Deafness and Sign Language: How Do They Affect Brain Development?

An invisible disability, deafness affects 1.5 billion people worldwide according to the World Health Organization (WHO). However, not everyone is deaf in the same way. Indeed, there are different forms of deafness classified according to the degree of hearing loss (mild, moderate, severe or profound deafness) or according to the location of the damage.

[Article issu de The Conversation, écrit par Delphine Fleurion, Enseignante chercheuse en neuropsychologie, psychologie du développement et surdité, Institut catholique de Lille (ICL)]

When deafness occurs in childhood, particularly before the period of language acquisition, the linguistic practice of deaf children can be very variable depending on their level of impairment or the sociocultural context in which they evolve. They can either develop a vocal language, a sign language, or both (bilingualism).

However, depending on the situation, the consequences in terms of brain development will not be the same.

What is sign language?

“Signing” consists of transmitting a message in bodily space, via movements of the upper limbs and facial expressions.

There are over 150 different sign languages ​​across the world. Each of them is composed of its own lexicon, a spatial grammar (in the West, for example, the past is signed behind the signer, while the future is signed in front) and a phonology (a hand shape can mean two different concepts depending on its location on the body: the signs BETE and CHEF are, for example, phonologically similar).

Following the Milan Congress, the third international congress for the improvement of the conditions of the deaf, held in 1880, it was decided that the oral method of education should be favored, which de facto resulted in by a ban on sign language. Deaf children therefore did not receive teaching in their natural language.

The latter nevertheless continued to communicate with each other by signing, leading to dialect variations within countries, or even within the same region. For this reason, some elderly deaf people still practice gestural patois recognizable mainly by deaf people who attended the same school.

Currently, it is difficult to estimate the number of people who use sign language. In France, the data ranges between 80,000 and 120,000 deaf speakers of French Sign Language (LSF).

Complex brain treatments

Sign language is processed by the same brain areas that are activated for vocal language in hearing people: those involved in comprehension (Wernicke's area) and expression (Broca's area). On the other hand, the sensory channels are different: sign language uses visual input and gestural output modalities.

Several cognitive processes are necessary to create a mental image or memorize the location and position of the hands. More complex cognitive processes are also involved, for example those involved in mental rotation (when we imagine the movements of an object without manipulating it) and interpersonal transfer (which consists, for example, of playing the role of what is being signed, to interpret the action instead of describing it).

Deafness and practice of sign language reshape the brain

Following the onset of deafness, an initial reorganization of neural networks takes place in the brain. Since the auditory cortex is not used to process auditory information, the auditory areas are recruited for the processing of visual information, thanks to cerebral plasticity, in other words the brain's ability to restructure itself. This plasticity is all the more marked as deafness occurs early in the child's development.

In the case where the deaf child is exposed to sign language early, a second type of reorganization is observed at the cerebral level. This visuogestural language indeed leads to a particular development of visual, spatial and attentional skills as well as higher functions such as theory of mind (the ability to understand and anticipate the thoughts of others) or the organization of knowledge in memory.

It should be noted that numerous psycholinguistic studies have revealed the existence of developmental similarities between the acquisition of oral language and that of signed language: gestural babble, first signs around 1 year of age, association of two signs around 2 years of age.

What happens if you are exposed to sign language late? For the moment, there is a lack of scientific data to assess the consequences on the development of non-verbal cognitive functions. Recent work seems to indicate that there are significant differences in the resolution of spatial memory tasks between native and late signers. However, one of the limitations of the study is the lack of a control group, a frequent bias in studies focusing on deafness.

However, depending on the environment in which the deaf child grows up, their exposure to sign language will be more or less late.

Learning sign language too late

When a deaf child grows up in a family context favorable to the daily practice of sign language, linguistic input occurs naturally and the mother tongue is rich and fluid. Such an environment allows native-speaking deaf children to easily develop their social communication skills. It also prepares for the acquisition of a second language, written or vocal, by promoting cerebral connectivity.

However, this small group of native speakers is not representative of the entire deaf population. Indeed, each year in France, 700 newborns are born deaf (i.e. approximately 1 in 1000 births). Among them, only 5 to 10% have parents who are themselves deaf. For parents who would like to learn to sign, LSF training is unfortunately rarely offered or covered. The number of deaf children exposed to natural sign language from birth is therefore very low.

Doctors generally recommend that families have a [implant cochléaire] as early as possible, in order to prioritize the hearing rehabilitation of their baby to develop speech, without mentioning sign language. However, the effectiveness of these rehabilitations and re-educations is not systematic. They allow the deaf child to access sound, but not to become a hearing child. Indeed, auditory function is not established or reestablished like that of a hearing person, because intensive training is necessary to acquire spoken language.

Result: among the 90 to 95% of deaf individuals whose parents are hearing, many are likely to discover sign language late, often after the sensitive period for language acquisition, which extends during the 5 first years of life, a period during which the brain has a high potential for cerebral plasticity.

However, we know that late exposure to language has harmful consequences not only on language acquisition, but also on the child's cognitive and social development.

A language that needs to be taught more

Since 2005, LSF has been recognized as a language in its own right, but bilingual teaching is not effective in all specialized schools for deaf children. Ideally, it should be practiced within classes by the teacher himself, or by the French-LSF interpreter present alongside him.

In the absence of appropriate teaching, the consequences on the acquisition of academic knowledge (reading, written comprehension, calculation, etc.) are considerable. In 1998, the Ministry of Health reported that approximately 80% of deaf people could not read correctly or did not understand what they read. According to other sources such as associations, the current illiteracy rate is between 50% and 80%. However, in view of the recommendations of the High Authority of Health and the new teaching systems in favor of a bilingual approach for deaf children, a new census could be carried out.