Neurobiological bases of reading disorder Part I: Etiological investigations

Abstract

While many studies have focused on identifying the neural and behavioral characteristics of decoding-based reading disorder (RD, aka developmental dyslexia), the etiology of RD remains largely unknown and understudied. Because the brain plays an intermediate role between genetic factors and behavioral outcomes, it is promising to address causality from a neural perspective. In the current, Part I of the two-part review, we discuss neuroimaging approaches to addressing the causality issue and review the results of studies that have employed these approaches. We assume that if a neural signature were associated with RD etiology, it would (a) manifest across comparisons in different languages, (b) be experience independent and appear in comparisons between RD and reading-matched controls, (c) be present both pre- and post-intervention, (d) be found in at-risk, pre-reading children and (e) be associated with genetic risk. We discuss each of these five characteristics in turn and summarize the studies that have examined each of them. The available literature provides evidence that anomalies in left temporo-parietal cortex, and possibly occipito-temporal cortex, may be closely related to the etiology of RD. Improved understanding of the etiology of RD can help improve the accuracy of early detection and enable targeted intervention of cognitive processes that are amenable to change, leading to improved outcomes in at-risk or affected populations.

Keywords: behavioral intervention; causal inference; cross-cultural; developmental dyslexia; etiology; imaging genetics; neuroimaging; preliterate; reading disorder; reading-matched design.

Figure 1

Five main approaches that can support etiological inference of reading disorder (RD) are presented schematically, including: (a) comparing RD with controls in different languages; (b) comparing RD with both age-matched and reading level-matched normal controls; (c) comparing neural impairments in RD between pre- and post-intervention; (d) comparing pre-readers with high- and low-risk for developing RD; and (e) investigating associations between neural and genetic variations. For each approach, we give an example illustrating one of several possible outcomes when a neural anomaly is etiological (in the upper panel) or non-etiological (in the lower panel). Note: region x/y could be any part of brain; brain activation could be measured by blood-oxygen-level dependent signal during a specific task, or by other techniques such as event-related potentials; brain structure could be measured by for example, cortical thickness, volume or surface area for gray matter, or fractional anisotropy for white matter fibers; TD, typically developing control group; RD, reading disorder group; Age-Matched, comparing RD with chronological age-matched controls; Reading-Matched, comparing RD with reading level-matched controls; Low-risk, preliterate children without a family history for RD; High-risk, preliterate children with a family history for RD