fMRI of syntactic processing in typically developing children: structural correlates in the inferior frontal gyrus

Abstract

Development of syntactic processing was examined to evaluate maturational processes including left language lateralization functions and increased specialization of brain regions important for syntactic processing. We utilized multimodal methods, including indices of brain activity from fMRI during a syntactic processing task, cortical thickness measurements from structural MRI, and neuropsychological measures. To evaluate hypotheses about increasing lateralization and specialization with development, we examined relationships between cortical thickness and magnitude and spatial activation extent within the left inferior frontal gyrus (IFG) and its right hemisphere homologue. We predicted that increased activation in the left and decreased activation in the right IFG would be associated with increased syntactic proficiency. As predicted, a more mature pattern of increased thickness in the right pars triangularis was associated with decreased activation intensity and extent in the right IFG. These findings suggest a maturational shift towards decreased involvement of the right IFG for syntactic processing. Better syntactic skills were associated with increased activation in the left IFG independent from age, suggesting increased specialization of the left IFG with increased proficiency. Overall, our findings show relationships between structural and functional neurodevelopment that co-occur with improved syntactic processing in critical language regions of the IFG in typically developing children.

Keywords: Syntax; fMRI; language; lateralization; multimodal; typical development.

Fig. 1

Summary of methods for obtaining structural (i.e. segmentation, cortical thickness) and functional (segmentation-based ROI mean activation and volume) measurements. The T1-weighted image is processed through FreeSurfer's recon-all command to obtain a thickness surface map and segmentation labels of anatomical regions. Thickness values were first converted into a 3D volume that follows the contours of the white matter surface, and then the ROIs were applied to get average thickness values within the IFG. For thickness, a combination of both white and gray matter segmentations for each ROI was used to ensure that all values in the white surface-based thickness ribbon were captured. FSL-processed activation maps were first registered to T1-weighted activation maps and then the ROIs were applied to the resultant transformed image to get average thickness and extent values within the IFG ROIs. Gray matter segmentations for each ROI were used to capture values on the cortical surface.

Fig. 2

Syntax task: group average activation maps. Lateral and medial whole brain maps with activation on the surface of an individual brain registered to MNI-152 space. Axial sections displaying group average activation maps during the syntax vs. rest contrast. Regions of significant activation (Z > 2.3 and a uncorrected cluster significance threshold of p = 0.05) for the syntax > rest contrast. Z values indicate z coordinate in MNI-152 space.

Fig. 3

Syntax task ROI analysis: group average activation maps. Axial sections displaying group average activation maps during the syntax vs. rest contrast. Regions of significant activation in the left IFG (Z > 2.3 and a corrected cluster significance threshold of p = 0.05) for the syntax > rest contrast. Z values indicate z coordinate in MNI-152 space.

Figure 4

Syntax > semantic conditions ROI analysis: condition contrast activation maps. Axial sections displaying condition contrast maps for the syntax > semantic contrast. Regions of significant activation (Z > 1.7 and an uncorrected cluster significance threshold of p = 0.05) for the syntax > rest contrast. Z values indicate z coordinate in MNI-152 space.

Fig. 5

Axial maps illustrate overlapping values in the IFG for voxels that were more active during syntax than rest (from Fig. 2), and/or voxels where activation during syntax vs. rest were correlated with performance on the CELF-4 sentence assembly (SA) subtest (pink = group mean activation, yellow = voxels where activation was correlated with CELF-4 SA performance independent from age, red = overlapping regions). Yellow arrow indicates location of values (Z) in the IFG (x = −54, y = 12, z = 16 in MNI-152 space) plotted against age-residualized values for CELF-4 SA raw scores in the graph. Only significant voxels within the IFG are shown; IFG mask applied to the image.

Fig. 6

RH pars triangularis mean thickness (mm) plotted against mean activation (z) and activation volume (extent) in three RH ROIs of the IFG (pars orbitalis, pars opercularis, and pars triangularis) for each individual.