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dyslexia. We’ll examine how these variations in brain function or structure may be responsible for both the dyslexia-associated challenges we’ve listed and the dyslexic advantages we’ll discuss in later chapters. Let’s begin, in this chapter, by looking at two dyslexia-associated variations in information processing (or cognition ).

Phonological Processing
    The first pattern we’ll discuss is a variation in the brain’s phonological (or “word sound”) processing system . This system is used to process phonemes, the basic sound components in words. English has approximately forty-four phonemes, and just as the letters of our alphabet can be strung together to form printed words, phonemes can be strung together to form all the spoken words in English.
    For the past thirty years most reading specialists have favored the phonological impairment theory as the most likely explanation of the brain basis of dyslexic reading and spelling problems. Think back for example to the definition of dyslexia cited in chapter 1, which states that dyslexia-associated reading and spelling difficulties “typically result from a deficit in the phonological component of language.”
    There are several good reasons for believing that phonological impairments play a key role in causing dyslexic reading and spelling challenges. Problems with phonological processing have been found in at least 80 to 90 percent of individuals with dyslexia, and they can clearly contribute to many of the challenges mentioned earlier in this chapter. The role that phonological processing impairments play in the reading and spelling challenges many individuals with dyslexia display has been especially well worked out. While we’ll save a fuller discussion of this role for our chapter on reading, there are several key points we should mention here.
    The phonological processing system plays a key role in analyzing and manipulating the sound structures of words. Many of these functions are important for matching word sounds and the letters used to represent them—that is, for mastering the rules of phonics which underlie decoding (or sounding out words) and encoding (or spelling words). Two of the most important phonological processing (or phonological awareness ) tasks underlying these skills are sound segmentation (or the ability to split incoming words into their component sounds) and sound discrimination (or the ability to distinguish word sounds from one another). Most individuals with dyslexia struggle with one or both of these tasks and as a result have difficulty mastering the basic skills underlying reading and spelling.
    Even though phonological processing involves low-level or fine-detail language processing—that is, processing of the most basic building blocks of language—it forms the foundation for the entire language structure and supports many of the higher language functions. That’s why severe problems with phonological processing can cause difficulty at all levels of language, such as mastering word meanings, learning how words interact when used in groups (that is, grammar and syntax), and understanding how words work together to form “discourse-level” messages like paragraphs or essays. When the higher-order language problems resulting from phonological impairments are severe, they are called “specific language impairment,” but the underlying process remains the same.
    The phonological processing system also plays an important role in many attention-related functions, including working memory and executive functioning. Working memory is the kind of short- to intermediate-term memory that helps us “keeps things in mind” for active conscious processing—very much like the random-access memory or RAM on your computer. The phonological processing system forms a phonological loop (or short-term memory tracing) that keeps auditory-verbal information alive in active