Chapter

Perisylvian Pathways

Marco Catani and Michel Thiebaut de Schotten

in Atlas of Human Brain Connections

Published on behalf of Oxford University Press

Published in print March 2012 | ISBN: 9780199541164
Published online November 2012 | e-ISBN: 9780191753268 | DOI: http://dx.doi.org/10.1093/med/9780199541164.003.0074
Perisylvian Pathways

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Johann Christian Reil was the first to identify a group of fibres running deeply in the white matter of the temporal, parietal, and frontal regions around the Sylvian fissure, which he described as the Ungenannte Marksubstanz (unnamed white matter) (Figure 7.1A) (Reil, 1812). In 1822 Karl Burdach confirmed Reil's finding and designated the perisylvian tracts collectively as the Fasciculus Arcuatus, because of the arching shape of its longest fibres (Burdach, 1822). The work of the German neuroanatomists was subsequently revisited by Dejerine (1895) who believed that the arcuate fasciculus was mainly composed of short U-shaped associative fibres connecting neighbouring perisylvian regions (Figure 7.1B). Dejerine was also the first to use the term superior longitudinal fasciculus and arcuate fasciculus interchangeably. In the meantime, post-mortem studies of patients with aphasia led Paul Broca (1861) and Carl Wernicke (1874) to discover the cortical ‘centres’ dedicated to speech production (Broca's area in the frontal lobe) and auditory comprehension (Wernicke's area in the temporal lobe). Wernicke was also the first to postulate that language relies on the integrity of a ‘psychic reflex arc’ between temporal and frontal regions. However, the arcuate fasciculus was not part of Wernicke's original anatomical model, as he thought that the temporal and frontal language areas were indirectly connected by fibres passing through the external capsule and relaying in the cortex of the insula (Wernicke, 1874). It was Monakow (1885), and later Dejerine (1895), to propose the arcuate fasciculus as the tract directly connecting Broca's and Wernicke's area. This anatomical model was subsequently revitalized by Norman Geschwind—with an additional emphasis on the importance of the connections to the angular gyrus—and became known as the Wernicke–Geschwind model (Geschwind, 1965, 1970; Catani and ffytche, 2005). Although the existence of the arcuate fasciculus has been confirmed in humans by many studies using blunt dissections and axonal staining of degenerating axons, these methods have not shed much light on its detailed anatomy. For this reason more powerful methods for tracing axonal pathways have been used in the monkey to identify a homologue of the arcuate fasciculus (Petrides and Pandya, 1988; Schmahmann and Pandya, 2006). But the absence of language in non-human primates raises doubts on the possibility of translating tout court these findings to man. The advent of diffusion tensor imaging tractography has therefore opened a new era in the field. The first tractography studies applied to the perisylvian pathways showed that the anatomy of the arcuate fasciculus is more complex than previously thought (Figures 7.1C,D,E and 7.2) (Catani et al., 2002, 2005a; Parker et al., 2005). In addition to the long segment directly connecting Wernicke's and Broca's territories (i.e. the arcuate fasciculus sensu strictu), there is an indirect pathway consisting of an anterior and a posterior segment linking the inferior parietal lobule (Geschwind's territory) to Broca's territory and Wernicke's territory, respectively. Additional support for the existence of the three perisylvian segments comes from human intraoperative electrocorticography (Matsumoto et al., 2004), functional connectivity (Schmithorst and Holland, 2007), and recent post-mortem dissections (Lawes et al., 2008).

Chapter.  12201 words.  Illustrated.

Subjects: Neuroscience

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