Arcuate Fasciculus Size and Trajectory
The size and trajectory of the human arcuate fasciculus is unique. The arcuate fasciculus is a white-matter fiber tract that links lateral temporal cortex with frontal cortex via a dorsal projection that arches around the Sylvain fissure. Lesion studies indicate that this pathway is critically involved with human language. Nineteenth century neurologists used gross dissection to reveal its general trajectory in the human brain, but recent diffusion tensor imaging (DTI) studies have revealed its specific terminations. The temporal projection of the arcuate reaches beyond classic Wernicke’s area in the superior temporal gyrus (STG), to the middle (MTG) and inferior (ITG) temporal gyri, areas known to be involved in lexical-semantic processing. On the other hand, the frontal projection reaches beyond classic Broca’s area in the pars opercularis (BA 44) and pars triangularis (BA 45), to the ventral premotor cortex (BA 6), pars orbitalis (BA 47) and the middle frontal gyrus (BA 9), regions also known to be involved in various aspects of language comprehension and production. On average, the human arcuate fasciculus is larger in the left than the right cerebral hemisphere, consistent with the fact that the left hemisphere is dominant for language in most humans. In chimpanzees, arcuate terminations are considerably more restricted, being focused on the superior temporal gyrus posteriorly, and on the ventral aspects of premotor cortex (BA 6) and pars opercularis (BA 44) anteriorly. In macaques, the arcuate is believed to project most strongly to dorsal prefrontal cortex rather than to Broca’s area homologue. The expanded pathway in humans may support the transmission of word-meaning information stored in the MTG and ITG to Broca’s area and surrounding cortex for both sentence comprehension and sentence construction during spontaneous speech.