The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Structural Asymmetries in the Human Brain Assessed via MRI


Anthropology, Indiana University

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The human brain is structurally and functionally different between hemispheres. Developmental, evolutionary, and genetic factors are thought to influence these asymmetries. Behavioral traits such as manual dexterity, motor control, and aspects of language are usually lateralized in the brain, but the extent to which these can be directly linked to specific anatomical asymmetries has been the subject of debate. Analyses of fossil hominin endocasts have revealed anatomical asymmetries that are assumed to reflect asymmetries in underlying brain regions. Clarifying where - and by how much - extant human brains are asymmetrical will allow better interpretations of these fossil asymmetries, both with respect to suspected brain asymmetries as well as possible functional/behavioral implications. Two areas of particular interest are Broca’s and Wernicke’s areas, because they play key roles in language production in modern humans. Previous research has suggested that these areas are asymmetric, but studies to date have had small sample sizes and often use brain scans of unhealthy patients. To this end, we investigated the various left-right differences of the human brain through a voxel-based morphometric analysis of MRI scans of 72 healthy, female subjects. Left-right reversed versions of individual brains were mapped into their corresponding original versions, using non-rigid deformation methods. These mappings were then registered to a common atlas, and average degrees of left-right asymmetry were calculated for each voxel. Our results showed both Broca’s and Wernicke’s areas to have significant leftward asymmetry at the p-value of .001. Implications of this work for hominin evolution will be discussed.

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