Neural bases of orthographic long-term memory and working memory in dysgraphia

Abstract

Spelling a word involves the retrieval of information about the word's letters and their order from long-term memory as well as the maintenance and processing of this information by working memory in preparation for serial production by the motor system. While it is known that brain lesions may selectively affect orthographic long-term memory and working memory processes, relatively little is known about the neurotopographic distribution of the substrates that support these cognitive processes, or the lesions that give rise to the distinct forms of dysgraphia that affect these cognitive processes. To examine these issues, this study uses a voxel-based mapping approach to analyse the lesion distribution of 27 individuals with dysgraphia subsequent to stroke, who were identified on the basis of their behavioural profiles alone, as suffering from deficits only affecting either orthographic long-term or working memory, as well as six other individuals with deficits affecting both sets of processes. The findings provide, for the first time, clear evidence of substrates that selectively support orthographic long-term and working memory processes, with orthographic long-term memory deficits centred in either the left posterior inferior frontal region or left ventral temporal cortex, and orthographic working memory deficits primarily arising from lesions of the left parietal cortex centred on the intraparietal sulcus. These findings also contribute to our understanding of the relationship between the neural instantiation of written language processes and spoken language, working memory and other cognitive skills.

Keywords: dysgraphia; orthography; spelling; working memory.

© The Author (2015). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

Figures

Post-stroke dysgraphia can reflect impairments in orthographic long-term and/or working memory. Rapp et al. analyse the lesion distribution of 33 individuals with deficits affecting either or both memory processes. Results reveal that while the frontal and ventral temporal lobes support orthographic long-term memory, the parietal lobule supports orthographic working memory.

Figure 1

The cognitive architecture of spelling. Schematic depicting the cognitive architecture of spelling, highlighting central (core) and peripheral processes. P = phonological/phonology; O = orthographic/orthography; WM = working memory.

Figure 2

Lesion overlap and voxel-based lesion mapping analysis for all groups. Lesions are presented on left hemisphere sagittal slices with the x-value in MNI coordinates reported below the slices. The colour scale reflects the number of overlapping lesions; only voxels with a minimum of three lesions are depicted. (A) Overlapping lesions for the set of the 17 individuals with orthographic LTM (O-LTM) deficits. (B) Overlapping lesions for the set of the 10 individuals with othographic working memory (O-WM) deficits. (C) Overlapping lesions for the set of the six individuals with both deficit types (O-Mixed) deficits. (D) The results of Analysis 1, testing (at each voxel) for differences in presence/absence of lesion for individuals with deficits affecting orthographic LTM or orthographic working memory. This analysis included orthographic LTM, orthographic working memory, and orthographic mixed deficit lesions. Clusters of significant difference are presented on left hemisphere sagittal slices, with the x-value in MNI coordinates reported below each slice. All clusters are FDR (false discovery rate) corrected for multiple comparisons at a P < 0.05. The colour scale reflects the z-values of the significant clusters. Positive z-values indicate clusters specifically associated with the orthographic LTM deficit; negative z-values indicate clusters specifically associated with the orthographic working memory deficit.

Figure 3

Voxel-based lesion mapping comparison of orthographic LTM and working memory deficit groups. The results of Analysis 2, testing (at each voxel) for differences in presence/absence of lesion for individuals with deficits affecting orthographic LTM or working memory. This analysis was restricted to individuals with only orthographic LTM or working memory deficits. Clusters of significant difference are presented on medial-to-lateral left hemisphere sagittal slices (on left), and superior-to-inferior axial slices (on right); x (for sagittal slices) and z (for axial slices) MNI coordinates are reported to the right of each slice. All clusters are FDR (false discovery rate) corrected for multiple comparisons at a P < 0.05. The colour scale reflects the z-values of the significant clusters. Positive z-values reflect the orthographic LTM deficit clusters; negative z-values reflect the orthographic working memory deficit clusters.

Figure 4

Voxel-based lesion mapping results depicted alongside spelling neuroimaging findings. Comparison of the lesion mapping results from the current study (left) with meta-analysis of functional neuroimaging studies of spelling (Purcell et al., 2011b; used with permission). All clusters are projected onto the surface of a canonical brain with a search depth of 16 mm. Left = left hemisphere.