The long-standing 19th-century anatomical model of the brain's language network just got a 21st-century upgrade. Marco Catani, a psychiatrist at the King's College Institute of Psychiatry in London, and his colleagues have discovered a pathway that links the two primary language regions in the brain's left hemisphere with a third region long suspected to contribute to human linguistic prowess. Found with a modified magnetic resonance imaging technique known as diffusion tensor tractography, the pathway affirms that “the circuit for language is more complex than we thought,” Catani says.
In the classic scheme, a bundle of nerve fibers called the arcuate fasciculus (red, at right) directly ties together Wernicke's area, the site of spoken language comprehension (roughly behind the ear), to Broca's area, the location of language production (behind the eye). The newfound pathway consists of two shorter fiber bundles that initially follow the arcuate fasciculus but end in the Geschwind territory of the inferior parietal lobe (toward the back of the head), an area thought to play a role in making language meaningful.
Although the indirect pathway will need to be verified by dissection—Catani suspects its proximity to the direct pathway has obscured its existence in the past—the virtual evidence matches actual nerve bundles found in monkey brains. The inclusion of the inferior parietal lobe in the language circuit could provide clues to how children develop language—the region is one of the last to mature in toddlers, and its full development coincides with the acquisition of complex language skills.
The pathway could also elucidate how language evolved. Catani says the correlation between human and primate anatomies suggests that “language is not due to a new structure of the [human] brain but somehow has developed from preexisting connections.”