Sign language in the brain

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Sign language refers to any natural language which uses visual gestures produced by the hands and body language to express meaning. The brain's left side is the dominant side utilized for producing and understanding sign language, just as it is for speech. [1] 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).

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Signers with damage in Broca's area have problems producing signs. Those with damage in the Wernicke's area (left hemisphere) in the temporal lobe of the brain have problems comprehending signed languages. 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 not equal to spoken language until it was widely agreed upon that due to the similarities in cortical connectivity they are linguistically and cognitively equivalent.

In the 1980s research on deaf patients with left hemisphere stroke were examined to explore the brains connection with signed languages. The left perisylvian region was discovered to be functionally critical for language, spoken and signed. [1] [2] Its location near several key auditory processing regions led to the belief that language processing required auditory input and was used to discredit signed languages as "real languages." [2] This research opened the doorway for linguistic analysis and further research on signed languages. Signed languages, like spoken languages, are highly structured linguistic systems; they have their own sets of phonological, morphological and syntactic characteristics. Despite some differences between spoken and signed languages, the associated brain areas share a lot in common. [3]

Hemispheric similarities and differences between spoken and signed languages

Both the left and right hemisphere have brain structures associated with spoken and signed languages. Spoken and signed languages both depend on the same cortical substrate. [2] This shows that the left hemisphere is responsible for processing all facets of language, not just speech. 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, which is processed dominantly in the right hemisphere. [2] Those patients 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.” [4] 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.” [4] Around the time of the experiment, 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. 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. [5] Prior right hemisphere studies on spoken languages has led to prevailing theories in its role in discourse cohesion and prosody. The right hemisphere has been proposed to assist in detection, processing and discrimination of visual movement. [2] The right hemisphere has also been shown to play a role in the perception of body movements and positions. [2] All of these right hemisphere features are more prominent for signed languages than spoken languages, hence the argument that signed languages engage the right hemisphere more than spoken languages.

As brain imaging technology such as EEG became more developed and commonplace, it was eventually applied to sign language comprehension. Using EEG to record event-related potentials can correlate specific brain activity to language processing in real time. Previous application of ERP on hearing patients showed neural activity in the left hemisphere related to syntactic errors. [2] When electrodes are hooked up to deaf native signers, similar syntactic anomalies associated with an event-related potential were recorded across both left and right hemisphere. This shows that syntactic processing for American Sign Language (ASL) is not lateralized to the left hemisphere. [2]

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 detectable activity within Broca's Area, Wernicke's Area, the angular gyrus, dorsolateral prefrontal cortex, and superior temporal sulcus. [6] Right hemisphere activity was detectable in less than 50% of trials for hearing subjects reciting English sentences. When deaf subjects were tasked with reading English, none of the left hemisphere structures seen with hearing subjects were visible. [6] Deaf subjects also displayed obvious middle and posterior temporal-parietal activation within the right hemisphere. [6] When hearing subjects were presented various signs designed to evoke emotion within native signers, there was no clear changes in brain activity in traditional language processing centers. Brain activity of deaf native signers when processing signs was similar to activity of hearing subjects processing English. However, processing of ASL extensively recruited right hemisphere structures including significant activation of the entire superior temporal lobe, the angular region, and inferior prefrontal cortex. Since native hearing signers also exhibited this right hemisphere activation when processing ASL, it has been proposed that this right hemisphere activation is due to the temporal visuospatial decoding necessary to process signed languages. [6]

In a similar study published in 2017, deaf individuals who use French Sign Language were studied during processing French Sign Language and written French. During the processes of each of the languages, there was bilateral activation in the occipital lobes, in the temporal lobes near the superior temporal sulcus, and in the frontal gyri. [7] The processing of sign language showed stronger activation in both occipital lobes, both posterior temporal lobes, and in the thalamus bilaterally. It also showed strong activation particularly in structures in the right hemisphere: the superior temporal sulcus, the fusiform gyrus, and the inferior frontal gyrus. [7] Opposed to processing sign language, when the individuals processed written French there was strong activation bilaterally and in the left hemisphere. The areas that showed bilateral activation were the inferior parietal lobes, fusiform gyri, and Brodmann Area 44, among others. The areas lateralized to the left hemisphere were the calcarine and fusiform gyrus, specifically at the location for visual word form. [7]

Neurological differences between deaf and hearing groups

It is thought that there are significant neuroanatomical differences among congenitally deaf humans versus those who become deaf later in life. [8] Therefore, it is widely thought that research into the differences in connections and projections of neurons in deaf humans must block into two groups—congenitally deaf and deaf after birth. Structural brain imaging has commonly shown white matter volume of the auditory cortices differs between deaf and hearing subjects, regardless of the first language learned. [8] Deaf humans are thought to have a larger ratio of gray matter to white matter in certain auditory cortices, such as left and right Heschl's gyrus and Superior Temporal gyrus. [9] This heightened ratio is thought to exist due to less overall white matter in Heschl's gyrus and the Superior Temoral gyrus among deaf humans. Overall, the auditory cortices of deaf humans have an increased gray-white matter ratio as a result of the lack of auditory stimuli which is commonly thought to lead to less myelination and fewer projections to and from the auditory cortices. [9]

It has been thought that congenitally deaf people could provide insight into brain plasticity; the decreased auditory connectivity and brain volume for auditory processing provides an opportunity for enhancement in the visual cortices which are of greater importance to deaf humans. [10] The Calcarine sulcus acts as the hub for the Primary Visual Cortex in humans. Congenitally deaf humans have measurably higher volume of Calcarine cortex than hearing humans. [10] The increased volume and size of visual cortices of deaf individuals can lead to heightened visual processing. Deaf humans have demonstrated, via event-related potential, an increased sensitivity and reactivity to new visual stimuli—evidence of brain plasticity leading to behavioral enhancement. [11]

Differences between signers and non-signers

In one experiment published in 1992, visual mental imagery was studied in ASL signers—deaf and hearing—and hearing non-signers. These hearing signers were born to deaf parents, and ASL was their first language. Another aspect looked at in this study was the difference between native signers and those who learned sign language at a later age. In this experiment, native signers are considered deaf individuals who were born to deaf parents and therefore started absorbing the language in infancy. The other deaf signers' primary language is sign language, but they did not learn it until between the ages of two to sixteen. [12]

In the experiment of generating simple and complex images, deaf individuals were the quickest, followed by hearing signers and then hearing non-signers. This was expected; however, looking at a chart of the results, the hearing signers performed almost identically, in regards to the simple and complex images, to the deaf signers but just more slowly. [12] The hearing non-signers were right on track in following behind on the simple image, but their reaction time was vastly longer. [12] At least in this area, experience with a visual-spatial language provides quicker reaction times.

The results are consistent with abilities recruited for processing sign language being enhanced in the brain, compared to those abilities in non-signers. A couple of things the subjects were tested on were mental rotation and mirror reversals. Signers had an advantage in mirror reversals, but there was no difference between signers and non-signers performing mental rotation. Because of these results, it may not be true to say that signers have a better ability to transform images, but the ability may be in rotating images. Because of this experiment, the cause of enhanced abilities was questioned to be because of auditory deprivation or because of using a visual-spatial language. Hearing signers who learned sign language as a first language may be the key to answering this question. [12]

Related Research Articles

Language center Speech processing in the brain

In neuroscience and psychology, the term language center refers collectively to the areas of the brain which serve a particular function for speech processing and production. Language is a core system, which gives humans the capacity to solve difficult problems and provides them with a unique type of social interaction. Language allows individuals to attribute symbols to specific concepts and display them through sentences and phrases that follow proper grammatical rules. Moreover, speech is the mechanism in which language is orally expressed.

<span class="mw-page-title-main">Broca's area</span> Speech production region in the dominant hemisphere of the hominid brain

Broca's area, or the Broca area, is a region in the frontal lobe of the dominant hemisphere, usually the left, of the brain with functions linked to speech production.

<span class="mw-page-title-main">Agnosia</span> Medical condition

Agnosia is the inability to process sensory information. Often there is a loss of ability to recognize objects, persons, sounds, shapes, or smells while the specific sense is not defective nor is there any significant memory loss. It is usually associated with brain injury or neurological illness, particularly after damage to the occipitotemporal border, which is part of the ventral stream. Agnosia only affects a single modality, such as vision or hearing. More recently, a top-down interruption is considered to cause the disturbance of handling perceptual information.

Aphasiology is the study of language impairment usually resulting from brain damage, due to neurovascular accident—hemorrhage, stroke—or associated with a variety of neurodegenerative diseases, including different types of dementia. These specific language deficits, termed aphasias, may be defined as impairments of language production or comprehension that cannot be attributed to trivial causes such as deafness or oral paralysis. A number of aphasias have been described, but two are best known: expressive aphasia and receptive aphasia.

Temporal lobe One of the four lobes of the mammalian brain

The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.

Wernickes area Speech comprehension region in the dominant hemisphere of the hominid brain

Wernicke's area, also called Wernicke's speech area, is one of the two parts of the cerebral cortex that are linked to speech, the other being Broca's area. It is involved in the comprehension of written and spoken language, in contrast to Broca's area, which is primarily involved in the production of language. It is traditionally thought to reside in Brodmann area 22, which is located in the superior temporal gyrus in the dominant cerebral hemisphere, which is the left hemisphere in about 95% of right-handed individuals and 70% of left-handed individuals.

Auditory cortex Part of the temporal lobe of the brain

The auditory cortex is the part of the temporal lobe that processes auditory information in humans and many other vertebrates. It is a part of the auditory system, performing basic and higher functions in hearing, such as possible relations to language switching. It is located bilaterally, roughly at the upper sides of the temporal lobes – in humans, curving down and onto the medial surface, on the superior temporal plane, within the lateral sulcus and comprising parts of the transverse temporal gyri, and the superior temporal gyrus, including the planum polare and planum temporale.

Transverse temporal gyrus Gyrus of the primary auditory cortex of the brain

The transverse temporal gyri, also called Heschl's gyri or Heschl's convolutions, are gyri found in the area of primary auditory cortex buried within the lateral sulcus of the human brain, occupying Brodmann areas 41 and 42. Transverse temporal gyri are superior to and separated from the planum temporale by Heschl's sulcus. Transverse temporal gyri are found in varying numbers in both the right and left hemispheres of the brain and one study found that this number is not related to the hemisphere or dominance of hemisphere studied in subjects. Transverse temporal gyri can be viewed in the sagittal plane as either an omega shape or a heart shape.

Amusia is a musical disorder that appears mainly as a defect in processing pitch but also encompasses musical memory and recognition. Two main classifications of amusia exist: acquired amusia, which occurs as a result of brain damage, and congenital amusia, which results from a music-processing anomaly present since birth.

Language processing in the brain "algorithmia-node.js"

Language processing refers to the way humans use words to communicate ideas and feelings, and how such communications are processed and understood. Language processing is considered to be a uniquely human ability that is not produced with the same grammatical understanding or systematicity in even human's closest primate relatives.

Brodmann area 22

Brodmann area 22 is a Brodmann's area that is cytoarchitecturally located in the posterior superior temporal gyrus of the brain. In the left cerebral hemisphere, it is one portion of Wernicke's area. The left hemisphere BA22 helps with generation and understanding of individual words. On the right side of the brain, BA22 helps to discriminate pitch and sound intensity, both of which are necessary to perceive melody and prosody. Wernicke's area is active in processing language and consists of the left Brodmann area 22 and Brodmann area 40, the supramarginal gyrus.

<span class="mw-page-title-main">Speech</span> Human vocal communication using spoken language

Speech is human vocal communication using language. Each language uses phonetic combinations of vowel and consonant sounds that form the sound of its words, and using those words in their semantic character as words in the lexicon of a language according to the syntactic constraints that govern lexical words' function in a sentence. In speaking, speakers perform many different intentional speech acts, e.g., informing, declaring, asking, persuading, directing, and can use enunciation, intonation, degrees of loudness, tempo, and other non-representational or paralinguistic aspects of vocalization to convey meaning. In their speech speakers also unintentionally communicate many aspects of their social position such as sex, age, place of origin, physical states, psychological states, physico-psychological states, education or experience, and the like.

Auditory verbal agnosia (AVA), also known as pure word deafness, is the inability to comprehend speech. Individuals with this disorder lose the ability to understand language, repeat words, and write from dictation. Some patients with AVA describe hearing spoken language as meaningless noise, often as though the person speaking was doing so in a foreign language. However, spontaneous speaking, reading, and writing are preserved. The maintenance of the ability to process non-speech auditory information, including music, also remains relatively more intact than spoken language comprehension. Individuals who exhibit pure word deafness are also still able to recognize non-verbal sounds. The ability to interpret language via lip reading, hand gestures, and context clues is preserved as well. Sometimes, this agnosia is preceded by cortical deafness; however, this is not always the case. Researchers have documented that in most patients exhibiting auditory verbal agnosia, the discrimination of consonants is more difficult than that of vowels, but as with most neurological disorders, there is variation among patients.

Lateralization of brain function Specialization of some cognitive functions in one side of the brain

The lateralization of brain function is the tendency for some neural functions or cognitive processes to be specialized to one side of the brain or the other. The median longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. Although the macrostructure of the two hemispheres appears to be almost identical, different composition of neuronal networks allows for specialized function that is different in each hemisphere.

Brain asymmetry Term in human neuroanatomy referring to several things

In human neuroanatomy, brain asymmetry can refer to at least two quite distinct findings:

Auditory agnosia is a form of agnosia that manifests itself primarily in the inability to recognize or differentiate between sounds. It is not a defect of the ear or "hearing", but rather a neurological inability of the brain to process sound meaning. While auditory agnosia impairs the understanding of sounds, other abilities such as reading, writing, and speaking are not hindered. It is caused by bilateral damage to the anterior superior temporal gyrus, which is part of the auditory pathway responsible for sound recognition, the auditory "what" pathway.

Superior temporal sulcus Part of the brains temporal lobe

The superior temporal sulcus (STS) is the sulcus separating the superior temporal gyrus from the middle temporal gyrus in the temporal lobe of the brain. A sulcus is a deep groove that curves into the largest part of the brain, the cerebrum, and a gyrus is a ridge that curves outward of the cerebrum.

Cross modal plasticity Reorganization of neurons in the brain to integrate the function of two or more sensory systems

Cross modal plasticity is the adaptive reorganization of neurons to integrate the function of two or more sensory systems. Cross modal plasticity is a type of neuroplasticity and often occurs after sensory deprivation due to disease or brain damage. The reorganization of the neural network is greatest following long-term sensory deprivation, such as congenital blindness or pre-lingual deafness. In these instances, cross modal plasticity can strengthen other sensory systems to compensate for the lack of vision or hearing. This strengthening is due to new connections that are formed to brain cortices that no longer receive sensory input.

Verbal intelligence

Verbal intelligence is the ability to understand and reason using concepts framed in words. More broadly, it is linked to problem solving, abstract reasoning, and working memory. Verbal intelligence is one of the most g-loaded abilities.

<span class="mw-page-title-main">Auditosensory cortex</span>

Auditosensory cortex is the part of the auditory system that is associated with the sense of hearing in humans. It occupies the bilateral primary auditory cortex in the temporal lobe of the mammalian brain. The term is used to describe Brodmann area 42 together with the transverse temporal gyri of Heschl. The auditosensory cortex takes part in the reception and processing of auditory nerve impulses, which passes sound information from the thalamus to the brain. Abnormalities in this region are responsible for many disorders in auditory abilities, such as congenital deafness, true cortical deafness, primary progressive aphasia and auditory hallucination.

References

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  3. ↑ Poizner H, Klima ES, Bellugi U., What the hands reveal about the brain, 1987, Cambridge, MA The MIT Press
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  11. ↑ Bottari D, Caclin A, Giard M-H, Pavani F. Changes in Early Cortical Visual Processing Predict Enhanced Reactivity in Deaf Individuals. Sirigu A, ed. PLoS ONE. 2011;6(9):e25607. doi : 10.1371/journal.pone.0025607.
  12. 1 2 3 4 Emmory, Karen, Kosslyn, Steven M., Bellugi, Ursula (1993-02-01). "Visual imagery and visual-spatial language: Enhanced imagery abilities in deaf and hearing ASL signers". Cognition. 46 (2): 139–181. doi:10.1016/0010-0277(93)90017-P. ISSN   0010-0277. PMID   8432094. S2CID   628189.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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