Neuroscientists have demonstrated that deaf people engage the same regions of the brain when signing as hearing people do when speaking. More interestingly, those not deaf from birth who have subsequently learnt to sign achieve this in the same way neurologically that bilingual speakers of any language do. In contrast, the standard gestures used in regular speech do not elicit the same neural response. This of course makes sense. Sign languages, unlike simply pointing or nodding one’s head, have complex grammar rules not present in simple gesticulation.
These rules are often very different to the spoken language of the primary country in which they are used. British Sign Language (BSL) for example, does not use the subject-verb-object (SVO) sentence structure typical of English. Rather, BSL typically uses topic-comment structure common in East Asian languages. Where this is absent, BSL follows OSV word order, quite remarkable given this is the least common sentence structure among spoken languages globally.
Similar to spoken languages such as Mandarin, BSL also lacks verb conjugation to express tense. Instead, expressions indicating time are used more frequently than in English in order to convey when an action occurred.
These differences of course arise because sign languages did not, with some exceptions, develop amongst communities of hearing people. For this reason, sign languages also differ greatly from one another. Standardised national sign languages arose in Britain and France around the same time in the eighteenth century. Naturally, sign languages existed before this, but they were far more localised.
In France, for example, a particular Parisian dialect was discovered by chance by the philanthropist and priest Charles-Michel de l'Épée, when he went into the home of two deaf sisters to escape the rain. Here, he learnt that they used a sign language to communicate both with each other and the wider deaf community within the city. He endeavoured to learn it, created a school for the deaf, and encouraged the church to engage with them. His advocacy work encouraged the spread of this dialect in France and abroad. In 1791, France became the first country to guarantee deaf people legal rights and protections, such as the right to interpreters, and defence in court.
French Sign Language went on to heavily influence most European sign languages, as well as those further afield in the United States, Brazil, and Canada. It is for this reason that British and American sign language are mutually unintelligible despite both countries speaking English.
Perhaps the most challenging concept for people who do not sign however is the use of non-manual motions and expressions to convey meaning. Some words use identical manual motions, but are distinguished by differing facial expressions, such as doctor and battery in Dutch Sign Language. This can present difficulties for some second language learners who are sometimes inclined to vocalise signs to aid with memorisation, but this can often distort their meaning.
The reason for this is simple: while many features of different languages can be difficult to grasp or comprehend with increasing distance from one’s own, for many they all share a common feature in that they all consist of sounds produced from the mouth. Appreciating that this is not essential took a very long time for society to achieve. In fact, before his chance encounter, the aforementioned Charles-Michel de l'Épée believed that deaf individuals could not achieve salvation as they lacked all comprehension for language and therefore the Gospel. While this thinking is now outdated, sign languages are often still be viewed as little more than a substitute, rather than languages in their own right, which can lead to social ostracism and an inaccessible society.