**8. Conclusions**

84 Neuroimaging – Cognitive and Clinical Neuroscience

Recent developments in brain imaging techniques include brain *tractography* with MRI, which has been promptly used to study human language. Brain tactography can be achieved through *diffusion tensor* and *diffusion spectrum* techniques. The main difference between the two of them depends on the deterministic vs. probabilistic approaches used to analyze the movement of water molecules within the main tracts substantiating cortico-cortical connections (de Schotten et al., 2011). It must be noted here, however, that the novelty and the relative scarcity of studies approaching language with this technique may explain certain

Although it is well known after Karl Wernicke, the relevance for human language of the *arquate fasciculus* (AF) connecting Broca's and Wernicke's areas appears largely strengthened by tractographic techniques. The data also stress the relevance for language processing of other fascicles connecting anterior and posterior brain areas. A detailed description of all

One of the first studies applying tractography to approach language was developed by Catani et al. (2005). These authors reported a direct strong connection between Broca's and Wernicke's areas through the FA, but given the fact that the areas actually connected covered a wider territory than the classical Broca's and Wernicke's areas (though, as shown earlier, the precise limits for these two areas may vary depending on the author), Catani and colleagues suggested to call them Broca's and Wernicke's *territories*, respectively. Their results also revealed the existence of two additional but indirect pathways connecting Broca's and Wernicke's territories. One would run laterally, consisting of an anterior segment connecting Broca's territory and the inferior parietal cortex. The other would be a posterior segment connecting the inferior parietal cortex with Wernicke's territory. Given the apparent relevance of these two indirect segments, and the fact that the inferior parietal cortex appears the main meeting point for these indirect connections, Catani et al. (2005) suggested to call this region the *Geschwind's territory*, in the memory of Norman Geschwind, who already proposed a relevant role of the inferior parietal cortex in language. This region largely overlaps with semantic areas

Fig. 4. Approximate locations of the system to process discourse

inconsistencies between studies (for an extensive review, see Friederici, 2009).

involved in word and discourse processing, as we have seen.

**7. All together** 

these connections is also emerging.

The moment arrives to summarize and interpret the major milestones that could be elucidated from the preceding exposition. In the following, we will also express a number of reflections on human language using brain function as a main perspective.

A first and relatively robust conclusion that can be extracted so far is that the human brain contains at least two major "centers of gravity", or main hubs in the networks devoted to language processing. These foci are, on the one hand, the left inferior frontal gyrus (IFG) and, on the other, the left superior temporal gyrus (STG), the latter probably spreading to the superior temporal sulcus (STS) and posterior portions of the middle temporal gyrus (MTG) as well as to some parts of the inferior parietal cortex. Both foci are highly and densely interconnected by means of several tracts, the most outstanding one being the arquate fasciculus (AF). Most of the primary functions and processes involving these foci are seemingly phonological/articulatory and syntactic in nature. These two main hubs are located in perisylvian areas and appear critical for human language. Actually, the main loci of the cerebral lesions yielding core symptoms highly specific of language are the perisylvian areas; the most conspicuous aphasias are usually the consequence of lesions affecting either these regions or the AF (e.g., LaPointe, 2011).

If the depiction in the preceding paragraph can be taken as relatively robust, the same is not the case when we attempt to subdivide each hub (IFG and STG). An approximate

Functional and Structural Magnetic Resonance Imaging of Human Language: A Review 87

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depiction seems that as we move from more posterior/dorsal regions to more anterior/ventral in the left IFG, a gradient of activations can be found to be specifically involved (in this order) in phonology/articulation, syntax, and semantics. A similar gradient could be found in the STG when moving from the primary auditory association areas in or around Hesch's gyrus, spreading widely to both anterior and posterior regions in the STG, probably covering also parts of the inferior parietal cortex. In the latter case, the gradient seems to cover, following this order, phonology/articulation and syntax. If we want to expand these functions to semantic processes, then STS and at least several portions of the MTG should be included.

From there, the system spreads to notably many other brain regions, comprising, posteriorly, large portions of the whole temporal lobes, including the temporal poles and part of the basal regions, as well as significant portions of the parietal cortex. Frontally, the system spreads to more anterior regions, including large extensions of the prefrontal cortex; among them, an area showing the most substantial increase in size in humans when compared to other primates and importantly involved in general intelligence. Significant medial regions, both in the prefrontal cortex and in the parietal cortex, are also included in this system. This *extended language network* (using an expression coined by Ferstl et al., 2008) largely overlaps with the *human default system*, a bilateral network in the human brain active when we are involved in "internal" mental tasks. If the linguistic message implies the visualization or representation of a given situation, then the corresponding primary or secondary areas of the neocortex can be activated, either motor or perceptual.

The system can therefore be viewed as a continuous flux of information spreading from perisylvian areas toward multiple, distant areas. In turn, it also seems that the limits between linguistic and non-linguistic processes within this system appear blurred. An overall rule seems to be that the closer we move toward the sylvian fissure, the more specifically linguistic the process is. But even in this case (as we have seen), these regions are not exclusively linguistic.

Finally, that the flux of information spreads from perisylvian areas toward extensive regions of the cerebral cortex (actually, nearly all portions of the cortex appear susceptible of being involved) does not necessarily mean that this spreading strictly follows a temporal (sequential) order. Actually, brain networks continuously fire at different frequencies (e.g., Buzsáki, 2006), and it is plausible that information fluxes continuously in a reciprocal way and almost simultaneously between perysilvian and more distant areas. This would be a possible underlying mechanism explaining the large number of mutual influences from one structural layer of language (phonology, syntax, and semantics) to each other, as reported in the literature (e.g., Pulvermüller et al., 2009b). Indeed, considering that there are about 10.000 connections per neuron in the cerebral cortex, firing up to 1.000 times per second and therefore performing a comparable number of calculations (Previc, 2009), a parallel or at least cascade mode of operation of the whole (extended) language network emerges as a very plausible picture. On the other hand, the centrality of auditory/verbal (i.e., phonological/articulatory) information in human language would be consistent with the position of the two main hubs involved in language processing and the direction of the information flux spreading from them as primary receptors of language information to widespread areas, even if the overall processes largely unfold in parallel.
