Connectivity of language domains unique to the human brain

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Summary: Researchers shed new light on how the human brain evolved to be capable of language. Compared to the brains of chimpanzees, the connection patterns of language areas in the human brain expanded more than previously thought.

Source: Radboud University

Neuroscientists have gained new insights into how our brain evolved into a language-capable brain. Compared to chimpanzee brains, the connection pattern of the language areas in our brain has expanded more than previously thought.

The researchers from Radboud University and the University of Oxford publish their findings in PNAS on July 4.

“At first glance, the brains of humans and chimpanzees look very similar. The striking difference between them and us is that while humans communicate through language, nonhuman primates don’t,” says co-first author Joanna Sierpowska.

Understanding what might have enabled this unique ability in the brain has inspired researchers for years. So far, however, their attention has been drawn primarily to a specific nerve tract connecting the frontal and temporal lobes called the arcuatus fasciculus, which is known to be involved in language function, alongside significant species differences.

“We wanted to focus on the connectivity of two cortical areas in the temporal lobe that are equally important for our ability to speak,” says Sierpowska.

To examine the differences between human and chimpanzee brains, the researchers used scans of 50 human brains and 29 chimpanzee brains, which were scanned in a similar manner to humans but under well-controlled anesthesia and as part of their routine veterinary examinations.

More specifically, they used a technique called diffusion-weighted imaging (DWI), which maps white matter, the neural pathways that connect areas of the brain.

Using these images, they examined the connectivity of two language-related brain centers (the anterior and posterior middle regions of the temporal lobe) and compared them between species.

“In humans, these two areas are considered critical to learning, using, and understanding language and are home to numerous white matter pathways,” says Sierpowska.

“It is also known that damage to these brain areas has adverse consequences for language function. So far, however, the question of whether their connection pattern is unique to humans has remained unanswered.”

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The researchers found that while the connectivity of the posterior midtemporal regions in chimpanzees is mainly restricted to the temporal lobe, in humans a new connection to the frontal and parietal lobes emerged using the arcuate fasciculus as an anatomical pathway. In fact, changes to both human language domains involve a range of expansions in connectivity within the temporal lobes.

This shows a woman's head
In fact, changes to both human language domains involve a range of expansions in connectivity within the temporal lobes. The image is in the public domain

“The results of our study suggest that the arcuatus fasciculus is certainly not the only driver of evolutionary changes that prepare the brain for full language ability,” says co-author Vitoria Piai.

“Our results are purely anatomical, so it’s difficult to say anything about brain function in this context,” says Piai.

“But the fact that this connection pattern is so unique to us humans suggests that it may be a crucial aspect of brain organization that enables our distinct language abilities.”

About this language and news from evolutionary neuroscientific research

Author: Harriette Koop
Source: Radboud University
Contact: Harriette Koop – Radboud University
Picture: The image is in the public domain

Original research: The results appear in PNAS

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