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Neural Structures of Language Processing

Much of the language function is processed in several association areas, and there are two well-identified areas that are considered vital for human communication: Wernicke's area and Broca's area. These areas are usually located in the dominant hemisphere (the left hemisphere in 97% of people) and are considered the most important areas for language processing. This is why language is considered a localized and lateralized function.

However, the non-dominant hemisphere also participates in this cognitive function, and there is ongoing debate on the level of participation of the less-dominant areas. The non-dominant hemisphere may be particularly involved in processing the prosody of spoken language.

Other factors are believed to be relevant to language processing and verbal fluency, such as cortical thickness, participation of prefrontal areas of the cortex, and communication between right and left hemispheres.

Wernicke's area

Main article: Wernicke's area

(NOTE TO SELF: INSERT PICTURE HERE Lateral surface of the brain with Brodmann's areas numbered.)

Wernicke's area is classically located in the posterior section of the superior temporal gyrus of the dominant hemisphere (Brodmann area 22), with some branches extending around the posterior section of the lateral sulcus, in the parietal lobe.

Wernicke's area is located between the auditory cortex and the visual cortex. The former is located in the transverse temporal gyrus (Brodmann areas 41 and 42), in the temporal lobe, while the latter is located in the posterior section of the occipital lobe (Brodmann areas 17, 18 and 19). While the dominant hemisphere is in charge of most of language comprehension, recent studies have demonstrated that the non-dominant (right hemisphere in 97% of people) homologous area participates in the comprehension of ambiguous words, whether they are written or heard. Receptive speech has traditionally been associated with Wernicke's area of the posterior superior temporal gyrus (STG) and surrounding areas. Current models of speech perception include greater Wernicke's area, but also implicate a "dorsal" stream that includes regions also involved in speech motor processing. First identified by Carl Wernicke in 1874, its main function is the comprehension of language and the ability to communicate coherent ideas, whether the language is vocal, written, or signed.

Broca's area

Main article: Broca's area

Broca's area is usually formed by the pars triangularis and the pars opercularis of the inferior frontal gyrus (Brodmann areas 44 and 45). It follows Wernicke's area, and as such they both are usually located in the left hemisphere of the brain. This area is involved mostly in the production of speech. Given its proximity to the motor cortex, neurons from Broca's area send signals to the larynx, tongue and mouth motor areas, which in turn send the signals to the corresponding muscles, thus allowing the creation of sounds.

A recent analysis of the specific roles of these sections of the left inferior frontal gyrus in verbal fluency indicates that Brodmann area 44 (pars opercularis) may subserve phonological fluency, whereas the Brodmann area 45 (pars triangularis) may be more involved in semantic fluency. Further analysis shows that Broca's area may have less involvement with information for producing individual words, but, instead, Broca's area is shown to coordinate language processing information for speech production on a greater scale.

Arcuate fasciculus

The arcuate fasciculus is the area of the brain between Wernicke's area and Broca's area that connects the two through bundles of nerve fibers. This portion of the brain serves as a transit center between the two areas dealing most largely with speech and communication.

Cortical thickness and verbal fluency

Recent studies have shown that the rate of increase in raw vocabulary fluency was positively correlated with the rate of cortical thinning. In other words, greater performance improvements were associated with greater thinning. This is more evident in left hemisphere regions, including the left lateral dorsal frontal and left lateral parietal regions: the usual locations of Broca's area and Wernicke's area, respectively.

After Sowell's studies, it was hypothesized that increased performance on the verbal fluency test would correlate with decreased cortical thickness in regions that have been associated with language: the middle and superior temporal cortex, the temporal–parietal junction, and inferior and middle frontal cortex. Additionally, other areas related to sustained attention for executive tasks were also expected to be affected by cortical thinning.

One theory for the relation between cortical thinning and improved language fluency is the effect that synaptic pruning has in signaling between neurons. If cortical thinning reflects synaptic pruning, then pruning may occur relatively early for language-based abilities. The functional benefit would be a tightly honed neural system that is impervious to "neural interference", avoiding undesired signals running through the neurons which could possibly worsen verbal fluency.

The strongest correlations between language fluency and cortical thicknesses were found in the temporal lobe and temporal–parietal junction. Significant correlations were also found in the auditory cortex, the somatosensory cortex related to the organs responsible for speech (lips, mouth) and frontal and parietal regions related to attention and performance monitoring. The frontal and parietal regions are also evident in the right hemisphere.

Sign Language Processing

Main Article: Sign language in the brain

Sign languages use a visual-manual modality of communication, incorporating motion, facial expression, and body position. Signed languages and spoken languages are processed similarly and in many of the same regions of the brain.