Dissociating the Human Language Pathways with High Angular Resolution Diffusion Fiber Tractography

TLDR

Anatomical tracer studies in nonhuman primates show that Broca’s area homologues 44 and 45 have distinct bidirectional corticocortical connections. This study uses high‑resolution diffusion imaging to demonstrate distinct projection patterns from Broca’s area subregions 44 and 45 in the living human brain. The authors employed high‑angular‑resolution diffusion imaging to trace the specific fiber tracts emanating from areas 44 and 45. Tractography revealed that area 44 projects to the rostral inferior parietal lobule via the third branch of the superior longitudinal fasciculus, while area 45 projects to the superior temporal gyrus via the extreme capsule, confirming that the two Broca subregions have distinct connectivity patterns and that the arcuate fasciculus links posterior superior temporal cortex to dorsolateral frontal areas.

Abstract

The anatomical connectivity of ventrolateral frontal areas 44 and 45, which in the human brain constitute Broca's region, has been revisited on the basis of experimental anatomical tracer evidence in the nonhuman primate that the homologues of areas 44 and 45 have distinct bidirectional corticocortical connections. Here we show, using high angular resolution diffusion imaging in the living human brain, a dissociation between the specific projections from the pars opercularis (area 44) and the pars triangularis (area 45) in the ventrolateral frontal lobe. As in the macaque monkey, area 44 has distinct connections with the rostral inferior parietal lobule via the third branch of the superior longitudinal fasciculus. In contrast, area 45 connects with the superior temporal gyrus, anterior to Heschl's gyrus, via the extreme capsule fiber system. These results highlight the differences in connectivity between areas 44 and 45 which had previously been thought to be uniformly connected with the posterior temporal region via the arcuate fasciculus. We also provide evidence in the human brain that the arcuate fasciculus, as in the macaque monkey brain, connects the posterior superior temporal region with dorsolateral frontal areas 8 and rostral 6 that lie above areas 44 and 45. Thus, monkey and human evidence suggests that the connections of areas 44 and 45 are much more differentiated than had previously been thought and provide the basis for studies searching for their differential contribution in function.