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, 414 (6863), 505

Asymmetric Broca's Area in Great Apes

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Asymmetric Broca's Area in Great Apes

C Cantalupo et al. Nature.

Abstract

Brodmann's area 44 delineates part of Broca's area within the inferior frontal gyrus of the human brain and is a critical region for speech production, being larger in the left hemisphere than in the right - an asymmetry that has been correlated with language dominance. Here we show that there is a similar asymmetry in this area, also with left-hemisphere dominance, in three great ape species (Pan troglodytes, Pan paniscus and Gorilla gorilla). Our findings suggest that the neuroanatomical substrates for left-hemisphere dominance in speech production were evident at least five million years ago and are not unique to hominid evolution.

Figures

Figure 1
Figure 1
Asymmetry of Brodmann’s area 44 in great apes. a, Area 44 (shown in red in the two hemispheres of a chimpanzee brain), which is defined as the portion of cortical surface of the inferior frontal gyrus (IFG) bounded by the fronto-orbital (FO) and precentral-inferior (PCI) sulci (CS, central sulcus). Both the FO and PCI sulci can be seen in parasagittal (1 mm thick) slices obtained using magnetic resonance imaging. The dorsal and ventral borders of area 44 are marked in white. We base the definition of the ventral border on cytoarchitectonic studies that show area 44 expanding over the ventral extent of the IFG, even if the PCI sulcus usually terminates at a more dorsal level,. b, Individual asymmetry quotients (AQ; calculated as (RL)/[(R+L)×0.5], where R and L represent areas in the right and left regions, respectively; the sign of the quotient indicates the direction of asymmetry: positive, right-hemisphere asymmetry; negative, left-hemisphere asymmetry) in individual chimpanzees ((blue bars), bonobos (orange) and gorillas (green). On the basis of criteria used for humans, 20 apes showed left-hemisphere asymmetry, six showed right-hemisphere asymmetry and one had no asymmetry (χ2(2) = 21.56, P<0.001). The number of subjects with a left-hemisphere asymmetry was greater than the number of subjects with either right-hemisphere asymmetry (z=2.55, P=0.011) or no asymmetry (z=4.15, P<0.001).

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