Functional disruption in the organization of the brain for reading in dyslexia

Abstract

Learning to read requires an awareness that spoken words can be decomposed into the phonologic constituents that the alphabetic characters represent. Such phonologic awareness is characteristically lacking in dyslexic readers who, therefore, have difficulty mapping the alphabetic characters onto the spoken word. To find the location and extent of the functional disruption in neural systems that underlies this impairment, we used functional magnetic resonance imaging to compare brain activation patterns in dyslexic and nonimpaired subjects as they performed tasks that made progressively greater demands on phonologic analysis. Brain activation patterns differed significantly between the groups with dyslexic readers showing relative underactivation in posterior regions (Wernicke's area, the angular gyrus, and striate cortex) and relative overactivation in an anterior region (inferior frontal gyrus). These results support a conclusion that the impairment in dyslexia is phonologic in nature and that these brain activation patterns may provide a neural signature for this impairment.

Figure 1

Number of activated pixels for brain regions where activation patterns across tasks differ significantly between NI and DYS readers. Activations (mean ± SEM) are shown on ordinate and tasks are on abscissa. We performed an overall ANOVA and followed up those interactions that were significant (minimizing type I error). Data are also shown for regions with marginal P values (minimizing type II error). Significance levels of the task by group effect (Huynh–Feldt corrected P values): STG, F(3, 171) = 4.3 and P = 0.009; BA 17, F(3, 171) = 4.0 and P = 0.012; IFG, F(3, 171) = 3.8 and P = 0.012; angular gyrus, F(3, 171) = 2.7 and P = 0.054; BA 46/47/11, F(3, 171) = 2.4 and P = 0.071; ILES, F(3, 171) = 2.2 and P = 0.094. The six anatomic regions (with center or ROI given in x, y, and z coordinates of Talairach) are (i) posterior STG, BA 22 (53, −43, 11); (ii) angular gyrus, BA 39, angular gyrus of the inferior parietal lobule (47, −45, 33); (iii) ILES, BA 18, 19, inferior occipital gyrus, inferior aspect of lateral occipital gyrus (36, −80, −5); (iv) BA 17, striate cortex (8, −89, 3); (v) IFG, BA 44 posterior aspect (pars operculum) of IFG and BA 45 middle aspect (pars triangularis) of IFG (47, 18, 18); (vi) BA 47, 11, 46, anterior inferior aspect of IFG, lateral and medial orbital gyri, and superior aspect of IFG and inferior aspect of middle frontal gyrus (33, 36, 0). Coordinates are shown for right hemisphere where x is positive (x is negative for left hemisphere).

Figure 2

Composite activation maps in DYS and NI readers for the C and NWR judgment tasks. As shown, DYS and NI readers differ in the degree of activation produced in different brain regions during phonologic (NWR) compared with orthographic (C) coding; DYS readers demonstrate a pattern of relative overactivation anteriorly in IFG in contrast to relative underactivation posteriorly, in STG and the angular gyrus. Composite maps (with z-axis Talairach position) are shown for the left anterior region (IFG, z = 33) and two regions in the left posterior system [post STG (STG, z = 12) and the angular gyrus (ANG, z = 23)]. Composite maps are based on brain activations representing C and NWR. The median t value was obtained for each pixel in each of the Talairach-transformed images of the 29 DYS and 32 NI readers, respectively. Those t values greater than 0.2 were cluster-filtered (cluster size = 3) and overlaid on composite anatomic images that were obtained by adding Talairach-transformed anatomical images from the two groups. The cluster criterion used in this composite differs from that used in the statistical analysis; when combining multiple activation maps from different subjects, it is necessary to change the threshold and cluster criterion to compensate for imprecise overlap of activation regions between subjects.

Figure 3

Relative increase in activation during phonologic compared with orthographic coding in different brain regions in NI and DYS readers. As shown in the key, the shadings represent the relative magnitude of the increase in activation (mean pixel counts) for a given ROI calculated as: (NWR − C/C) = R. In posterior regions [e.g., posterior BA 22 (STG) and BA 39 (angular gyrus)], the relative change in activation is large (>2, shown in black) in NI readers but very small in DYS readers (<0.5, shown as lightest gray). A contrasting pattern is shown in anterior regions, for example, in BA 44 and 45 (IFG), where NI readers demonstrate an increase in activation (0.5–1) and DYS readers demonstrate an even greater increase (>2). There are regions where NI and DYS readers show similar increases in activation, for example, BA 6 and anterior STG (BA 41, BA 42, anterior BA 22). Brain regions shown in white were not part of the 17 ROIs examined; numbers represent BAs.