A set of specialized cells that serve a specific function (e.g., visual system).
Either of the two symmetrical halves of the cerebrum, as divided by the longitudinal cerebral fissure.
An area in the brain, usually in the left hemisphere, that is responsible for the organization of motor speech patterns or language output.
The region in the brain where the parietal and temporal lobes meet.
Of, relating to, or distributed to the occipital and temporal lobes of the brain. See occipital lobe. See temporal lobe.
A condition that involves the loss of a previously possessed ability to read, despite intact vision.
An area of the brain that has been damaged by injury or disease.
The lobe in the brain responsible for determining spatial sense and navigation. Also responsible for visual perception.
The region of the brain thought to be involved in auditory perception, semantic processing and vision.
The study of the sound system used in language and its rules for combining sounds and patterns of stress and intonation.
An inherited specific learning disability that makes it extremely difficult to read, write, and spell despite at least average intelligence. It is characterized by abilities below the expected level given a child's age, school grade, and intelligence.
The smallest of the four lobes of the human brain; responsible for visual processing.
An area of the brain thought to be responsible for amassing and processing combinations of letters and their meanings into coherent words of a given language.
A system for representing the sounds of language by written symbols. This involves correct spelling patterns and rules.
The connection of neurons within the brain; the pathway used to achieve a specific goal.
Unit of speech or text that appears like it could be an actual word in a certain language while in fact it is not.
A specific pattern of brain function or dysfunction as indicative of a particular disorder.
The specific aspect of a language which exists beyond the specific sounds (i.e., beyond their phonemic composition). For example, syllable, intonation, stress.
A graphic symbol that represents an idea or concept.
Refers to the ability of the brain to adapt to new conditions; for example, if one area of the brain becomes nonfunctional, another area may take over its responsibilities to some extent.
Neural systems for reading
Data from laboratories around the world indicate that there are a number of interrelated neural systems
used in reading, at least two regions in the back of the brain (posterior) as well as distinct and related systems in the front of the brain (anterior). We refer to "systems" rather than "regions" because each of the areas of the brain related to reading generally encompasses more than a single brain region.
Three neural systems for reading are illustrated in this figure of the surface of the left hemisphere
: an anterior system in the region of the inferior frontal gyrus (Broca's area)
believed to serve articulation and word analysis; two posterior systems, one in the parieto-temporal region
believed to serve word analysis, and a second in the occipito-temporal region
(termed the word-form area) and believed to serve for the rapid, automatic, fluent identification of words. Reprinted from Shaywitz (2003) with permission.
These systems have been documented by many laboratories using brain imaging studies (see the following two recent reviews for full references: Shaywitz, Morris, & Shaywitz, 2008; Shaywitz & Shaywitz, 2008). A large literature on acquired inability to read (acquired alexia
) describes brain lesions
in one of the posterior reading systems, a system encompassing portions of the parietal lobe
and temporal lobe
, termed the parieto-temporal system, as pivotal in mapping the visual percept of print onto the sounds (phonology
) of spoken language. Many recent brain imaging studies in patients with developmental dyslexia
(see below) have documented the importance of this posterior reading system in word analysis (i.e., going from the written words to the sounds of spoken language).
A second posterior reading system is of particular importance for skilled, fluent reading (i.e., reading that is rapid and automatic). This system encompasses portions of the occipital lobe
and temporal lobe and has been termed the visual word-form area (VWFA)
(Cohen et al., 2000; Dehaene, Cohen, Sigman, & Vinckier, 2005; Vinckier et al., 2007). A recent report (Gaillard et al., 2006) provides more convincing cause and effect evidence of the critical role of the VWFA in reading. In this case, a 46-year-old right-handed man developed alexia after resection of a large portion of the posterior brain involving the word form area. Following surgery, reading was difficult and slow though other cognitive skills (e.g., object naming, face recognition, language and writing to dictation) remained normal. Brain activation in the VWFA in response to words was normal prior to surgery but was not apparent following resection of the left occipito-temporal region. How the VWFA functions to integrate phonology (sounds) and orthography
(print) is as yet unknown. Another reading related neural circuit
involves an anterior system known as Broca's area, located in the inferior frontal gyrus. This system has long been associated with articulation and also serves an important function in silent reading and naming.
The reading systems in dyslexia in children and adults
Converging evidence from many laboratories around the world has demonstrated "a neural signature
for dyslexia," that is, a disruption of posterior reading systems during reading real words and pseudowords
, and often what has been considered as compensatory overactivation in other parts of the reading system.
What has been termed a neural signature for dyslexia is illustrated in this schematic view of left hemisphere brain systems in non-impaired (left) and dyslexic (right) readers. In non-impaired readers, the three systems shown in Figure 1 are shown. In dyslexic readers, the anterior system is slightly overactivated compared with systems of non-impaired readers; in contrast, the two posterior systems are underactivated. This pattern of underactivation in left posterior reading systems is referred to as the neural signature for dyslexia. Reprinted from Shaywitz (2003) with permission.
In a study of children with dyslexia, we used fMRI (functional Magnetic Resonance Imaging which is used to visualize the brain) to study 144 dyslexic and typical reading boys and girls as they read pseudowords and real words (Shaywitz et al., 2002). Results indicated that during phonologic analysis, typical readers demonstrate significantly greater activation than do children with dyslexia in predominantly left hemisphere sites (representing the anterior reading system around Broca's area and two posterior sites, one in the parieto-temporal system, and the other in the occipito-temporal system). Studies of children with dyslexia are particularly important because they indicate that dysfunction in left hemisphere posterior reading circuits is already present in children with dyslexia and cannot be ascribed simply to a lifetime of poor reading. Further, this study found that while the posterior reading systems were disrupted in dyslexic readers, compensatory systems developed.
Data from fMRI studies in children with dyslexia reported by our group converge with reports from many investigators using functional brain imaging which show a failure of left hemisphere posterior brain systems to function properly during reading, findings which are apparent regardless of the language spoken. For example, in a study funded by the European Union, Paulesu et al. (2001) studied dyslexic readers in England, France and Italy. Regardless of whether the native language spoken was English, French or Italian, dyslexic readers exhibited a disruption in posterior reading systems.
Development of reading systems in dyslexia and the importance of the occipito-temporal reading system
While converging evidence points to three important neural systems for reading, few studies have examined age-related changes in these systems in typical readers or in children with dyslexia. In a recent report (Shaywitz et al., 2007) we used fMRI to study age-related changes in reading in a cross-sectional study of 232 dyslexic and non-impaired boys and girls as they read pseudowords. Findings indicated that a system for reading which develops with age in dyslexic readers differs from that in non-impaired readers: dyslexic readers develop a more posterior and medial system while non-impaired readers develop a more anterior and lateral system within the left VWFA. Interestingly, this difference in activation patterns between the two groups of readers has parallels to reported brain activation differences observed during reading of two Japanese writing systems: Kana and Kanji. Left anterior lateral occipito-temporal activation, similar to that seen in non-impaired readers, occurred during reading Kana (Nakamura, Dehaene, Jobert, Le Bihan, & Kouider, 2007). Kana script employs symbols that are linked to the sound or phonologic element
(comparable to English and other alphabetic scripts). In Kana and in alphabetic scripts, children initially learn to read words by learning how letters and sounds are linked and then, over time, these linkages are integrated and permanently instantiated as a word form.
In contrast, posterior medial occipito-temporal activation, comparable to that observed in dyslexic readers, was noted during reading of Kanji script (Nakamura et al., 2007). Consideration of the mechanisms used for reading Kanji compared to Kana provides insight into potentially different mechanisms that develop with age in dyslexic readers contrasted to non-impaired readers. Kanji script uses ideographs
where each character must be memorized as it represents a concept or idea, but is not linked to specific sounds. This suggests that the posterior medial occipito-temporal system functions as a memory-based system. It is reasonable to suppose that as children with dyslexia mature, this posterior medial system supports memorization rather than the progressive sound-symbol linkages observed in non-impaired readers. And there is evidence that dyslexic readers are not able to make good use of sound-symbol linkages as they mature and instead, come to rely on memorized words. For example, phonologic deficits continue to characterize struggling readers even as they enter adolescence and adult life; as reported by us previously (Shaywitz et al., 2003), persistently poor adult readers read words by memorization so that they are able to read familiar words but have difficulty reading unfamiliar words.
Thus, these results support and now extend previous findings to indicate that the system responsible for the integration of letters and sounds, the VWFA, is the neural circuit that develops with age in non-impaired readers. And conversely, dyslexic readers, who struggle to read new or unfamiliar words, come to rely on an alternate system, the posterior medial occipito-temporal system, which functions via memory networks.
Effects of reading interventions on neural systems for reading
Given the converging evidence of a disruption of posterior reading systems in dyslexia, an obvious question relates to the plasticity
of these neural systems, that is, whether they are malleable and can be changed by an effective reading intervention. We hypothesized that the provision of an evidence-based, phonologically mediated reading intervention would improve reading and the development of the neural systems serving reading. The experimental intervention was structured to help children gain phonological knowledge (i.e., develop an awareness of the internal structure of spoken words) and, at the same time, develop their understanding of how the orthography (written letters) represents the phonology (sounds in words).
Two groups of dyslexic readers as well as a control group were studied. One group of dyslexic readers received an experimental intervention that provided second and third grade poor readers with 50 minutes of daily, individual tutoring that was explicit and systematic and focused on helping children understand the alphabetic principle (i.e., how letters and combinations of letters represent the small segments of speech known as phonemes) and provided many opportunities to practice applying the letter-sound linkages taught. The other group of dyslexic readers received "community intervention," that is, a variety of interventions commonly provided within the school. However, specific, systematic, explicit phonologically-based interventions comparable to the experimental intervention were not used in any of reading programs that were provided to the community group. The children were imaged on three occasions: pre-intervention, immediately post-intervention, and one year after the intervention was complete.
Children who received the experimental intervention not only improved their reading, but compared to their pre-intervention brain activation patterns, these children demonstrated an increase in activation in the anterior system as well as in the parieto-temporal and occipito-temporal systems (similar to typical readers).
In summary, these data demonstrate that an intensive evidence-based reading intervention brings about significant changes in brain organization so that brain activation patterns resemble those of typical readers. These data have important implications for public policy regarding teaching children to read: the provision of an evidence-based reading intervention at an early age improves reading and facilitates the development of those neural systems which underlie reading. Further studies are necessary to clarify the long-term impact of these interventions, particularly on the development of fluency, and the neural systems serving fluent reading. Furthermore, the demonstration of decreased activation in the VWFA associated with fluent, rapid reading supports the need for students with dyslexia to be provided with the accommodation of extra time on tests.
Cohen, L., Dehaene, S., Naccache, L., Lehericy, S., Dehaene-Lambertz, G., Henaff, M.A., et al. (2000). The visual word form area: Spatial and temporal characterization of an initial stage of reading in normal subjects and posterior split-brain patients. Brain, 123, 291-307.
Dehaene, S., Cohen, L., Sigman, M., & Vinckier, F. (2005). The neural code for written words: A proposal. Trends in Cognitive Sciences, 9, 335-341.
Gaillard, R., Naccache, L., Pinel, P., Clemenceau, S., Volle, E., Hasboun, D., et al. (2006). Direct intracranial, FMRI, and lesion evidence for the causal role of left inferotemporal cortex in reading. Neuron, 50, 191-204.
Nakamura, K., Dehaene, S., Jobert, A., Le Bihan, D., & Kouider, S. (2005). Subliminal convergence of Kanji and Kana words: Further evidence for functional parcellation of the posterior temporal cortex in visual word perception. Journal of Cognitive Neuroscience, 17, 954-68.
Paulesu, E., Demonet, J.-F., Fazio, F., McCrory, E., Chanoine, V., Brunswick, N., et al. (2001). Dyslexia-cultural diversity and biological unity. Science, 29, 2165-2167.
Shaywitz, B., Skudlarski, P., Holahan, J., Marchione, K., Constable, R., Fulbright, R., et al. (2007). Age-related changes in reading systems of dyslexic children. Annals of Neurology, 61, 363-370.
Shaywitz, B. A., Shaywitz, S. E., Pugh, K. R., Mencl, W. E., Fulbright, R. K., Skudlarski, P., et al. (2002). Disruption of posterior brain systems for reading in children with developmental dyslexia. Biological Psychiatry, 52, 101-110.
Shaywitz, S. (2003). Overcoming dyslexia: A new and complete science-based program for reading problems at any level. New York, NY: Alfred A. Knopf.
Shaywitz, S., Shaywitz, B., Fulbright, R., Skudlarski, P., Mencl, W., Constable, R., et al. (2003). Neural systems for compensation and persistence: Young adult outcome of childhood reading disability. Biological Psychiatry, 54, 25-33.
Shaywitz, S. E., Morris, M. K., & Shaywitz, B. A. (2008). The education of dyslexic children from childhood to young adulthood. Annual Review of Psychology, 59, 451-475.
Shaywitz, S. E., & Shaywitz, B. A. (2008). Paying attention to reading: The neurobiology of reading and dyslexia. Development and Psychopathology, 20, 1329-1349.
Vinckier, F., Dehaene, S., Jobert, A., Dubus, J. P., Sigman, M., & Cohen, L. (2007). Hierarchical coding of letter strings in the ventral stream: Dissecting the inner organization of the visual word-form system. Neuron, 55, 143-156.