Meta-analysis gives a complete picture of the neural basis of sign language

The ability to speak is one of the essential characteristics that sets humans apart from other animals. Many people might be equal in speech and language. However, a cognitive scientific study of sign languages ​​from the 1960s paints a different picture: Today it is clear, sign languages ​​are completely autonomous languages ​​and have complex organization on several linguistic levels such as grammar. and meaning.

Previous studies on the processing of sign language in the human brain had already found some things and also differences between sign languages ​​and spoken languages. So far, however, it has been difficult to get a consistent picture of how both types of language are processed in the brain.

Researchers at CBS MPI now wanted to find out which brain regions are involved in sign language processing across different studies – and the extent of the overlap with brain regions that humans experience. audio using for spoken language processing. In a meta-study recently published in the journal Human brain mapping, collected data from worldwide signature language processing tests conducted.

Meta-analysis allows us to get a complete picture of the neural basis of sign language. Thus, for the first time, we were able to statistically and robustly identify the brain regions involved in sign language processing across all studies. “

Emiliano Zaccarella, last author of the paper and director of the group, Department of Neuropsychology, MPI CBS

The researchers found that specifically the area known as Broca in the left brain of the left hemisphere is one of the regions involved in sign language processing in almost all studies evaluated . It has long been known that this brain region plays a key role in spoken language, where it is used for grammar and meaning. To better classify their results from the routine meta-analysis, the scientists compared their results with a database containing several thousand studies with brain scans.

The Leipzig-based researchers were indeed able to confirm a link between spoken and signed language in the Broca area. They also managed to show the role of the right frontal brain – compared to the Broca area on the left side of the brain. This has been repeated in many of the assessed sign language studies, as it processes non-linguistic objects such as the spatial or social information of its peers.

This means that the movements of the hands, face and body – in which signs are included – are in principle perceived alike by deaf and hard of hearing people. Except in the case of deaf people, however, they also activate the language network in the left hemisphere of the brain, including the Broca area. So they see the movements as gestures with linguistic content – rather than real movement sequences, as would be the case with hearing people.

The results show that the Broca region of the left hemisphere is a key node in the human brain language network. Depending on whether people use language in the form of signs, sounds or writing, it works with other networks.

The Broca area therefore processes not only spoken and written language, as has been known so far, but also abstract linguistic information in any form of language in general. “So the brain specializes in language per se, not speaking,” explained Patrick C. Trettenbrein, first published author and doctoral student at CBS MPI. In a follow-up study, the research team is now aiming to find out if the different parts of the Broca area also specialize in the meaning or grammar of sign language in deaf people, similar to hearing people.