An interesting comparison can be made between poor readers who have experienced brain trauma associated with an injury or a stroke, and individuals who experience reading difficulties in the absence of brain trauma. The research literature has described the first type of individual as having “acquired dyslexia” and the second type of individual as having “developmental dyslexia.”
There are several variants of both kinds of dyslexia but the theoretical description of why individuals with acquired dyslexia and why individuals with developmental dyslexia have reading problems is strikingly different. To understand the differences we need to understand a little bit about how we can recognize words when we read. When learning to read we store two types of “word memories” in our heads. The first type is the word image of a word. This is essentially a photograph of the way a word appears. Having developed such a representation we can look at a word, and then activate the word image of the word. Having done this we still need a further process: we need to transform the word image into the sound of the word. In essence, we activate the image of the word and then determine how the word sounds in speech.
The second type of word memories involves the storage of both word images and the phonemes that make up words. Phonemes are the smallest unit of sound in spoken languages. For a detailed presentation of the role of phonemes in learning to read, see the developing skilled reading link at the bottom of this page. As reading skill develops, the way that normal readers activate the sound of a word changes. Initially, the individual uses both the word image and the phonological memory for a word. But as skill improves, the activation of the phonological representation overrides the activation of word images. This occurs because activation of the phonological representation is faster.
Now we come to acquired and developmental dyslexia. The brain trauma that results in dyslexia like symptoms (acquired dyslexia) inhibits the activation of phonological representations and word image representations with particular damage to word image representations. In contrast, individuals with developmental dyslexia have difficulty accessing the phonological representations of words. In particular, they have difficulty recognizing words that have irregular pronouncing patterns (e.g., iron) and letter sequences that are not words (e.g., sloke) but can be pronounced. Both types of dyslexia present as an individual with impaired reading skills, but the underlying causes of the impairment are quite different.
For a more detailed explanation of the causes of developmental dyslexia follow the developing skilled reading link below.