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In this issue of EJN, Kelly et al. succeeded in demonstrating that autoradiographic anatomical tracing studies in the macaque monkey predicted patterns of connectivity into and out of Broca’s ‘language’ region in man. Resting state connectivity during functional magnetic resonance imaging was measured in humans, as anatomical tracing cannot be done and diffusion tensor imaging cannot delineate the exact cortical origins and terminations of pathways.

The Kelly et al. (2010) study was executed with great rigor. The human regions of interest were mapped on the basis of individuals’ unique sulcal and gyral morphology, which were precisely defined and translated from the monkey into human cytoarchitectural nomenclature. This individualized method was then followed by a seed-based analysis and a data-driven spectral clustering analysis. Great methodological transparency was provided (e.g. by illustrating how the number of clusters designated to be found (e.g. 2 vs. 4) affected the structure of the findings).

The monkey target regions were BA 44 and 45 (traditionally considered Broca’s Area) and BA 6 (which subserves orofacial movements). However, recent research suggest that BA 47 (pars orbitalis) should now be included in ‘Broca’s Complex’ (Hagoort et al., 2004; Hagoort, 2005) as it is activated during both semantic processing (Bookheimer, 2002) and in the extraction of structural order or ‘syntax’ from oral (Friederici et al., 2006) and signed (Petitto et al., 2000) speech.

Many of the projections from Broca’s area (Kelly et al., 2010) and from Broca’s complex (Xiang et al., 2010) are not with regions traditionally considered linguistic. Indeed, BA 47 has been implicated in the extraction of temporal coherence from music (Levitin & Menon, 2003). Taken together these findings may help to liberate Broca’s complex from being primarily a language-specific node. Instead, consistent with the cognitive neuroscience perspective, regions inside of and outside of Broca’s complex participate in variety of functional networks beyond just ‘language’.

In terms of future research, it would be good to validate the relationship between anatomical tracing and resting state functional connectivity of Broca’s Complex within at least one species of monkey. To my knowledge this has not been done even though resting state activity in man and monkey have been compared (Rilling et al., 2007).

After that, it would be interesting to compare the hypothesis–driven findings guided by the new anatomical tracings of ‘Broca’s complex’ in the monkey to a recent resting state functional connectivity analysis of ‘Broca’s complex’ in man. This would help to assess the extent to which data-driven predictions from the monkey affect data analysis in the human.

Another suggestion may be to move away from just measuring connectivity of the default brain at rest and, instead, examine resting state connectivity during the immediate period before a trial has begun but while the individual is preparing for a specific cognitive task (such as word rhyming). Frye et al. (2008, 2010) have performed such a connectivity analysis with magnetoencephalographic data analyzed by means of Granger Causality. This method computes not only the strength of connectivity between regions but also the strength of the direction of activity in or out of a specific cortical area.

References

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  2. References
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  • Friederici, A.D., Fiebach, C.J., Schlesewsky, M., Bornkessel, I.D. & Yves von Cramon, D. (2006) Processing linguistic complexity and grammaticality in the left frontal cortex. Cereb. Cortex, 16, 17091717.
  • Frye, R., Wu, M.H., Zouridakis, G. & Liederman, J.. (2008) Causal prefrontal-temporoparietal interactions are correlated with word-rhyming performance in reading disabilities. Biomagnetism: Interdisciplinary Research and Exploration. Proceedings of Biomag , Sapporo, Japan.
  • Frye, R.E., Wu, M.H., Liederman, J., McGraw Fisher, J. & Zouridakis, S. (2010) The directional connectivity of the preparatory phonological decoding networkIn typical and dyslexic readers as revealed by Granger causality. Front. Syst. Neurosci., (under review).
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  • Levitin, D.J. & Menon, V. (2003) Musical structure is processed in “language” areas of the brain: a possible role for Brodmann Area 47 in temporal coherence”. NeuroImage, 20, 242252.
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