Meta-study provides comprehensive 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 would probably intuitively assimilate speech and language. However, cognitive science research on sign languages ​​since the 1960s paints a different picture: it is clear today that sign languages ​​are completely autonomous languages ​​and have a complex organization at several linguistic levels such as grammar. and meaning.

Previous studies of the processing of sign language in the human brain had already found similarities and also differences between sign languages ​​and spoken languages. Until now, however, it has been difficult to get a consistent picture of how the two forms of language are processed in the brain.

MPI CBS researchers now wanted to know which regions of the brain are actually involved in processing sign language in different studies – and how much overlap is with the regions of the brain that hearing people use for language processing. speak. In a recently published meta-study in the journal Human brain mapping, they pooled data from sign language processing experiments conducted around the world.

A meta-study gives us the opportunity to gain an overview 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 in all studies. “

Emiliano Zaccarella, last author of the article and group leader, Department of Neuropsychology, MPI CBS

The researchers found that the so-called Broca area in the frontal brain of the left hemisphere is one of the regions involved in the processing of sign language in almost all of the studies evaluated. This region of the brain has long been known to play a central role in spoken language, where it is used for grammar and meaning. In order to better categorize their results from the current meta-study, the scientists compared their results with a database of several thousand studies with brain scans.

The Leipzig-based researchers were indeed able to confirm that there is an overlap between spoken language and sign language in the Broca region. They also succeeded in showing the role played by the right frontal brain – the counterpart of the Broca area on the left side of the brain. This has also appeared repeatedly in many evaluated sign language studies, as it deals with non-linguistic aspects such as the spatial or social information of its counterpart.

This means that the movements of the hands, face and body – of which the signs consist – are in principle perceived in the same way by deaf and hearing people. However, it is only in the case of deaf people that they additionally activate the linguistic network in the left hemisphere of the brain, including the Broca region. They therefore perceive gestures as gestures with linguistic content – rather than as pure sequences of movement, as would be the case with hearing people.

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

Broca’s space therefore deals not only with spoken and written language, as we know so far, but also abstract linguistic information in any form of language in general. “The brain is therefore specialized in language itself, not in speaking,” explains Patrick C. Trettenbrein, first author of the publication and doctoral student at MPI CBS. In a follow-up study, the research team is now aiming to find out whether different parts of the Broca region are also specialized in sign language meaning or grammar in deaf people, such as hearing people. .