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Orofacial somatomotor responses in the macaque monkey homologue of Broca's area

Abstract

In the ventrolateral frontal lobe of the human brain there is a distinct entity, cytoarchitectonic area 44 (Broca's area), which is crucial in speech production1,2,3,4. There has been controversy5,6 over whether monkeys possess an area comparable to human area 44. We have addressed this question in the macaque monkey by combining quantitative architectonic analysis of the cortical areas within the ventrolateral frontal region with electrophysiological recording of neuron activity and electrical intracortical microstimulation. Here we show that, immediately in front of the ventral part of the agranular premotor cortical area 6, there is a distinct cortical area that is architectonically comparable to human area 44 and that this monkey area 44 is involved with the orofacial musculature. We suggest that area 44 might have evolved originally as an area exercising high-level control over orofacial actions, including those related to communicative acts, and that, in the human brain, area 44 eventually also came to control certain aspects of the speech act.

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Acknowledgements

We thank M. Bouchard for technical assistance. This work was supported by grants from Canadian Institutes of Health Research and the James S. McDonnell Foundation.

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Correspondence to Michael Petrides.

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplementary Methods (DOC 26 kb)

Supplementary Discussion

Clarification of the confusion in the definition of area 45. (DOC 33 kb)

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Further reading

Figure 1: Region of interest and location of experimental results in the monkey.
Figure 2: Photomicrographs of areas 45, 44 and 6V focused on granular layer IV and adjacent layers III and V.
Figure 3: Mean area of the largest cells in layers III and V of areas 45, 44 and 6V.
Figure 4: Quantitative assessment of the relative granularity of layer IV in architectonic areas 45, 44 and 6V.

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