Structural connectivity of Broca's area and medial frontal cortex.

Despite over 140 years of research on Broca's area, the connections of this region to medial frontal cortex remain unclear. The current study investigates this structural connectivity using diffusion-weighted MRI tractography in living humans. Our results show connections between Broca's area and Brodmann's areas (BA) 9, 8, and 6 (both supplementary motor area (SMA) in caudal BA 6, and Pre-SMA in rostral BA 6). Trajectories follow an anterior-to-posterior gradient, wherein the most anterior portions of Broca's area connect to BA 9 and 8 while posterior Broca's area connects to Pre-SMA and SMA. This anterior-posterior connectivity gradient is also present when connectivity-based parcellation of Broca's area is performed. Previous studies of language organization suggest involvement of anterior Broca's area in semantics and posterior Broca's area in syntax/phonology. Given corresponding patterns of functional and structural organization of Broca's area, it seems well warranted to investigate carefully how anterior vs. posterior medial frontal cortex differentially affect semantics, syntax and phonology.