Abstract
The left inferior frontal gyrus (IFG) has been reported to be critically involved in syntactic processing, not only in first language (L1), but in second language (L2). Indeed, the leftward lateralization of the IFG has been shown to be correlated with the performance of a syntactic task in L2. Given that posterior language-related regions are systematically connected with the left IFG, the next question is which of the dorsal and ventral pathways is more critical to the individual syntactic abilities in L2. Here we used diffusion magnetic resonance imaging (MRI) and tractography with newly developed semi-automatic methods of defining seeds and selecting regions of interest (ROIs). We calculated mean thickness and fractional anisotropy (FA) in each ROI for the arcuate fasciculus (Arcuate) of the dorsal pathway, as well as for the inferior fronto-occipital fasciculus (IFOF) of the ventral pathway. In Experiment I, we performed partial correlation analyses between FA and the accuracy of the syntactic task, removing the effects of the accuracy of a spelling task, gender, and handedness. Among the two pathways in each hemisphere, only FA of the left Arcuate was significantly correlated with individual accuracy of the syntactic task. In Experiment II, we recruited monozygotic twins and examined to what extent their L2 abilities and their structural properties were similar. Within twin pairs, the highest significant correlation was observed for reaction times of the spelling task, while the correlation for the accuracy of the syntactic task was marginal; these two correlation coefficients were significantly different. Moreover, the thickness of the left Arcuate was highly correlated within pairs, while its FA, as well as the thickness/FA in the ventral pathways, was not significantly correlated. The correlation coefficient for the thickness of the left Arcuate was significantly larger than that of the left IFOF. These results suggest that the thickness of the left Arcuate is more associated with the shared genetic/environmental factors, whereas both of mutually correlated FA in the left Arcuate and individual syntactic abilities in L2 may be less prone to these shared factors.
Highlights
It has been widely accepted that the cortical regions supporting language functions are mainly connected by dorsal and ventral pathways (Hickok and Poeppel, 2007; Friederici, 2011)
We improved the seed definition in a semi-automatic manner to reliably track the arcuate fasciculus (Arcuate) and inferior fronto-occipital fasciculus (IFOF) (Figure 1), and successfully identified those fiber tracts in both hemispheres of all participants. We objectively selected their regions of interest (ROIs), thereby minimizing the variances in thickness among participants (Figure 2), and further clarified that the Arcuate was significantly thicker in the left hemisphere than in the right, while the IFOF was marginally thicker in the right hemisphere (Table 3)
We revealed that the mean fractional anisotropy (FA) in the ROI of the left Arcuate was significantly correlated with the accuracy of the syntactic error-detection (Syn) task (Figure 4A)
Summary
It has been widely accepted that the cortical regions supporting language functions are mainly connected by dorsal and ventral pathways (Hickok and Poeppel, 2007; Friederici, 2011). Our voxel-based morphometry (VBM) study with the same tasks showed that the accuracy of the Syn task, but not of the Spe task, was correlated with the leftward lateralization of a single region’s volume in the F3t (Nauchi and Sakai, 2009) These functional MRI and VBM studies have provided a consistent view that the individual differences both in the function and anatomy of the left IFG were related to the individual syntactic abilities. In order to reveal the roles of connectivity including the left IFG in L2 acquisition, it is important to separate syntactic abilities from other linguistic or general ones, and to further clarify whether the structural properties of the dorsal and/or the ventral pathways reflect individual syntactic abilities. In the present study we used a 3 T scanner and conducted a pilot experiment to examine the effects of imaging parameters
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