Lateralization of ventral and dorsal auditory-language pathways in the human brain

Abstract

Recent electrophysiological investigations of the auditory system in primates along with functional neuroimaging studies of auditory perception in humans have suggested there are two pathways arising from the primary auditory cortex. In the primate brain, a 'ventral' pathway is thought to project anteriorly from the primary auditory cortex to prefrontal areas along the superior temporal gyrus while a separate 'dorsal' route connects these areas posteriorly via the inferior parietal lobe. We use diffusion MRI tractography, a noninvasive technique based on diffusion-weighted MRI, to investigate the possibility of a similar pattern of connectivity in the human brain for the first time. The dorsal pathway from Wernicke's area to Broca's area is shown to include the arcuate fasciculus and connectivity to Brodmann area 40, lateral superior temporal gyrus (LSTG), and lateral middle temporal gyrus. A ventral route between Wernicke's area and Broca's area is demonstrated that connects via the external capsule/uncinate fasciculus and the medial superior temporal gyrus. Ventral connections are also observed in the lateral superior and middle temporal gyri. The connections are stronger in the dominant hemisphere, in agreement with previous studies of functional lateralization of auditory-language processing.

Exploiting peak anisotropy for tracking through complex structures

“…However, DTI fails when voxels contain complex structure, such as crossing fibres, because it assumes a single fibre direction in each voxel. Multi-tensor models [7,11] overcome this limitation to some extent, but require knowledge of the number of fibre populations prior to fitting. More recently, various multiplefibre reconstruction algorithms have been developed, such as Persistent Angular Structure (PAS) MRI [15], Spherical Deconvolution [14] and QBall Imaging [35], which resolve multiple fibre orientations in a single voxel without prior knowledge of the number of fibre directions.…”

Symmetries in human brain language pathways correlate with verbal recall

“…However, Dorsaint-Pierre et al (10) have recently found that the leftward lateralization of the planum temporale does not correlate with language lateralization as assessed by the sodium amytal procedure. On the other hand, several studies have reported a higher prevalence of leftward asymmetry of perisylvian white matter volumes, suggesting that language networks represent a more likely anatomical substrate for lateralization of language functions rather than cortical areas alone (11)(12)(13)(14). However, the behavioral correlates of lateralization of white matter pathways remain unknown.…”

“…DT-MRI tractography is a noninvasive technique that, instead of visualizing axons directly, reconstructs their trajectories by measuring the diffusivity of water along different directions and tracing a pathway of least hindrance to diffusion (parallel to the dominant fiber orientation) to form continuous pathways. DT-MRI facilitates the study of large-scale brain networks, including those subserving language (11)(12)(13)(20)(21)(22). Recent DT-MRI investigations have revealed three important parts of the language network (21).…”

“…Recent DT-MRI investigations have revealed three important parts of the language network (21). First, there is the well established direct pathway (21) connecting Wernicke's territory in the left temporal lobe with Broca's territory in the left frontal lobe through the arcuate fasciculus (11)(12)(13)(20)(21)(22). In addition, there is an indirect pathway (21) consisting of two segments, an anterior segment linking Broca's territory with the inferior parietal lobe (Geschwind's territory) (21), and a posterior segment linking Geschwind's territory with Wernicke's territory (13,20,21).…”

“…First, there is the well established direct pathway (21) connecting Wernicke's territory in the left temporal lobe with Broca's territory in the left frontal lobe through the arcuate fasciculus (11)(12)(13)(20)(21)(22). In addition, there is an indirect pathway (21) consisting of two segments, an anterior segment linking Broca's territory with the inferior parietal lobe (Geschwind's territory) (21), and a posterior segment linking Geschwind's territory with Wernicke's territory (13,20,21). Not only does this arrangement support the more flexible architecture of parallel processing (23), but it is also in keeping with recent functional neuroimaging findings (8,16,24) and contemporary models of language disorders (5,23,25).…”