Dyslexia is a specific learning disability that primarily affects an individual’s ability to read and spell, despite having average or above-average intelligence. This condition is neurological in origin and represents a difference in how the brain processes language. It is not caused by a lack of motivation, effort, or sensory impairment. Understanding the characteristics of dyslexia requires examining the underlying neurological and cognitive mechanisms. These sections define the primary characteristics, from brain-based differences to observable difficulties in literacy and associated traits.
The Neurological Basis of Dyslexia
Dyslexia stems from structural and functional differences in the brain’s language processing centers, primarily in the left hemisphere. Brain imaging studies reveal reduced activation in key regions responsible for processing the sounds of language, such as the temporo-parietal region and the inferior frontal gyrus. This under-activation is often compensated for by increased reliance on less efficient brain regions, including those in the right hemisphere. The white matter tracts connecting these language areas, such as the arcuate fasciculus, may also show reduced integrity, disrupting communication efficiency. Dyslexia is highly heritable, often running in families, with genetic studies identifying associated genes like DCDC2 and KIAA0319.
Primary Characteristics in Reading and Spelling
The most recognizable characteristics of dyslexia are the observable difficulties in achieving accurate and fluent word recognition and proficient spelling. These difficulties are often first noticed when a child begins formal reading instruction. Early indicators include persistent trouble learning common nursery rhymes or recognizing the letters of the alphabet.
In the school-aged child, the core difficulty manifests as slow, laborious, and often inaccurate reading, severely impacting reading fluency. The student may frequently confuse small function words (like “to” and “of”) or make consistent errors such as transposing letters (“felt” for “left”) or making letter reversals (“b” for “d”). Reading comprehension is often affected because mental effort is consumed by the task of decoding the words themselves.
Spelling is typically disorganized and inconsistent, with the same word often being spelled multiple ways within a single piece of writing. Students struggle to decode unfamiliar words because they cannot reliably map the letters to their corresponding sounds. This difficulty with word-level reading and spelling continues into adolescence and adulthood, often requiring significant time and effort to complete reading assignments.
The Core Cognitive Deficit: Phonological Processing
The underlying factor driving reading and spelling difficulties is a specific cognitive barrier known as a deficit in phonological processing. This refers to the ability to recognize and manipulate the sound structure of spoken language. The most significant component of this deficit is poor phonological awareness, which is the consciousness of the individual sound units, or phonemes, that make up words.
For example, a person with this deficit struggles to segment a word like “cat” into its three distinct phonemes, or to blend those sounds back together to form the word. Since reading requires connecting these phonemes to their visual letter representations (graphemes), a weakness in this area makes decoding almost impossible. This difficulty in mapping sounds to symbols is the primary barrier to developing fluent word recognition.
The phonological deficit also affects phonological memory, which is the ability to hold a sequence of speech sounds in short-term memory. The inability to automatically access and manipulate these sound codes makes the learning and retrieval of letter-sound correspondences a taxing and effortful process.
Associated Non-Literacy Characteristics
While the core of dyslexia relates to reading, several other characteristics often accompany the condition, though they are not universal. One common associated trait is a deficit in rapid automatized naming (RAN), which is the speed at which a person can retrieve and articulate the names of familiar visual stimuli like letters, numbers, or colors. This difficulty impacts the automaticity required for fluent reading.
Difficulties with verbal working memory are also frequently observed, particularly with the phonological loop component. This can lead to trouble following multi-step verbal instructions or remembering lists of facts, names, or dates. Sequencing difficulties may also manifest, causing problems with tasks that require ordering, such as recounting a story or learning the order of the months of the year.
These associated characteristics can affect executive function skills, including organization, time management, and planning complex tasks. Although these difficulties are not a direct cause of the reading impairment, they often compound the academic and daily life challenges faced by individuals with dyslexia.