Pathways to language: fiber tracts in the human brain

doi:10.1016/j.tics.2009.01.001

Trends in Cognitive Sciences

Volume 13, Issue 4, April 2009, Pages 175-181

https://doi.org/10.1016/j.tics.2009.01.001 Get rights and content

The human language function is not only based on the grey matter of circumscribed brain regions in the frontal and the temporal cortex but moreover on the white matter fiber tracts connecting these regions. Different pathways connecting frontal and temporal cortex have been identified. The dorsal pathway projecting from the posterior portion of Broca's area to the superior temporal region seems to be of particular importance for higher-order language functions. This pathway is particularly weak in non-human compared to human primates and in children compared to adults. It is therefore considered to be crucial for the evolution of human language, which is characterized by the ability to process syntactically complex sentences.

Section snippets

White matter, grey matter and language

Knowledge about the language–brain relationship has dramatically improved since the advent of brain imaging techniques. The functional relevance of different regions in the grey matter as revealed by functional magnetic resonance imaging (fMRI) has led to several quite elaborate neurocognitive models on speech and language processing (e.g. Refs 1, 2, 3, 4, 5). These models define functional neural networks for the processing of different aspects of language such as phonology, syntax and

Broca's area

Subdivisions of the IFG into different ‘cytoarchitectonically’ defined areas are acknowledged by all scientists, and some even propose a functional separation of these (Box 1). The cytoarchitectonic differentiation proposed by Brodmann [8] was based on the layering of the isocortex and the presence of particular cell types therein (Figure 1). According to these criteria, Broca's area is distinguished into a more posterior part of the inferior frontal gyrus (i.e. Brodmann Area [BA] 44) from the

White matter fiber tracts connecting the language areas

Research trying to identify the fiber pathways between Broca's area and Wernicke's area date back to 1895 when Dejerine [17] defined the arcuate fasciculus (AF) as the prominent fiber tract connecting these two areas (see long segment in Figure 2) based on post-mortem dissections.

Today, DTI (Box 2) can be used to investigate fiber pathways in vivo in the human brain 18, 19, 20, 21, 22. Two issues are of great importance when considering DTI studies in the language context. The first issue

Language pathways in the left hemisphere

Studies on the issue of hemispheric differences in the human brain were triggered by the observation that functional language lateralization to the left hemisphere is found in 95% of right-handers and to the right-hemisphere in 15% of left-handers 23, 24. Those studies which analyzed the volume of the language-related areas reported a leftward asymmetry in the volume of the human planum temporale 25, 26, 27, 28 and of Broca's area 27, 28, 29, 30.

Comparative studies between human and non-human

Phylogeny and ontogeny of the language pathways

When considering the importance of these different pathways for language and its evolution, two additional pieces of information are of particular relevance. Because it is impossible to observe language evolution directly, we can only rely on inferences from comparisons between human and non-human primates and on inferences from the language–brain relationship during human ontogeny. Thus, additional information could be gained from considering phylogenetic and ontogenetic data. The comparison

Conclusion

The data reviewed indicate that there are several pathways connecting the language-relevant brain areas. They indicate, however, that in particular the dorsal pathway connecting the posterior part of Broca's area (i.e. BA 44) and the posterior STG/STS is crucial for the human language capacity which is characterized by the ability to process complex sentence structures. First, non-human primates which are not able to learn and process hierarchically structured sequences [56] do not seem to

Acknowledgement

I would like to thank Yves von Cramon for sharing his neuroanatomical expertise with me.

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Trends in Cognitive Sciences

ReviewPathways to language: fiber tracts in the human brain

Section snippets

White matter, grey matter and language

Broca's area

White matter fiber tracts connecting the language areas

Language pathways in the left hemisphere

Phylogeny and ontogeny of the language pathways

Conclusion

Acknowledgement

Trends Cogn. Sci.

Trends Cogn. Sci.

Trends Cogn. Sci.

Biophys. J.

Neuroimage

Brain Res. Cogn. Brain Res.

Neuroimage

Neuroimage

Neuroimage

Trends Cogn. Sci.

Neuroimage

Neuroimage

Neuroimage

Curr. Opin. Neurobiol.

Neuroimage

Neuroimage

Neuron

Neuron

Neuron

Neuroimage

The extended argument dependency model: a neurocognitive approach to sentence comprehension across languages

Psychol. Rev.

Opinion - The cortical organization of speech processing

Nat. Rev. Neurosci.

On aphasia

Brain

The organization of language and the brain

Science

Broca's region revisited: cytoarchitecture and intersubject variability

J. Comp. Neurol.

Connectivity-based parcellation of Broca's area

Cereb. Cortex

Multimodal architectonic mapping of human superior temporal gyrus

Anat. Embryol. (Berl.)

Effective and structural connectivity in the human auditory cortex

J. Neurosci.

The role of left inferior frontal and superior temporal cortex in sentence comprehension: localizing syntactic and semantic processes

Cereb. Cortex

The constraints functional neuroimaging places on classical models of auditory word processing

J. Cogn. Neurosci.

Bilateral speech comprehension reflects differential sensitivity to spectral and temporal features

J. Neurosci.

Anatomie des centres nerveux

In vivo fiber tractography using DT-MRI data

Magn. Reson. Med.

Morphometry of in vivo human white matter association pathways with diffusion-weighted magnetic resonance imaging

Ann. Neurol.

Review
Pathways to language: fiber tracts in the human brain