Ming Han

Overview

 (please put your name on this wiki :) :cotoiale)  (ming: thanks)

Purple - Laurent

            Neurolinguistic research is amazing on two counts. First, the discipline has enjoyed a long history, developing and adapting ever more inventive methods of experimentation. It began with Broca's speech research in the mid-1800s, ^[1] studying aphasic patients, to research conducted in present day, using modern techniques such as positron emission topography (PET), functional MRI (fMRI), diffusion tensor MRI tractography (DTI), functional connectivity MRI (fcMRI), transcranial magnetic stimulation and so on. Secondly, equipped with new techniques and a long history, a huge body of scientific research has gathered to date in trying to understanding how language is organized in the human brain. The vast amount of data available is overwhelming. This neurowiki aims to summarize present understanding and consensus over language organization, while giving attention to various methods used in neurolinguistic research today. But first, the neurowiki will begin by introducing the classical model of language organization that has long been in existence, and has only recently come into question.

 

Table of Content

 

1. Classical model:

            1.1 Broca’s area

            1.2 Wernicke’s area

 

2. Current understanding

2.1 Broca's complex

            2.2 Posterior language zone

           

3. New language areas

            3.1 Geschwind's territory

            3.2 Basal ganglia and cerebellum

 

4. Connections and lanaguage network models

            4.1 Connections

            4.2 The MUC framework: memory, unification and control

 

 

 

1. Classical Model

 

            The classical model of language organization was associated with the connectionist school of thought. The connectionists believed that language localized in the brain in specific modules dedicated to language (which neuroscientists today do agree with). However, early conceptions of language revolved around what was immediately observable. Language was thus divided into a collection of observable activities. These activities were the comprehension of speech and reading, as well as production of speech and writing. Following this line of thought, modules associated with language was divided into language production zones and language comprehension zones. This line of thinking also stems from aphasic research, as the loss of certain brain tissues led to the loss of certain observable language activities. As we will see later, current thinking has moved from observable language activities to the study of actual abstract linguistic functions such as phonology, syntax, lexicon and semantics, and correlating brain areas based on these linguistically defined concepts. (it would be great if you wanted to include a citation: cotoiale) (Ming: just some of my thoughts on how the study of language evolved, :)

 

 

1.1   Broca’s Area:

 

            Broca's area is essentially the left inferior frontal gyrus (IFG), and traditionally consists of pars opercularis (BA44) and pars triangularis (BA45). Lesion of this area produced non-fluent aphasia. The famous example was Paul Broca’s patient “Tan”, whom had lesion in the inferior frontal lobe, and could only utter the word tan. [1] Thus this region of the brain was associated with language production. Although later studies have shown lesions to Broca’s area to affect mostly speech production, and not writing. ^[2]

 

 

1.2   Wernicke’s Area:

 

            Wernicke's Area resides at the left posterior superior temporal gyrus (STGp), corresponding to BA22. Carl Wernicke conducted studies in the late 1800s, assessing lesions to the left STGp, and found that such lesions resulted in fluent, receptive aphasia. Later scientists dubbed this region of the brain the language comprehension center. Although later experiments casted doubts on Wernicke’s claims, as some studies have shown that selective lesions of Wernicke’s area did not produce profound receptive aphasias. [3]

 

(Jin.S I think it might better to have pictures of those areas although all of HMB300 students are well aware of neuroanatomy)

 

2. Current understanding of language localization

           

            The classical model was lacking on two levels. First, the categorization of language into receptive and production zones was very simplistic and general. Second, due to its broad categorization, the classical model failed to account for aphasic symptoms accurately, such as the study discussed in section 1.1, 1.2. ^[2][3]

 

 

            The new direction for language localization did not totally abandon the classical model. Neurolinguists utilized the receptive and production dichotomy, and built around it. What was a drastic change however, was the incorporation of linguistic concepts such as phonology, syntax, lexicon and semantics. It is interesting to note that these new concepts are not in conflict with the old model, but merely helped to expand it beyond the classical observable language abilities approach. (Reconciling of the new and classical model will be discussed later)

 

            Modern technology is one thing that constantly refreshes the way we look at the brain; this is especially the case for neurolinguistics. The field has come a long way from the days of Paul Broca and his famous aphasic studies, and new techniques such as fcMRI are now starting to look at functional coherence of brain regions, ^[4] a step up from cytoarchitecture studies of Brodmann areas (although BA is still a very valuable tool).

 

            Another deviation from the classical model involves new designations for the two traditional language areas (Broca and Wernicke’s area). Broca’s area was given the name Broca’s complex ^[5], as a body of evidence accumulated that the language zone involved numerous areas outside of the traditional Broca’s area, the term was also given to emphasize the several anatomically and functionally distinct regions within Broca’s area, making up a complex. Wernicke’s area has not received the same attention as Broca’s area, it has to some degree retained its name, although neurolinguists are beginning to address it simply as the posterior superior temporal gyrus (STGp), since other areas surrounding it have also been found to be important in language processing, and STGp is simply a part of the posterior language zone.

 

 

2.1 Broca’s complex:

 

            In a recent review by Peter Hagoort, Broca’s complex was hypothesized to be the unification space for language, ^[5] in contrast with the classical model, which classified Broca’s area as a language production zone. Based on evidence from functional studies and fcMRI evidence, ^[4] Broca’s complex included the original areas of pars triangularis (BA 45) and pars opercularis (BA 44), but also extended into pars orbitalis (BA 47) anterior to Broca’s area, and BA 6 (see figure). ^[5]

 

Figure 1. Anatomical Map of Broca's Complex.

From Friederici (2009) ^[6]

 

From meta-analysis of functional imaging studies, Hagoort concluded that the unification functions of Broca’s complex divided into three subsections: syntactic, semantic and phonological unification. ^[5] And these three subdivisions corresponded with specific areas within the Broca’s complex. From anterior to posterior: Semantic unification occurred between BA 47 and 45, syntactic unification took place between BA 45 and 44, whereas phonological unification took place at pars opercularis (BA 44). Hagoort argues that all three linguistic structures contain levels of hierarchy: in that case of syntactic unification, basic lexical elements are combined into grammatic sentences. Contextual and lexical associations occurred under semantic unification, whereas phonological unification is the organization of speech sounds. ^[5]

 

            The new model describes language function of the frontal cortex with increased specificity. Hagoort’s formulation of Broca’s complex as a hub for language unification makes anatomical sense. Broca’s complex is situated in between the motor cortex and the prefrontal cortex. The prefrontal cortex is involved in planning complex cognitive behaviors, and motor cortex is involved directly with language production.

 

            This new model agrees with the classical model in seeing Broca’s area as a language production zone, since to produce language, one has to organize basic elements such as lexicons and speech sounds into complex structures before one can utter or write sentences. However, the new model also suggests that language production is not the sole function of this brain region, since language comprehension requires the decoding and re-unification of language elements. Functional studies have shown activation of the inferior frontal region in response to sentence comprehension. Activity of this region also increased with increasing sentence complexity. ^[7]

 

 

2.2 Posterior language zone:

 

            Recent studies have shown that much of the temporal lobe is implicated in language processing, and linguistic functions are not exclusive to Wernicke’s area. The general area of the posterior language zone is located close to the primary auditory cortex, as well as the primary visual cortex, a perfect place for receiving incoming information.

 

            The posterior language zone consists of several subsections, each implicated in a certain aspect of language processing. The superior temporal gyrus is subdivided into three anatomically distinct regions: anterior STG, Heschl’s gyrus, and posterior STG (Wernicke’s area) (see figure 2). ^[8] Heschl’s gyrus is widely known to be a region of the brain that receives acoustic signals, these signals can then be passed down to the posterior region, where it has been shown to be responsible for transforming acoustic signals into phonetic signals. ^[8] Finally, the anterior portion of the STG has been implicated in higher order syntactic processing of speech sounds. ^[9]

 

Figure 2. Anatomical Map of Posterior Language Areas.

From Hickok & Poeppel (2000) ^[10]

 

            The posterior language zone also includes middle temporal gyrus (MTG) and inferior temporal gyrus (ITG) (see figure 2). Both areas are implicated in word level processing, where phonetic signals from the posterior STG are compared with lexical-semantic representations, hypothesized to exist within the ITG. ^[8] The proximity of the ITG to memory regions such as the hippocampus seems to suggest that during language acquisition, lexicon and semantics may be encoded and stored in this part of the cortex, along with visual, auditory and perhaps even emotional information, and utilized by both language production as well as language comprehension networks. Interestingly, some studies have shown normal performance on picture naming tasks without activation of Broca or Wernicke’s area. ^[11] Instead, STG was activated in addition to parahippocampal gyrus, as well as fusiform area. This seems to suggest that a lexical memory region exists separate from higher order hierarchical processing involving Broca and Wernicke’s area.

 

 

3. New language areas

 perhaps you could include some information about the right hemisphere input to language here. things like prosody, metaphors, etc. [abarnett] see Brownell et al., 1990 (Ming: Right hemisphere input to language function is really interesting, and seems to supplement a lot of things in addition to basic functions traditionally associated with language . I guess the lateralization of language function debate is still raging a bit, I'm definitely going to look into this article, will get back to you) wooooo, just found a wiki article that separates language lateralization quite well: http://en.wikipedia.org/wiki/Cerebral_hemisphere#Hemisphere_lateralization

(After some thought, I think language and the right hemisphere deserves its own wiki, in the mean time I think the above link provides the general idea)

 

3.1 Geschwind’s territory:

 

            Recent DTI study has found strong connections between Broca’s area, Wernicke’s area, and Geschwind’s territory, residing in the inferior parietal cortex. ^[12] The study suggested that Geschwind’s territory might be involved in semantics, although evidence for this has been scarce. There is currently some evidence that this area is involved in sensorimotor control of writing. ^[13]

 

 

(Xingci: Any theories regarding how it might be involved in the sensorimotor control of writing?) (Ming: inferior parietal cortex is close to somatosensory, visual and even auditory cortex, so anatomically it could be expected to mediate all these functions required in writing and controlling/coordinating finger movements, but this is total speculation on my part. An old article by Atsushi Yamadori - Writing and hemispheric coordination, appear to also point to angular gyrus in his agraphia research. In addition, he posit the notion that writing is not innate like spoken language, and that significant other portions of the brain are utilized in the difficult task of writing, he speculates right hemisphere involvement as well.)

 

 

3.2 Basal ganglia and Cerebellum:

 

            Both areas are known to be involved in motor functions, involvement in cognition, and especially language processing has only been reported recently. A recent fMRI study looked at connectivity of basal ganglia and cerebellum to Broca’s area and temporal cortex. The study found strong connections between these areas. They hypothesized that basal ganglia may be implicated in initiating cortical function, whereas the cerebellum amplified basal ganglia signaling, facilitating cortical decision-making, ^[15] and perhaps influencing things such as picking the correct choice of words. Other studies have questioned involvement of basal ganglia and cerebellum in cognitive processing, either stating the fact that the results are far from conclusive, or providing results which showed no apparent activation with response to cognitive tasks. ^[16][17]

 

4. Connections and language network models

 

4.1 Connections:

 

            Much of neuroanatomical research has focus on language localization in grey matter; the only white matter fibre tract known to be involved in language was the arcuate fasciculus. Although neuroscientists have always suspected language areas in the brain to be highly connected, studying connectivity in the brain has always been difficult. Thanks to new techniques such as DTI, this is no longer the case. A recent review of studies done on language connectivity by Angela Friederici, expanded on the traditional concept of one fibre tract connecting two language regions. The language areas discussed in that review corresponded quite accurately with all of the language areas discussed in this neurowiki. Below will be a summary of the main points from the review of Friederici.

 

            Connection between anterior and posterior language zones involves two pathways: dorsal and ventral. ^[6]

 

            The dorsal pathway is carried by the well-known arcuate fasciculus. This pathway runs from pars opercularis (BA 44) to the posterior regions of STG/ STS. The dorsal pathway has been shown to communicate phonological and syntactic information, with emphasis on processing of complex, hierarchical concepts. ^[6]

 

            The ventral pathway separated into 2 streams. One stream runs from the deep operculum to the anterior STG, carried by the uncinate fasciculus. The other stream runs from pars triangularis (BA 45) to the anterior part of Heschl’s gyrus, carried by the extreme capsule. The first pathway communicates syntax, similar to the dorsal pathway, but only at a local and linear level. The second pathway is implicated in communication of semantic information. ^[6]

 

 

4.2 The MUC framework: memory, unification and control:

 

            Peter Hagoort, the same person that functionally differentiated the 3 regions of Broca’s complex, devised the MUC framework. The framework gives us a broad overview of the components thought to underlie language processing: memory, unification and control. ^[14] Unification was discussed earlier in this neurowiki, with Broca’s complex being the region of the brain where unification takes place. Memory component of the MUC framework is thought to reside in the temporal cortex, specifically in the middle to inferior temporal gyri. ^[14] The third component – control, includes areas involved in cognition, emotion as well as attention. Control components might include the anterior cingulated cortex, dorsal lateral prefrontal cortex, basal ganglia, cerebellum and even amygdala.

 

            This model is unique in the fact that it unites broad aspects of language, and gives us a coherent picture of how language is organized in the brain, based on consensus of accumulated experimental evidence.

 

5. Concluding remarks:

 

            The current model of language organization utilized evidence from functional imaging studies, aphasic studies, as well as connectivity studies, in establishing a coherent picture of what areas are involved in language processing, what function do these areas perform, as well as how these areas are connected to produce and comprehend language.

 

            Current consensus on language organization has improved upon the classical model on several levels. First, the new model changed the way we categorize language, from observable language activities, to how language is analyzed and synthesized, based on abstract linguistic concepts.  Secondly, shift in the way we categorize language has led to the redefinition of the boundaries of old language centers. Finally, not only is the new model sufficient to encompass broad aspects of language processing, it is also able to describe functional anatomy of language with great specificity.

 

 

 great wiki :)

(a.zhang: ditto above =P awesome wiki!)

(Jin.S: I love this wiki)

Rena: Language is such an interesting subject. Did you find any studies on lesioning in language centers and the compensation of the brain to replace language functionality after loss? Any theraph done with patients in that area?

 

Reference:

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            suivies d’une observation d’aphemie. Bull Soc Anat Paris. 6: 330 –357.

2. Basso, A., Taborelli, A., Vignolo, L.A. (1978) Dissociated disorders of speaking and writing in aphasia. Journal of Neurology, Neurosurgery and Psychiatry. 41: 556-563.

3. Dronkers NF., Redfern B B., Knight R T. (2000) "The neural architecture of language disorders". in Bizzi, Emilio; Gazzaniga, Michael S.. The New cognitive neurosciences (2nd ed.). Cambridge, Mass: MIT Press. pp. 949–58.

4. Xiang, H.D., Fonteijn, H.M., Norris, D.G., Hagoort, P. (2010) Topographical Functional Connectivity Pattern in the Perisylvian Languag Networks. Cerebral Cortex. 20: 549-560.

5. Hagoort, P. (2005) Broca’s complex as the unification space for language. In: Cutler A, editor. Twenty-first century psycholinguistics: four cornerstones. Mahwah (NJ): Lawrence Erlbaum. p. 157--173.

6. Friederici, A.D. (2009) Pathways to language: fiber tracts in the human brain. Trends in Neuroscience. 13(4): 175-181.

7. Bornkessel, I., Zysset, T.S., Friederici, A.D., Yves von Cramon, D., Schlesewskyb, M. (2005) Who did what to whom? The neural basis of argument hierarchies during language comprehension. NeuroImage. 26: 221– 233

8. Poeppel, D., Idsardi, W.J., van Wassenhove, V. (2008) Speech perception at the interface of neurobiology and linguistics. Philosophical Transactions of the  Royal Society B. 363: 1071–1086

9. Angela D. Friederici, A.D., Rüschemeyer, S.A., Hahne A., Fiebach, C.J. (2003) The Role of Left Inferior Frontal and Superior Temporal Cortex in Sentence Comprehension: Localizing Syntactic and Semantic Processes. Cerebral Cortex. 13(2): 170-177.

10. Hickok, G. & Poeppel, D. (2000) Towards a functional neuroanatomy of speech perception. Trends in Cognitive Science. 4: 131–138.

11. Etard, O., Mellet, E., Papathanassiou, D., Benali, K., Houde, O., Mazoyer, B., Tzourio-Mazoyer, N. (2000) Picture naming without Broca's and Wernicke's area. Brain Imaging. 11(3): 617-622.

12. Catani, M., Jones, D.K., & Ffytche, D.H. (2005) Perisylvian language networks of the human brain. Annals of Neurology. 57 (1): 8–16.

13. Brownsett, S.L.E. & Wise, R.J.S. (2010) The Contribution of the Parietal Lobes to

            Speaking and Writing. Cerebral Cortex. 20: 517-523.

14. Hagoort, P. (2005) On Broca, brain, and binding: a new framework. Trends in Cognitive Sciences. 9(9): 416-423.

15. Bootha, J.R., Wood, L., Lua, D., Houk, J.C., Bitan, T. (2007) The role of the basal ganglia and cerebellum in language processing. Brain Research. 1133: 136–144.

16. Jung De Smet, H., Baillieux, H., De Deyn, P.P., Mariën, P., Paquier, P. (2007) The Cerebellum and Language: The Story So Far. Folia Phoniatr Logop. 59:165–170.

17. Longworth, C.E., Keenan, S.E., Barker, R.A, Marslen-Wilson, W.D., Tyler, L.K. (2005) The basal ganglia and rule-governed language use: evidence from vascular and degenerative conditions. Brain. 128: 584–596.