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

摘要

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 ventrolateralfrontal lobe. Asinthe macaque monkey, area 44 has distinct connections withthe rostralinferior parietal lobule viathethird branch of the superior longitudinal fasciculus. In contrast, area 45 connects with the superior temporal gyrus, anterior to Heschl’s gyrus, viathe extreme capsulefiber system. These results highlightthe 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.