Learning Disorder or Developmental Learning Disorder with impairment in reading. These classifications confirm the condition is a neurodevelopmental disorder of biological origin.