Language processing in the brain
In psycholinguistics, 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.[1]
"Language processing" redirects here. For the processing of language by computers, see Natural language processing.
Throughout the 20th century the dominant model[2] for language processing in the brain was the Geschwind-Lichteim-Wernicke model, which is based primarily on the analysis of brain-damaged patients. However, due to improvements in intra-cortical electrophysiological recordings of monkey and human brains, as well non-invasive techniques such as fMRI, PET, MEG and EEG, a dual auditory pathway[3][4] has been revealed and a two-streams model has been developed. In accordance with this model, there are two pathways that connect the auditory cortex to the frontal lobe, each pathway accounting for different linguistic roles. The auditory ventral stream pathway is responsible for sound recognition, and is accordingly known as the auditory 'what' pathway. The auditory dorsal stream in both humans and non-human primates is responsible for sound localization, and is accordingly known as the auditory 'where' pathway. In humans, this pathway (especially in the left hemisphere) is also responsible for speech production, speech repetition, lip-reading, and phonological working memory and long-term memory. In accordance with the 'from where to what' model of language evolution,[5][6] the reason the ADS is characterized with such a broad range of functions is that each indicates a different stage in language evolution.
The division of the two streams first occurs in the auditory nerve where the anterior branch enters the anterior cochlear nucleus in the brainstem which gives rise to the auditory ventral stream. The posterior branch enters the dorsal and posteroventral cochlear nucleus to give rise to the auditory dorsal stream.[7]: 8
Language processing can also occur in relation to signed languages or written content.
Current neurolinguistics models[edit]
Anatomy[edit]
In the last two decades, significant advances occurred in our understanding of the neural processing of sounds in primates. Initially by recording of neural activity in the auditory cortices of monkeys[18][19] and later elaborated via histological staining[20][21][22] and fMRI scanning studies,[23] 3 auditory fields were identified in the primary auditory cortex, and 9 associative auditory fields were shown to surround them (Figure 1 top left). Anatomical tracing and lesion studies further indicated of a separation between the anterior and posterior auditory fields, with the anterior primary auditory fields (areas R-RT) projecting to the anterior associative auditory fields (areas AL-RTL), and the posterior primary auditory field (area A1) projecting to the posterior associative auditory fields (areas CL-CM).[20][24][25][26] Recently, evidence accumulated that indicates homology between the human and monkey auditory fields. In humans, histological staining studies revealed two separate auditory fields in the primary auditory region of Heschl's gyrus,[27][28] and by mapping the tonotopic organization of the human primary auditory fields with high resolution fMRI and comparing it to the tonotopic organization of the monkey primary auditory fields, homology was established between the human anterior primary auditory field and monkey area R (denoted in humans as area hR) and the human posterior primary auditory field and the monkey area A1 (denoted in humans as area hA1).[29][30][31][32][33] Intra-cortical recordings from the human auditory cortex further demonstrated similar patterns of connectivity to the auditory cortex of the monkey. Recording from the surface of the auditory cortex (supra-temporal plane) reported that the anterior Heschl's gyrus (area hR) projects primarily to the middle-anterior superior temporal gyrus (mSTG-aSTG) and the posterior Heschl's gyrus (area hA1) projects primarily to the posterior superior temporal gyrus (pSTG) and the planum temporale (area PT; Figure 1 top right).[34][35] Consistent with connections from area hR to the aSTG and hA1 to the pSTG is an fMRI study of a patient with impaired sound recognition (auditory agnosia), who was shown with reduced bilateral activation in areas hR and aSTG but with spared activation in the mSTG-pSTG.[36] This connectivity pattern is also corroborated by a study that recorded activation from the lateral surface of the auditory cortex and reported of simultaneous non-overlapping activation clusters in the pSTG and mSTG-aSTG while listening to sounds.[37]
Downstream to the auditory cortex, anatomical tracing studies in monkeys delineated projections from the anterior associative auditory fields (areas AL-RTL) to ventral prefrontal and premotor cortices in the inferior frontal gyrus (IFG)[38][39] and amygdala.[40] Cortical recording and functional imaging studies in macaque monkeys further elaborated on this processing stream by showing that acoustic information flows from the anterior auditory cortex to the temporal pole (TP) and then to the IFG.[41][42][43][44][45][46] This pathway is commonly referred to as the auditory ventral stream (AVS; Figure 1, bottom left-red arrows). In contrast to the anterior auditory fields, tracing studies reported that the posterior auditory fields (areas CL-CM) project primarily to dorsolateral prefrontal and premotor cortices (although some projections do terminate in the IFG.[47][39] Cortical recordings and anatomical tracing studies in monkeys further provided evidence that this processing stream flows from the posterior auditory fields to the frontal lobe via a relay station in the intra-parietal sulcus (IPS).[48][49][50][51][52][53] This pathway is commonly referred to as the auditory dorsal stream (ADS; Figure 1, bottom left-blue arrows). Comparing the white matter pathways involved in communication in humans and monkeys with diffusion tensor imaging techniques indicates of similar connections of the AVS and ADS in the two species (Monkey,[52] Human[54][55][56][57][58][59]). In humans, the pSTG was shown to project to the parietal lobe (sylvian parietal-temporal junction-inferior parietal lobule; Spt-IPL), and from there to dorsolateral prefrontal and premotor cortices (Figure 1, bottom right-blue arrows), and the aSTG was shown to project to the anterior temporal lobe (middle temporal gyrus-temporal pole; MTG-TP) and from there to the IFG (Figure 1 bottom right-red arrows).
Auditory ventral stream[edit]
The auditory ventral stream (AVS) connects the auditory cortex with the middle temporal gyrus and temporal pole, which in turn connects with the inferior frontal gyrus. This pathway is responsible for sound recognition, and is accordingly known as the auditory 'what' pathway. The functions of the AVS include the following.
Linguistic theories[edit]
Language-processing research informs theories of language. The primary theoretical question is whether linguistic structures follow from the brain structures or vice versa. Externalist models, such as Ferdinand de Saussure's structuralism, argue that language as a social phenomenon is external to the brain. The individual receives the linguistic system from the outside, and the given language shapes the individual's brain.[181]
This idea is opposed by internalist models including Noam Chomsky's transformational generative grammar, George Lakoff's Cognitive Linguistics, and John A. Hawkins's efficiency hypothesis. According to Chomsky, language is acquired from an innate brain structure independently of meaning.[182] Lakoff argues that language emerges from the sensory systems.[183] Hawkins hypothesizes that cross-linguistically prevalent patterns are based on the brain's natural processing preferences.[184]
Additionally, models inspired by Richard Dawkins's memetics, including Construction Grammar and Usage-Based Linguistics, advocate a two-way model arguing that the brain shapes language, and language shapes the brain.[185][186]
Evidence from neuroimaging studies points towards the externalist position. ERP studies suggest that language processing is based on the interaction of syntax and semantics, and the research does not support innate grammatical structures.[187][188] MRI studies suggest that the structural characteristics of the child's first language shapes the processing connectome of the brain.[189] Processing research has failed to find support for the inverse idea that syntactic structures reflect the brain's natural processing preferences cross-linguistically.[190]
The evolution of language[edit]
The auditory dorsal stream also has non-language related functions, such as sound localization[191][192][193][194][195] and guidance of eye movements.[196][197] Recent studies also indicate a role of the ADS in localization of family/tribe members, as a study[198] that recorded from the cortex of an epileptic patient reported that the pSTG, but not aSTG, is selective for the presence of new speakers. An fMRI[199] study of fetuses at their third trimester also demonstrated that area Spt is more selective to female speech than pure tones, and a sub-section of Spt is selective to the speech of their mother in contrast to unfamiliar female voices.
It is presently unknown why so many functions are ascribed to the human ADS. An attempt to unify these functions under a single framework was conducted in the 'From where to what' model of language evolution[200][201] In accordance with this model, each function of the ADS indicates of a different intermediate phase in the evolution of language. The roles of sound localization and integration of sound location with voices and auditory objects is interpreted as evidence that the origin of speech is the exchange of contact calls (calls used to report location in cases of separation) between mothers and offspring. The role of the ADS in the perception and production of intonations is interpreted as evidence that speech began by modifying the contact calls with intonations, possibly for distinguishing alarm contact calls from safe contact calls. The role of the ADS in encoding the names of objects (phonological long-term memory) is interpreted as evidence of gradual transition from modifying calls with intonations to complete vocal control. The role of the ADS in the integration of lip movements with phonemes and in speech repetition is interpreted as evidence that spoken words were learned by infants mimicking their parents' vocalizations, initially by imitating their lip movements. The role of the ADS in phonological working memory is interpreted as evidence that the words learned through mimicry remained active in the ADS even when not spoken. This resulted with individuals capable of rehearsing a list of vocalizations, which enabled the production of words with several syllables. Further developments in the ADS enabled the rehearsal of lists of words, which provided the infra-structure for communicating with sentences.
Sign language in the brain[edit]
Neuroscientific research has provided a scientific understanding of how sign language is processed in the brain. There are over 135 discrete sign languages around the world- making use of different accents formed by separate areas of a country.[202]
By resorting to lesion analyses and neuroimaging, neuroscientists have discovered that whether it be spoken or sign language, human brains process language in general, in a similar manner regarding which area of the brain is being used. [202]Lesion analyses are used to examine the consequences of damage to specific brain regions involved in language while neuroimaging explore regions that are engaged in the processing of language.[202]
Previous hypotheses have been made that damage to Broca's area or Wernicke’s area does not affect sign language being perceived; however, it is not the case. Studies have shown that damage to these areas are similar in results in spoken language where sign errors are present and/or repeated. [202]In both types of languages, they are affected by damage to the left hemisphere of the brain rather than the right -usually dealing with the arts.
There are obvious patterns for utilizing and processing language. In sign language, Broca’s area is activated while processing sign language employs Wernicke’s area similar to that of spoken language [202]
There have been other hypotheses about the lateralization of the two hemispheres. Specifically, the right hemisphere was thought to contribute to the overall communication of a language globally whereas the left hemisphere would be dominant in generating the language locally.[203] Through research in aphasias, RHD signers were found to have a problem maintaining the spatial portion of their signs, confusing similar signs at different locations necessary to communicate with another properly.[203] LHD signers, on the other hand, had similar results to those of hearing patients. Furthermore, other studies have emphasized that sign language is present bilaterally but will need to continue researching to reach a conclusion.[203]
Writing in the brain[edit]
There is a comparatively small body of research on the neurology of reading and writing.[204] Most of the studies performed deal with reading rather than writing or spelling, and the majority of both kinds focus solely on the English language.[205] English orthography is less transparent than that of other languages using a Latin script.[204] Another difficulty is that some studies focus on spelling words of English and omit the few logographic characters found in the script.[204]
In terms of spelling, English words can be divided into three categories – regular, irregular, and “novel words” or “nonwords.” Regular words are those in which there is a regular, one-to-one correspondence between grapheme and phoneme in spelling. Irregular words are those in which no such correspondence exists. Nonwords are those that exhibit the expected orthography of regular words but do not carry meaning, such as nonce words and onomatopoeia.[204]
An issue in the cognitive and neurological study of reading and spelling in English is whether a single-route or dual-route model best describes how literate speakers are able to read and write all three categories of English words according to accepted standards of orthographic correctness. Single-route models posit that lexical memory is used to store all spellings of words for retrieval in a single process. Dual-route models posit that lexical memory is employed to process irregular and high-frequency regular words, while low-frequency regular words and nonwords are processed using a sub-lexical set of phonological rules.[204]
The single-route model for reading has found support in computer modelling studies, which suggest that readers identify words by their orthographic similarities to phonologically alike words.[204] However, cognitive and lesion studies lean towards the dual-route model. Cognitive spelling studies on children and adults suggest that spellers employ phonological rules in spelling regular words and nonwords, while lexical memory is accessed to spell irregular words and high-frequency words of all types.[204] Similarly, lesion studies indicate that lexical memory is used to store irregular words and certain regular words, while phonological rules are used to spell nonwords.[204]
More recently, neuroimaging studies using positron emission tomography and fMRI have suggested a balanced model in which the reading of all word types begins in the visual word form area, but subsequently branches off into different routes depending upon whether or not access to lexical memory or semantic information is needed (which would be expected with irregular words under a dual-route model).[204] A 2007 fMRI study found that subjects asked to produce regular words in a spelling task exhibited greater activation in the left posterior STG, an area used for phonological processing, while the spelling of irregular words produced greater activation of areas used for lexical memory and semantic processing, such as the left IFG and left SMG and both hemispheres of the MTG.[204] Spelling nonwords was found to access members of both pathways, such as the left STG and bilateral MTG and ITG.[204] Significantly, it was found that spelling induces activation in areas such as the left fusiform gyrus and left SMG that are also important in reading, suggesting that a similar pathway is used for both reading and writing.[204]
Far less information exists on the cognition and neurology of non-alphabetic and non-English scripts. Every language has a morphological and a phonological component, either of which can be recorded by a writing system. Scripts recording words and morphemes are considered logographic, while those recording phonological segments, such as syllabaries and alphabets, are phonographic.[205] Most systems combine the two and have both logographic and phonographic characters.[205]
In terms of complexity, writing systems can be characterized as "transparent" or "opaque" and as "shallow" or "deep". A "transparent" system exhibits an obvious correspondence between grapheme and sound, while in an "opaque" system this relationship is less obvious. The terms "shallow" and "deep" refer to the extent that a system's orthography represents morphemes as opposed to phonological segments.[205] Systems that record larger morphosyntactic or phonological segments, such as logographic systems and syllabaries put greater demand on the memory of users.[205] It would thus be expected that an opaque or deep writing system would put greater demand on areas of the brain used for lexical memory than would a system with transparent or shallow orthography.