A unifying theoretical framework of three working memory components provides a systems perspective for discussing past and new findings in a 12-year research program that point to heterogeneity in the genetic and brain basis and behavioral expression of dyslexia: (a) codes for word-form storage and processing, (b) time-sensitive phonological and orthographic loops for maintaining information in working memory or outputting it, and (c) executive functions for language (e.g., rapid automatic switching of attention). Results, which span the genetic to neurological to behavioral levels of analysis, point to possible impairment in any one or combination of these working memory components in individuals with dyslexia. A DNA variation on chromosome 15 may be linked with the phonological word-form in the first working-memory component. A DNA variation on chromosome 6 may be linked with slow rapid automatic switching, inattention ratings, and impaired goal-directed activity ratings in the third working-memory component. Brain and behavioral findings support (a) Triple Word Form Theory: phonological, orthographic, and morphological word-forms and their parts contribute to learning to read and spell words; and (b) Cross-Word Form Mapping: in the process of learning to read and spell words children compute the inter-relationships among the three word-forms and their parts. However, children with dyslexia may require more focus on the morphological word-form and its parts and their relationships with the other two word-forms and their parts than do normal readers. Also, children with dyslexia have unusual difficulties in sustaining phonological loop function in working memory over time; their impaired orthographic loop function may interfere with learning to write alphabet letters and spell, which may be as impaired as word decoding and reading. Impaired executive functions may interfere with the efficiency of working memory in processing oral and written language.

BACKGROUND AND PURPOSE:Diffusion tensor imaging is a tool that can be used to study white matter microstructure in dyslexia. We tested the hypothesis that dyslexics have a white matter structural change (as measured by directional diffusion of water, which can be affected by disruption in white matter tracts) between brain regions that previous functional connectivity studies showed were associated with phonologic processing.

During fMRI imaging, 12 good and 8 poor writers aged 11 wrote a newly taught pseudoletter and a highly practiced letter. Both letters were formed from the same components, but the pseudoletter had a novel configuration not corresponding to a written English letter form. On the first fMRI contrast between the newly taught pseudoletter and highly practiced letter, based on a group map, good and poor writers significantly activated many common regions; but the poor writers showed spatially more extensive brain activation than did the good writers. The additional regions of significant activation may reflect inefficiency in learning a new letter form. For the second contrast between the highly practiced and newly taught letters, individual brain activation analyses, based on exact clusters, showed that good and poor writers differed significantly in activation only in left fusiform. This individual fusiform activation correlated significantly with behavioral measures of automatic letter writing and expressive orthographic coding. Multiple regression in which both individual fusiform activation and individual orthographic coding were entered explained significant variance in written composition. Results are discussed in reference to the role of the orthographic loop, from internal letter form to external letter writing by hand, in writing letters and composing. The overall results are consistent with prior brain and behavioral studies of writing.

Good and poor fifth-grade writers differed, after controlling for multiple comparisons, in 42 brain regions on group maps and then individual brain analyses for fMRI contrast between tapping adjacent fingers sequentially and same finger repeatedly. Of these, 11 regions were correlated with both handwriting and spelling (transcription). Gender differences on the fMRI contrast, with girls more activated, occurred only in left superior parietal, which was correlated with handwriting and spelling. Significance of serial-organization of fingers for handwriting and spelling is discussed.