The Neuroanatomical Pathways of Language.

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Brain-related models of language have existed for more than 150 years, since Wernicke (1874) and Lichtheim (1884) published their models based on different aphasia syndromes. These models assumed separate centers for acoustic-sensory aspects supporting language comprehension and motor aspects supporting language production, as well as connections between them.

Today, the existing brain imaging methodology allows us to localize different aspects of language in circumscribed brain regions and to specify their functional and structural connectivities in vivo. For the first time, this enables us to describe neural networks supporting semantic and syntactic processes in more detail and to formulate a functional neuroanatomicalmodel of language.

The relevance of white matter fiber bundles for different language perception and production was recognized for the first time  in the late of  19th century when they were proposed to form possible connections between the different language centers (Wernicke, 1874).

In his model, Wernicke proposed a speech production center (based on the work of Broca, 1861), a sensory language center, and a connection between the centers supporting their interaction. Broca’s area in the inferior frontal cortex and Wernicke’s area in the superior temporal cortex as the classical language areas are subparts of the network that supports language functions at different levels (Vigneau et al., 2006; Hickok and Poeppel, 2007; Friederici, 2011).

The connections between the classic language regions that have been identified over the past decades are multifold. Initially, two broad functional processing streams connecting temporal and frontal areas were proposed (Hickok and Poeppel, 2000). Without basing their model on structural magnetic resonance imaging evidence of the fiber tracts connecting the language-related brain regions, Hickok and Poeppel (2004) discussed a functional “dorsal stream” as being responsible for sensory-to-motor mapping in speech processing and a functional “ventral stream” as supporting sound-to-meaning mapping.

Based on neuroanatomical studies in non-human primates and functional studies in human and non-human primates, Rauschecker and Scott (2009) proposed a model that assumes a dorsal stream going forward from the temporal cortex to the premotor cortex, a ventral stream going forward from the temporal cortex to Broca’s area and from there going backward via a dorsal stream to the parietal and temporal cortex. The authors take these processing streams to underlie human speech processing during perception and production.

The arcuate fasciculus, which connects the prefrontal cortex to the posterior superior temporal gyrus dorsally, partly runs closely in parallel with the superior longitudinal fasciculus from prefrontal to parietal regions, but not in its posterior portion, curving into the posterior temporal cortex. But since the arcuate fasciculus and the superior longitudinal fasciculus do run partly in parallel between the prefrontal and parietal cortex, some researchers refer to this fiber tract as the acute fasciculus/superior longitudinal fasciculus (AF/SLF) .

There are two major dorsally located fiber bundles that are relevant for speech and language, respectively. These two fiber bundles can be differentiated by their termination regions in the frontal cortex and by the particular language subfunctions of these termination regions.

The first dorsal fiber bundle connects the temporal cortex with the premotor cortex through the mediation of the inferior parietal lobe, whereas the second bundle connects the temporal cortex to Broca’s area, in particular BA 44 (Perani et al., 2011).

The fiber bundle that terminates in the premotor cortex appears to be functionally relevant for acoustic-motor mapping, whereas the one that terminates in BA 44 appears to support the processing of syntactically complex sentences . The language regions in the frontal cortex and in the temporal cortex are connected not only by dorsally located pathways but also by at least two ventrally located pathways.

            A fiber system usually referred to as the extreme capsule fiber system (ECFS), but also named the inferior-fronto-occipital fasciculus (IFOF), as it connects inferior frontal regions with the temporal and occipital cortex. Second, there is the uncinate fasciculus, which connects the frontal operculum to the anterior temporal cortex.

As these two fasciculi run closely parallel their respective function is hard to disentangle, except with respect to their termination points. Thus, neuroanatomically there are at least two dorsal and two ventral fiber tracts connecting the temporal to the inferior frontal cortex. The fiber tracts can be separated with respect to their target regions in the frontal cortex. These different fiber tracts appear to support different language functions.