When compared to chimp brains, our brain's pattern of connections between language areas has expanded more than previously thought.
Neuroscientists now understand how our brain evolved into a language-ready brain.
For years, researchers have been intrigued by what in the brain could have enabled this extraordinary ability. However, until now, their focus has been primarily on the arcuate fasciculus, a nerve tract connecting the frontal and temporal lobes that, in addition to showing significant differences between species, is well-known to be involved in language function.
“At first glance, the brains of humans and chimpanzees look very much alike. The perplexing difference between them and us is that we humans communicate using language, whereas non-human primates do not,” says co-first author Joanna Sierpowska.
The researchers used scans of 50 human brains and 29 chimp brains scanned in the same way as humans to study the differences between the human and chimp brains. It was performed under well-controlled anaesthesia as part of their routine veterinary examinations. They used a technique known as diffusion-weighted imaging (DWI) to image white matter, or the nerve pathways that connect brain areas.
The team used these images to compare the connectivity of two language-related brain hubs (the anterior and posterior middle areas of the temporal lobe).
Whereas the connection of the posterior middle temporal regions in chimps is primarily limited to the temporal lobe, the researchers discovered that in humans, a new link towards the frontal and parietal lobes evolved via the arcuate fasciculus as an anatomical avenue. Indeed, modifications to both human language areas contain a suite of temporal lobe connection expansions.
According to Sierpowska, both of these areas are critical for learning, using, and understanding language in humans and contain numerous white matter pathways. It is also known that damage to these brain areas has a negative impact on language function. However, the question of whether their pattern of connections is unique to humans has remained unanswered until now.
According to co-author Vitoria Piai, the findings suggest that the arcuate fasciculus is not the only main driver of evolutionary changes that prepare the brain for full-fledged language capabilities.
“Our findings are purely anatomical, so it is hard to say anything about brain function in this context,” says Piai. “But the fact that this pattern of connections is so unique for us humans suggests that it may be a crucial aspect of brain organization enabling our distinctive language abilities.”