Temporal-callosal pathway diffusivity predicts phonological skills in children

Abstract

The development of skilled reading requires efficient communication between distributed brain regions. By using diffusion tensor imaging, we assessed the interhemispheric connections in a group of children with a wide range of reading abilities. We segmented the callosal fibers into regions based on their likely cortical projection zones, and we measured diffusion properties in these segmented regions. Phonological awareness (a key factor in reading acquisition) was positively correlated with diffusivity perpendicular to the main axis of the callosal fibers that connect the temporal lobes. These results could be explained by several physiological properties. For example, good readers may have fewer but larger axons connecting left and right temporal lobes, or their axon membranes in these regions may be more permeable than the membranes of poor readers. These measurements are consistent with previous work suggesting that good readers have reduced interhemispheric connectivity and are better at processing rapidly changing visual and auditory stimuli.

Fig. 1.

FA and phonological awareness scatterplots for the four posterior callosal regions. The Inset icon shows the rough location of each of the four regions: superior parietal (S), posterior parietal (P), temporal (T), and occipital (O).

Fig. 2.

Eigenvalue analysis of the temporal-callosal segment. (A) The ellipsoid representation of the diffusion tensor. The first eigenvalue represents the diffusivity along the longest axis of the ellipsoid, the second represents the diffusivity along the next longest axis, and the third represents diffusivity along the shortest axis of the ellipsoid. (B) The FA and MD correlations with phonological awareness are carried by the second (triangles) and third (circles) eigenvalues. The first eigenvalue (squares) is not correlated with phonological awareness. For reference, note that the typical MD in the nearby cerebrospinal fluid was roughly equivalent to that of freely diffusing water at body temperature [3 μm²/msec (50)].

Fig. 3.

Radial diffusivity and phonological awareness in the temporal-callosal segment accounted for 27% of the behavioral variance (r = 0.52; P < 0.0002; df = 47). Radial diffusivity was computed as the mean of the second and third eigenvalues.

Fig. 4.

Pathway selection. (A) All pathways in the right and left hemisphere are estimated; only those pathways that pass through the corpus callosum are analyzed. (B) The callosal pathways are segmented based on their intersection with one of seven possible planar regions, shown as the colored planes. (C) The segmentation of all the callosal pathways is shown by using the same color scheme as the planar segmentation regions. These are occipital (green), posterior parietal (yellow), superior parietal (blue), temporal (purple), superior frontal (red), anterior frontal (orange), orbitofrontal (cyan). (D) The outline of the callosum is color coded by the projection zone of the fibers within each segment. The black point below each corpus callosum indicates the location of the AC. The mean (SD) cross-sectional segment areas are as follows (in mm²): occipital, 39.9 (10.8); temporal, 29.2 (14.4); posterior parietal, 30.7 (14.7); superior parietal, 33.5 (15); superior frontal, 103.8 (23.4); anterior frontal, 96.4 (20.8); orbital, 20.6 (8.5). All brains are AC–PC aligned, but otherwise left in native space. (Scale bar: 1 cm.)