Reading is a process which involves a number of brain areas. These areas of the brain tend to be different in individuals with dyslexia, helping to explain the challenges they face. The image below helps to show where some of these areas of the brain are located. When someone reads a word, the visual image is processed in the rear of the brain. This region is known as the occipital lobe, where the lines and curves of the letters are analyzed. From here the information is sent through two main potential pathways.
The upper pathway, the dorsal stream, runs to the left parieto-temporal region (between the parietal and temporal lobes) as well as the frontal lobe to a lesser extent. These regions are used for phonetic decoding, the process of breaking a word down into individual sounds. This pathway is used almost exclusively by children learning to read.
The lower pathway, the ventral stream, goes to the occipito-temporal region (between the occipital and temporal lobes). This region becomes more active as children become more proficient at reading. It is associated with automatic word recognition and thus reading fluency, the ability to read accurately and quickly. Words are seen as a whole by this region, rather than broken down phonetically.
While all three of these regions are recruited by strong readers, they primarily rely on the occipito-temporal region to allow for fluent reading. Those individuals with dyslexia tend to rely heavily on the left frontal lobe, as well as recruiting the right hemisphere as well. While this compensation by utilizing other brain areas allows them to read, it is far less fluent.
Reference:
Mather, N, & Wendling, B.J. (2012). Essentials of dyslexia assessment and intervention. Hoboken, NJ: John Wiley & Sons, Inc.