Dyslexia is a neurobiological learning difference that affects a person’s ability to read and spell words. This difficulty arises from differences in how the brain processes the sound structure of language, and it is not connected to a person’s overall intelligence or ability to learn. Research indicates that the prevalence of dyslexia among school-aged children falls between 5% and 17% of the population.
Developmental Versus Acquired Dyslexia
The classification of reading difficulties is based on the timing of onset, distinguishing between developmental and acquired forms. Developmental dyslexia is present from birth, often hereditary, and persists as a lifelong condition. It is rooted in genetic and neurodevelopmental factors that affect how the brain organizes itself for language processing.
Acquired dyslexia, also known as alexia, results from neurological damage after a person has already learned to read. This form typically occurs following a stroke, traumatic brain injury, or certain diseases affecting the brain. Individuals with acquired dyslexia lose previously established reading skills, and the presentation of symptoms can vary depending on the location and extent of the brain damage.
Phonological Dyslexia
Phonological dyslexia is the most common and widely studied profile of developmental dyslexia, characterized by a core deficit in phonological awareness. This awareness is the ability to recognize, manipulate, and work with the smallest units of sound, called phonemes, within spoken language. The difficulty is not with hearing, but with processing the sounds that make up words.
Individuals with this profile struggle to establish the link between letters (graphemes) and their corresponding sounds (phonemes), a process known as grapheme-phoneme correspondence. This makes the fundamental task of decoding—sounding out unfamiliar words—laborious and inaccurate. For instance, they may have trouble blending the sounds /c/, /a/, and /t/ together to form the word “cat.”
This impairment in sound processing leads to slow and non-fluent reading, especially when encountering new or longer words. When reading, a person may substitute a word with a visually similar one or an unrelated word, or they may struggle with non-words or pseudowords that cannot be recognized by sight. The challenge with mapping sounds to print translates into significant difficulties with spelling. Because the brain’s language processing centers for phonology are less efficient, reading becomes an act of intense effort rather than an automatic skill.
Surface Dyslexia
Surface dyslexia presents a different pattern of reading difficulty, where the primary challenge lies with the orthographic pathway for reading. This pathway is responsible for recognizing whole words instantly, storing them as “sight words” in the brain’s visual word form area. Individuals with this profile find it hard to memorize the visual shape of a word for quick, automatic retrieval.
The difficulty is particularly pronounced with irregular words, which do not follow standard phonetic rules, such as “yacht,” “colonel,” or “debt.” A reader with surface dyslexia will often attempt to sound out these irregular words phonetically, resulting in errors like pronouncing “yacht” as yatch-et or “colonel” as col-o-nel. Their reliance on sounding out every word, even familiar ones, leads to reading that is slow and choppy, lacking fluency.
In contrast to the phonological profile, individuals with surface dyslexia can often successfully decode words that follow regular phonetic patterns. However, their inability to build a robust sight word vocabulary means that they treat every word as if it is being seen for the first time. The effort involved in continuously decoding words drains cognitive resources, which can impact reading comprehension.
Related Profiles and Co-occurring Learning Differences
The Rapid Automatized Naming (RAN) deficit is characterized by a difficulty in quickly and automatically retrieving and articulating the names of a series of familiar visual stimuli, such as letters, numbers, colors, or objects. A slower RAN score is strongly correlated with reduced reading fluency, suggesting an issue with processing speed or the efficiency of accessing verbal information from memory. While a RAN deficit can exist independently, it frequently co-occurs with the phonological deficit, sometimes leading to a “double-deficit” profile that represents a more severe reading impairment.
Other conditions, such as dysgraphia and dyscalculia, are separate Specific Learning Disorders (SLDs) that affect writing and mathematics, respectively. Dysgraphia involves difficulties with the mechanical aspects of writing, while dyscalculia involves problems with number sense and mathematical reasoning. These distinct SLDs are frequently diagnosed alongside dyslexia, reflecting overlapping neurobiological factors, but they are not considered types of dyslexia itself.