Magnetic source imaging: A suitable tool of exploring the neurophysiology of typical and impaired reading ability

Abstract

This chapter outlines the basic features and applications of Magnetic Source Imaging (MSI) as a tool for studying the functional organization of the brain circuits that support phonological decoding and word recognition. MSI provides temporal resolution in the order of milliseconds and adequate spatial resolution in order to (a) determine if the degree, peak latency, and temporal progression of regional activity vary systematically with individual differences in reading ability, and (b) identify cortical regions where activity predicts reading achievement scores. A series of small and larger-scale studies are described utilizing clinically validated source-parameter estimation techniques as well as minimum norm estimation procedures which permit modeling of distributed cortical magnetic sources. Results demonstrate that reading-impaired children show reduced activation of posterior temporal and inferior parietal regions during late stages of stimulus processing, mainly in the left hemisphere. Specific task demands, i.e. lexical characteristics and memory involvement may determine additional features of brain activation profiles, including hemispheric distribution of group effects, and the degree of prefrontal involvement in the reading process. These effects are present regardless of comorbidity with attention deficit hyperactivity disorder (ADHD), and also highlight the importance of early activity in the left fusiform gyrus for both word and pseudoword reading ability. The key role of these regions into the developing brain mechanism for reading receives further support from intervention studies, demonstrating “normalizing” changes in the degree and latency of regional activation.