The development of reading impairment: A cognitive neuroscience model

Abstract

This review discusses recent cognitive neuroscience investigations into the biological bases of developmental dyslexia, a common disorder impacting approximately 5 to 17 percent of the population. Our aim is to summarize central findings from several lines of evidence that converge on pivotal aspects of the brain bases of developmental dyslexia. We highlight ways in which the approaches and methodologies of developmental cognitive neuroscience that are addressed in this special issue-including neuroimaging, human genetics, refinement of cognitive and biological phenotypes, neural plasticity and computational model-can be employed in uncovering the biological bases of this disorder. Taking a developmental perspective on the biological bases of dyslexia, we propose a simple cascading model for the developmental progression of this disorder, in which individual differences in brain areas associated with phonological processing might influence the specialization of visual areas involved in the rapid processing of written words. We also discuss recent efforts to understand the impact of successful reading interventions in terms of changes within cortical circuits associated with reading ability.

Left cortical specialization for visual letter strings predicts rudimentary knowledge of letter-sound association in preschoolers

“…Each word was vertically flipped and its letters were segmented into simple features by using Adobe Photoshop. These segments were then rearranged to form pseudoletters, respecting the total number of characters (4) and the overall size (width × height) of the original word (24). Pseudoletters therefore contained junctions, ascending/descending features, and close-up shapes ( Fig.…”

“…1), critically depends on the left ventral occipito-temporal cortex (2). This left hemispheric specialization is thought to arise during children's reading acquisition through reentrant mapping of sound representations in the left temporal and frontal cortices to letter representations in ipsilateral posterior areas (3)(4)(5). Even though the exact onset of this leftward specialization is not known, this process appears to take considerable time during formal reading acquisition (6)(7)(8).…”

The visual word form area: expertise for reading in the fusiform gyrus

“…I), among other regions, in response to visual words and pseudowords as measured by fMRI [44,45] and MEG responses within the first 200 ms [14]. Such differences have not yet been associated with neuropathology in this region, but may instead reflect the absence of a specialization that accrues over the course of years of successful reading experience [55]. …”

Socioeconomic background modulates cognition–achievement relationships in reading

“…Studies of twins have suggested that phonological skills have both genetically heritable and environmentally influenced components (Fisher & DeFries, 2002;Knopnik et al, 2002), and a deficit in phonological processing is now believed to be the primary core deficit in developmental dyslexia, a reading disability in which children exhibit a difficulty in accurately or fluently reading at age-appropriate levels (L. Bradley & Bryant, 1983). This phonological impairment is reflected in dysfunction of left perisylvian cortical regions during reading tasks (McCandliss & Noble, 2003). Importantly, experimental manipulations in which reading-disabled children were exposed to both phonological skill training and explicit instruction in letter-sound correspondence rules have resulted in improved reading ability, relative to various readingdisabled control groups (L. Bradley, & Bryant, 1983;Foorman, Francis, Novy, & Liberman, 1991;Wagner & Torgesen, 1987), suggesting that phonological skill is not merely associated with reading, but actually plays a causal role in its development.…”