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Learning anatomy is similar to learning a language. Pity the poor student who attempts to commit to memory the names and locations of different anatomical structures over the course of a day, a week, a year. Just as language fluency requires knowledge of how to connect words together in a meaningful way, fluency in neuroanatomy requires a comprehensive understanding of the connections between different regions and an interest in connecting structure to function. Nearly 150 years ago, Paul Broca linked brain structure to function in his description of what is commonly known as Broca’s aphasia (Broca, 1861). His description of functional deficits that follow damage to the left ventrolateral prefrontal cortex were soon followed by Carl Wernicke’s description of language deficits following damage to left posterior temporal cortex (Wernicke, 1874), and the development by Brodmann of the most famous and most commonly known cytoarchitectonic map (Brodmann, 1909). Brodmann’s map attempted to provide a link between our understanding of the brain at the microscopic and macroscopic scales, and today we associate Brodmann’s areas 44 and 45 with the term ‘Broca’s area’. But what about the anatomical connections? In the human, it is fairly straightforward for neuroanatomists to study cytoarchitectonic divisions in post-mortem brain; however, attempts to use anterograde and retrograde tracers in post-mortem human tissue have not been very successful. For this reason, the gold standard for studying anatomical connections remains experimental tracing studies in animals, especially in the nonhuman primate. Advances in neuroimaging have provided us with new methods for studying neuroanatomical structure and function, including diffusion tensor imaging methods for examining white matter fiber tracts and functional magnetic resonance imaging methods for examining function. The development of methods for examining functional connectivity using resting state functional connectivity (RSFC) and task-dependent functional connectivity methods extend our ability to map anatomical and functional connectivity in the human.

In an article published in this issue of EJN, Clare Kelly and collaborators (Kelly et al., 2010) describe studies linking human brain RSFC data with anatomical tracing studies in nonhuman primates. More specifically, the authors examined the correspondence of patterns of connectivity between areas 6, 44 and 45 and posterior parietal and temporal regions in the human, measured with RSFC methods, to anatomical connectivity studies between the homologues of these areas in the macaque monkey, measured in a separate nonhuman primate autoradiographic tracing study by Petrides & Pandya (2009). The studies demonstrate strong correspondence between the connectivity in the human and nonhuman primate. The clustering of anatomical data supports the existence of different functional connectivity patterns for ventral area 6 that are distinct from areas 44 and 45. Ventral area 6 shows strong connections with the rostral supramarginal gyrus of the parietal lobe. In contrast to area 6, regions 44 and 45 show connections with the caudal supramarginal and angular gyri, and also with the middle temporal gyrus. These anatomical results are exciting because they support functional hypotheses such as the dual stream model, proposing that one circuit (area 6) allows mapping of acoustic speech sounds to articulatory acts, whereas a more ventral circuit links lateral temporal areas for speech comprehension with Broca’s area (Hickok & Poeppel, 2004). The mapping of speech sounds to articulatory acts in area 6 may be a human homologue to the mirror neuron network, as mirror neurons responding to both the perception and generation of actions are found in monkey homologues of area 6 (Rizzolatti et al., 1996) and the human anterior supramarginal gyrus (Fogassi et al., 2005). These data linking human and primate anatomy have an important impact on our understanding of the circuits for language processing.

References

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  2. References
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  • Kelly, C. & Uddin, L.Q., Shehzad, Z., Margulies, D.S., Xavier Castellanos, F., Milham, M.P. & Petrides, M. (2010) Broca's region: linking human brain functional connectivity data and non-human primate tracing anatomy studies. Eur. J. Neurosci., 32, 383398.
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