Morphological differences in the lateral geniculate nucleus associated with dyslexia

Mónica Giraldo-Chica, John P Hegarty 2nd, Keith A Schneider, Mónica Giraldo-Chica, John P Hegarty 2nd, Keith A Schneider

Abstract

Developmental dyslexia is a common learning disability characterized by normal intelligence but difficulty in skills associated with reading, writing and spelling. One of the most prominent, albeit controversial, theories of dyslexia is the magnocellular theory, which suggests that malfunction of the magnocellular system in the brain is responsible for the behavioral deficits. We sought to test the basis of this theory by directly measuring the lateral geniculate nucleus (LGN), the only location in the brain where the magnocellular and parvocellular streams are spatially disjoint. Using high-resolution proton-density weighted MRI scans, we precisely measured the anatomical boundaries of the LGN in 13 subjects with dyslexia (five female) and 13 controls (three female), all 22-26 years old. The left LGN was significantly smaller in volume in subjects with dyslexia and also differed in shape; no differences were observed in the right LGN. The functional significance of this asymmetry is unknown, but these results are consistent with the magnocellular theory and support theories of dyslexia that involve differences in the early visual system.

Keywords: Dyslexia; Lateral geniculate nucleus; Magnocellular; Parvocellular.

Figures

Graphical abstract
Graphical abstract
Fig. 1
Fig. 1
LGN images and masks. Top left: Coronal slice of proton density weighted image zoomed to the posterior thalamus. Middle left: Same image with LGN mask highlighted. Bottom left: Outline of a human LGN from a stained section (Andrews et al., 1997) with labeled parvocellular (P) and magnocellular (M) layers. Right: Coronal cross-sections through the centers of mass for the left (L) and right (R) LGN masks for all of the subjects in the study.
Fig. 2
Fig. 2
LGN volume. The mean volumes, measured in native space, of the left and right LGN are shown for each group. Error bars indicate the standard error of the mean. The circular symbols indicate the volumes for individual subjects.
Fig. 3
Fig. 3
Probability maps of the LGN anatomy in native space. Each row shows a separate coronal slice from the anterior LGN. In the inset, the slice locations are shaded green over a horizontal slice through the control LGN map. The slices are arranged from anterior (A) to posterior (P). The left two columns show the average map of all subjects in each group for the left (L) and right (R) LGN in the native space, registered by the center of mass. The color code indicates the probability of each voxel belonging to the LGN. The third column shows maps of the difference in probabilities between the dyslexia and control maps. The rightmost column indicates the statistical significance of the difference for the left LGN, corrected for multiple comparisons. There were no significant differences in the right LGN.
Fig. 4
Fig. 4
Probability maps of the location of the LGN in standard space. Each row shows a separate coronal slice, arranged from anterior (A) to posterior (P), y = −27.5 to −29 (MNI coordinates). In the inset, the slice locations are shaded green over a horizontal slice through the control LGN map. These slices were chosen for display because they showed the most pronounced differences between groups. The left two columns show, for each group, the location probability in standard space of the LGN across subjects, for the left (L) and right (R) LGN. The color code indicates the probability of the voxel to belong to each LGN. The MNI coordinates of the centers of mass of the probability distributions were: left dyslexia (−22.5, −27.5, −4.8), right dyslexia (23.5, −26.3, −3.9), left control (−22.6, −26.7, −5.5), right control (23.8, −25.8, −4.8). The right column shows the difference of the maps between groups (dyslexia − control).

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