- We know that the brain reorganises in deaf people when the auditory cortex does not receive sound, Mary Rudner explains. For a while, we’ve also known that the same language areas on the left side of the brain handle both signed and speech-based language. But, this is the first time it has been possible to separate the effects of being deaf and having access to sign language in one and the same study.
The researchers conducted experiments with three groups: individuals who have been deaf since birth and who use sign language, individuals who have been deaf since birth who use spoken language and who have never learnt a sign language, and people with normal hearing with no knowledge of sign language. All three groups watched sign language and were asked to do a related non-linguistic task which involved paying attention either to the shape of the hand, or where the sign was articulated in relation to the body.
At the same time, the activity in their brains was measured using functional magnetic resonance imaging (fMRI). In deaf people with a knowledge of sign language we can say that the sign language “spoke to them”, as Jerker Rönnberg put it, in the auditory cortex in the left hemisphere of the brain in exactly the same place where hearing individuals people process speech.
- As sign language is their native language, they could of course not resist ”listening” to the sign language even though they were meant to focus on non-linguistic information.
For the deaf individuals with no knowledge of sign language, on the other hand, no equivalent activity in the left hemisphere of the brain was apparent when they watched sign language. But, in both groups of deaf people the auditory cortex was activated in the right hemisphere.
- The left hemisphere of the deaf brain preserves its language processing function, says Mary Rudner, but only in relation to languages you know. In contrast, the right hemisphere auditory cortex is reorganised for general visual processing and not specifically for sign language.
The study’s design makes it unique, Jerker Rönnberg explains. It is the first time that deaf people with and without knowledge of sign language are involved in the same experiment at one and the same time. The groups are also well matched with one another in terms of age, gender and not least cognitive ability, so it is possible to make comparisons between them.
The study was carried out in London with both Swedes and British people, and it would have been almost impossible in Sweden, Jerker Rönnberg says.
- In Sweden there are very few deaf individuals who do not have access to sign language, “oral deaf” as they are known. Deaf people in Sweden generally use sign language, or if they belong to the younger generation their hearing function may have been restored, at least partially, via a cochlear implant. It is a result of the policy in this area, which has long been pursued in Sweden.
The Linköping-based researchers are working together with the Deafness, Cognition and Language Research Centre at University College London. The primary researcher is Velia Cardin who works there.
The article in Nature Communications is called “Dissociating cognitive and sensory neural plasticity in human superior temporal cortex”
Anika Agebjörn 2013-02-12