Language processing in the brain

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.

[8][2][9] The Wernicke–Lichtheim–Geschwind model is primarily based on research conducted on brain-damaged individuals who were reported to possess a variety of language related disorders.

This region then projects to a word production center (Broca's area) that is located in the left inferior frontal gyrus.

This lack of clear definition for the contribution of Wernicke's and Broca's regions to human language rendered it extremely difficult to identify their homologues in other primates.

[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.

[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).

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).

[97][98][99][100][101][102][103][104] One fMRI study[105] in which participants were instructed to read a story further correlated activity in the anterior MTG with the amount of semantic and syntactic content each sentence contained.

Studies of present-day humans have demonstrated a role for the ADS in speech production, particularly in the vocal expression of the names of objects.

Magnetic interference in the pSTG and IFG of healthy participants also produced speech errors and speech arrest, respectively[114][115] One study has also reported that electrical stimulation of the left IPL caused patients to believe that they had spoken when they had not and that IFG stimulation caused patients to unconsciously move their lips.

[116] The contribution of the ADS to the process of articulating the names of objects could be dependent on the reception of afferents from the semantic lexicon of the AVS, as an intra-cortical recording study reported of activation in the posterior MTG prior to activation in the Spt-IPL region when patients named objects in pictures[117] Intra-cortical electrical stimulation studies also reported that electrical interference to the posterior MTG was correlated with impaired object naming[118][82] Additionally, lesion studies of stroke patients have provided evidence supporting the dual stream model's role in speech production.

These findings from stroke patients further support the involvement of the dorsal stream pathway in speech production, complementing the stimulation and interference studies in healthy participants.

[83] The authors also reported that stimulation in area Spt and the inferior IPL induced interference during both object-naming and speech-comprehension tasks.

[130] Neuropsychological studies have also found that individuals with speech repetition deficits but preserved auditory comprehension (i.e., conduction aphasia) suffer from circumscribed damage to the Spt-IPL area[131][132][133][134][135][136][137] or damage to the projections that emanate from this area and target the frontal lobe[138][139][140][141] Studies have also reported a transient speech repetition deficit in patients after direct intra-cortical electrical stimulation to this same region.

[11][142][143] Insight into the purpose of speech repetition in the ADS is provided by longitudinal studies of children that correlated the learning of foreign vocabulary with the ability to repeat nonsense words.

Demonstrating the role of the descending ADS connections in monitoring emitted calls, an fMRI study instructed participants to speak under normal conditions or when hearing a modified version of their own voice (delayed first formant) and reported that hearing a distorted version of one's own voice results in increased activation in the pSTG.

[148] Further demonstrating that the ADS facilitates motor feedback during mimicry is an intra-cortical recording study that contrasted speech perception and repetition.

[151] The association of the pSTS with the audio-visual integration of speech has also been demonstrated in a study that presented participants with pictures of faces and spoken words of varying quality.

[155] Empirical research has demonstrated that visual lip movements enhance speech processing along the auditory dorsal stream, particularly in noisy conditions.

Conversely, IPL damage results in individuals correctly identifying the object but incorrectly pronouncing its name (e.g., saying "gof" instead of "goat," an example of phonemic paraphasia).

[161] An MEG study has also correlated recovery from anomia (a disorder characterized by an impaired ability to name objects) with changes in IPL activation.

Based on these associations, the semantic analysis of text has been linked to the inferior-temporal gyrus and MTG, and the phonological analysis of text has been linked to the pSTG-Spt- IPL[168][169][170] Working memory is often treated as the temporary activation of the representations stored in long-term memory that are used for speech (phonological representations).

[182] Studies have shown that performance on phonological working memory tasks correlates with properties of the left dorsal branch of the arcuate fasciculus (AF), which connects posterior temporal language regions with attention-regulating areas in the middle frontal gyrus.

Externalist models, such as Ferdinand de Saussure's structuralism, argue that language as a social phenomenon is external to the brain.

[184] 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.

ERP studies suggest that language processing is based on the interaction of syntax and semantics, and the research does not support innate grammatical structures.

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.

Studies have shown that damage to these areas are similar in results in spoken language where sign errors are present and/or repeated.

[207] 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.

The terms "shallow" and "deep" refer to the extent that a system's orthography represents morphemes as opposed to phonological segments.