A transcallosal fibre system between homotopic inferior frontal regions supports complex linguistic processing

Abstract

Inferior frontal regions in the left and right hemisphere support different aspects of language processing. In the canonical model, left inferior frontal regions are mostly involved in processing based on phonological, syntactic and semantic features of language, whereas the right inferior frontal regions process paralinguistic aspects like affective prosody. Using diffusion tensor imaging (DTI)-based probabilistic fibre tracking in 20 healthy volunteers, we identify a callosal fibre system connecting left and right inferior frontal regions that are involved in linguistic processing of varying complexity. Anatomically, we show that the interhemispheric fibres are highly aligned and distributed along a rostral to caudal gradient in the body and genu of the corpus callosum to connect homotopic inferior frontal regions. In the light of converging data, taking previous DTI-based tracking studies and clinical case studies into account, our findings suggest that the right inferior frontal cortex not only processes paralinguistic aspects of language (such as affective prosody), as purported by the canonical model, but also supports the computation of linguistic aspects of varying complexity in the human brain. Our model may explain patterns of right-hemispheric contribution to stroke recovery as well as disorders of prosodic processing. Beyond language-related brain function, we discuss how inter-species differences in interhemispheric connectivity and fibre density, including the system we described here may also explain differences in transcallosal information transfer and cognitive abilities across different mammalian species.

Keywords: corpus callosum; diffusion tensor imaging; homotopic; language processing; linguistic complexity.

Figure 1

Suprathreshold peak coordinates (at p < 0.001, uncorrected) in inferior frontal cortex that were used as seed coordinates for the interhemispheric probabilistic DTI‐based fiber tracking are marked with an *. A1 shows the peak coordinate in LIFGpo, A2 in LIFGpt, B1 in RIFGpo, and B2 in RIFGpt. All clusters are superimposed on the cytoarchitectonic probability atlas by Eickhoff et al. (2005) in SPM8

Figure 2

Mapping and rendering of the transcallosal white matter fiber tracts from left to right inferior frontal gyrus (pars opercularis, IFGpo, BA 44, in yellow) and left to right inferior frontal gyrus (pars triangularis, IFGpt, B45 and ventral BA44, in blue). A1, A2, B1, B2 = seed coordinates from Figure 1