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WIREs Cogn Sci
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Hemispheric asymmetry: contributions from brain imaging

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A series of studies using functional and structural magnetic resonance imaging, including diffusion tensor imaging measures also, to elucidate the aspects of hemispheric asymmetry are reviewed. It is suggested that laterality evolved as a response to the demands of language and the need for air‐based communication which may have necessitated a division of labor between the hemispheres in order to avoid having duplicate copies in both the hemispheres that would increase processing redundancy. This would have put pressure on brain structures related to the evolution of language and speech, such as the left peri‐Sylvian region. MRI data are provided showing structural and functional asymmetry in this region of the brain and how fibers connecting the right and left peri‐Sylvian regions pass through the corpus callosum. It is further suggested that the so‐called Yakelovian‐torque, i.e., the twisting of the brain along the longitudinal axis, with the right frontal and left occipital poles protruding beyond the corresponding left and right sides, was necessary for the expansion of the left peri‐Sylvian region and the right occipito‐parietal regions subserving the processing of spatial relations. Functional magnetic resonance imaging data related to sex differences for visuo‐spatial processing are presented showing enhanced right‐sided activation in posterior parts of the brain in both sexes, and frontal activation including Broca's area in the female group only, suggesting that males and females use different strategies when solving a cognitive task. The paper ends with a discussion of the role of the corpus callosum in laterality and the role played by structural asymmetry in understanding corresponding functional asymmetry. WIREs Cogni Sci 2011 2 461–478 DOI: 10.1002/wcs.122

Figure 1.

Structural axial MRI image showing a slice through the planum temporale (PT) area about 8–10 mm above the AC‐PC midline, with the PT area marked by manual tracings on the left and right sides. Note the prominent asymmetry with larger area in the left hemisphere. (Reprinted with permission from Ref 22. Copyright 2000 Elsevier Limited.)

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Figure 2.

The asymmetries found in the gross anatomy of the two brain hemispheres are shown. Protrusions of the hemispheres, anteriorly and posteriorly, are observed, as well as differences in the widths of the frontal (F) and occipital (O) lobes. A twisting effect is also observed, known as Yakovlevian torque, in which structures surrounding the right Sylvian fissure are ‘torqued forward’ relative to their counterparts on the left. (Reprinted with permission from Ref 42. Copyright 2003 Nature Publishing Group.)

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Figure 3.

Significantly thicker voxels in the left hemisphere (yellow/red areas) and in the right hemisphere (blue/purple areas) visualized on the lateral surface. Unpublished data from Bradley Peterson, Columbia University, USA, Kerstin von Plessen and Kenneth Hugdahl, University of Bergen, Norway.

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Figure 4.

Diffusion tensor imaging (DTI) of white matter tractography outlining neural pathways in the transverse (red color), longitudinal (green color), and vertical (blue color) axes. (Reprinted with permission from Ref 69. Copyright 2009 Oxford University Press.)

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Figure 5.

Lateralization of language pathways and behavioral correlates. (a) Distribution of the lateralization pattern of the direct long segment (red). (b) Significant correlation between the lateralization index (streamlines) of the direct segment and (c) performances on the CVLT (number of words correctly recalled). (Reprinted with permission from Ref 31. Copyright 2007 PNAS.)

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Figure 6.

Clusters of significant MR signal increases in the 3D mental rotation condition after subtraction of the 2D control stimulus condition. The images were height thresholded at a significance level of p < 0.001. See text and corresponding table for further details. (Reprinted with permission from Ref 12. Copyright 2006 Elsevier Publishing.)

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Figure 7.

Brain imaging data by functional magnetic resonance imaging (fMRI), showing asymmetrical neuronal activation favoring the left side in the peri‐Sylvian region to dichotic presentations of CV syllables. (Reprinted with permission from Ref 97. Copyright 2008 Elsevier Publishing.)

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Figure 8.

fMRI brain imaging data to presentations of isolated phoneme consonant sounds, isolated from a CV syllable. Note the prominent left‐sided asymmetry with significant activation seen only on the left side in the peri‐Sylvian region in the posterior temporal lobe. (Reprinted with permission from Ref 9. Copyright 2005 Elsevier Publishing.)

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