Sign language refers to a mode of communication, distinct from spoken languages, which uses visual gestures with the hands accompanied by body language to express meaning. It has been determined that the brain's left side is the dominant side utilized for producing and understanding sign language, just as it is for speech. Signers with damage in the Wernicke's area (left hemisphere) in the temporal lobe of the brain have problems comprehending signed languages, while those with damage in the Broca's area, which consists of parts of the temporal lobe, it's junction with the parietal lobe, as well as the Superior Temporal Gyrus, have problems producing signs. Despite complex differences between spoken and signed languages, the associated brain areas are near identical.
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The language of ASL: similarities to and differences from spoken languages
Unlike spoken languages, which are encoded in vocal-auditory changes, signed languages rely on visual-spatial changes in conjunction with non-manual markers to convey meaning. The neural organization underlying sign language abilities, however, has more in common with that of spoken language than it does with the neural organization underlying visuospatial processing. This shows that the left hemisphere is responsible for processing all facets of language, not just speech. When communicating in their respective languages, similar brain regions are activated for both deaf and hearing subjects with a few exceptions. During the processing of auditory stimuli for spoken languages there is uniform activation of the supratemporal plane of the temporal lobe, which is not seen in the processing of stimuli for signed languages. This difference in activity is hypothesized to exist because the supratemporal plane contains several systems directly involved in speech perception. [with deaf subjects having some additional activated region near the visual cortex when low level sensory processing is included in the analysis.] In other words, like spoken languages, signed languages are controlled by the left hemisphere of brain, in Broca’s and Wernicke's areas.,
Signed languages, like spoken languages, are highly structured linguistic system; they have their own sets of phonological, morphological and syntactic characteristics.
A characteristic specific to signed languages is that of “signing space.” Signing space refers to the area in front of the signer in which signs are expressed. It is often used to detail relationships between arguments in discourse, but is also used in place of verbal prepositions; rather than using words to explain spatial relationships, signing space allows for a visual configuration of these relationships.
Brain centers responsible for language processing
In 1861, Paul Broca studied patients with the ability to understand spoken languages but the inability to produce them. The damaged area was named Broca's area, and located in the left hemisphere’s inferior frontal gyrus (Brodmann areas 44, 45). Soon after, in 1874, Carl Wernicke studied patients with the reverse deficits: patients could produce spoken language, but could not comprehend it. The damaged area was named Wernicke's area, and is located in the left hemisphere’s posterior superior temporal gyrus (Brodmann area 22).
Early on, it was noted that Broca’s area was near the part of the motor cortex controlling the face and mouth. Likewise, Wernicke's area was near the auditory cortex. These motor and auditory areas are important in spoken language processing and production, but the connection to signed languages had yet to be uncovered. For this reason, the left hemisphere was described as the verbal hemisphere, with the right hemisphere deemed to be responsible for spatial tasks. This criteria and classification was used to denounce signed languages as equal with their spoken counterparts before it was more widely agreed upon that due to the similarities in cortical connectivity they are linguistically and cognitively equivalent.
Before signed languages received recognition, a debate arose among linguists over the validity of signed languages. What was the brain organization of these languages? It was hypothesized that the deaf-equivalent of Broca's aphasia arose from damage somewhere near the cortex controlling the movement of the hands, and the deaf-equivalent of Wernicke's aphasia arose from damage near the visual cortex. However, that has not turned out to be the correct description of sign language in the brain, a finding which has changed our understanding of how language is organized in the brain more generally. The discovery that the left perisylvian region, which lay close to several auditory processing regions, does not rely on auditory processing helped to validate signed languages as "real languages."
Furthermore, as science and medicine progressed and more advanced research, such as lesion studies, were able to be carried out the scientific community began to arrive at a consensus that the brain functions very similarly to spoken and signed languages. At about this time, theories began to float around the community that there may be an unexplained involvement of the right hemisphere in signed languages not seen in spoken languages. Prior right hemisphere studies on spoken languages has led to prevailing theories in its role in discourse cohesion and prosody. These theories were also adopted by signed language linguists and further imaging studies and neuropsychological testing confirmed the presence of activity in the right hemisphere. While it has become evident that the right hemisphere plays a similar role in both spoken and signed languages, there is suspicion that the right hemisphere may play a more significant role in signed languages than in spoken languages.
Left hemisphere damage
To determine the brain structures associated with processing and production of signed languages, signers with left- and right-hemisphere damage were studied. Those with left hemisphere damage (LHD), in areas ranging from the frontal lobe to the occipital lobe, exhibited both Broca’s and Wernicke’s aphasia symptoms. Patients performed poorly on many language-based tasks such as comprehending signs and sentences and fluently signing. Similar to hearing patients’ “slips of the tongue” after LHD, deaf LHD patients experienced paraphasias, or “slips of the hand.” These slips of the hand usually involve an incorrect hand shape in the correct location and with the correct movement, similar to a hearing patient substituting “bline” or “gine” for “fine.”
It was determined that deficits in sign language articulation were not due to general motor problems; the patients who had difficulties signing were nevertheless capable of producing meaningless hand and arm gestures to command.
Right hemisphere damage
Signers with right-hemisphere damage (RHD), again ranging from frontal lobes to occipital lobes, had no problems with fluency or correct sign comprehension and production. Even when nonlinguistic visuospatial abilities, like drawing or copying, were compromised, patients could communicate efficiently.
Some right hemisphere damage does lead to disruptions in sign languages, however. The topographical use of signing space is often imprecise in patients with RHD; the relation between the location of hands in signing space and the location of objects in physical space is often impaired. Rather than being misunderstood, however, subjects and objects in a sentence may simply be placed incorrectly relative to the other subjects and objects in a sentence, like saying “the pencil is in the book” rather than, “the pencil is on top of the book.”
Treatment for deaf aphasics
Learning sign language to communicate after stroke has been a treatment option for hearing aphasics, but currently there is not much literature on treatment for signers with aphasias and other communication deficits. Understanding the neural underpinnings of sign language is, however, a large step in treatment research.