Language System - Connectome Guide

Language Network

Why we think this network is worth considering in decision making:

Disturbances of language have been linked to anatomic disruption of the human language network architecture for nearly 150 years.

Evidence that this network is responsible for useful function in humans:

While details regarding its exact organization are still in refinement, the basic details regarding the anatomy of the human language system have been non-controversial for many decades, and there is a long and robust history of using models of the language system in clinical decision-making spanning back a very long time.

Exploring the language organization in the brain began in the mid-19^th century with Broca’s and Wernicke’s studies on patients with defined brain injuries and lesions in the mid-19^th century.^1,2 Further studies identified a neural tract, the arcuate fasciculus, for connecting Broca’s and Wernicke’s areas being one of the major components of language processing in humans.³ Later on, more studies confirmed and expanded these findings using additional techniques such as cortical intraoperative mapping,⁴ and subcortical mapping,⁵ and neuroimaging using DTI and fMRI. Using DTI, Rilling et al., (2008) found a noticeable temporal projection of arcuate fasciculus that is much smaller or absent in non-human primates.⁶ Further studies reported a strong left lateralization of arcuate fasciculus in humans.⁷

Additional invasive tests have been described to study impact on networks for clinical decision making. Historically injecting sodium amytal into one carotid artery (Wada Test) has been the gold standard for finding out the language lateralization in the brain.⁸ Studies found a very strong correlation between Wada test and language lateralization.⁹ More recently, similar findings have emerged using non-invasive neuro-imaging techniques. One study reported fMRI superiority over Wada test predicting postoperative language function in patients when fMRI and Wada test are not consistent with each other.¹⁰ Moreover, a recent guideline by the American Academy of Neurology suggested fMRI may be considered as a replacement of Wada test in epilepsy.¹¹

Consequences of damage to this network:

Evidence that damaging this network causes neurological decline.

Several studies demonstrated aphasic impairment as a result of damage to this network in stroke patients, epilepsy patients, and tumor patients.^12,13,14

Lately, speech network has been successfully mapped using rs-fMRI in an aphasic patient with left temporal tumor even in the absence of cognitive function. In the study, patient was scanned before and after a seizure that causes aphasia for several days.¹⁵ In aged patients it was found that higher functional connectivity in the left language areas associated with higher scores in behavioral tests, this finding suggests a role for functional preservation in these areas.¹⁶

44 ᐅ Summary

Area 44: part of the inferior frontal gyrus of the lateral frontal lobe. Translates abstract and intentional information in the prefrontal cortex to more detailed representations to help guide the production of verbal and manual actions. In addition to its known association with Broca's area, is sometimes represented as part of Broca's complex

ᐅ Where is it?

Area 44 is at the posterior most part of the inferior frontal gyrus. It is the anterior bank of pars opercularis of the IFG.

ᐅ What are its borders?

Area 44 borders area 45 anteriorly and area 6r posteriorly. Area 8C is its medial border and its inferior border is wedged between then upper borders of Areas 6R and 6V. Its superior edge borders IFSp and IFJa. Its opercular surface is FOP4.

ᐅ What are its functional connections?

Area 44 demonstrates functional connectivity to areas SFL, IFSp, IFJa, 45, 47s, 47L, 9a, 9m, 8AV, 8BL and 8C in the dorsolateral frontal lobe, area 8BM in the medial frontal lobe, area 55b in the premotor areas, areas FOP5, AVI and PSL in the insula- opercular region, areas TGd, STSdp and STSvp in the temporal lobe, areas PFm, and PGi in the inferior parietal lobe, and no areas in the medial parietal lobe.

ᐅ What are its white matter connections?

Area 44 is structurally connected to the arcuate/SLF and the FAT. Connections with the arcuate/SLF project posteriorly and wrap around the Sylvian fissure to the middle temporal gyrus to end at TE1a and TE1m. There are also projections from the arcuate/SLF before it terminates to parcellations A5 and STSdp. The majority of the inferior connections of the frontal aslant tract end at 44, the tract is connected superiorly to superior frontal gyrus parcellations SFL, 6ma and s6-8. Local short association bundles are connected with 45 and 8C. White matter tracts from 44 in the right hemisphere have less consistent connections with the arcuate/SLF.

ᐅ What is known about its function?

Area 44 translates abstract and intentional information in the prefrontal cortex to more detailed representations to help guide the production of verbal and manual actions. Area 44, in addition to its known association with Broca's area, is sometimes represented as part of "Broca's complex", including Brodmann Areas 45, 46, 47 and the mesial supplementary motor area of 6, which contribute to a frontal-subcortical circuit. The right pars opercularis has also been implicated in cognitive inhibition in the overall context of working memory.

44_a

A: lateral-medial

44_b

B: anterior-posterior

44_c

C: superior-inferior

44_dti

DTI image

45 ᐅ Summary

Area 45: part of the inferior frontal gyrus of the lateral frontal lobe. In addition to its known association with Broca's area, is sometimes represented as part of Broca's complex