Brain asymmetry

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In human neuroanatomy, brain asymmetry can refer to at least two quite distinct findings:

Neuroanatomical differences themselves exist on different scales, from neuronal densities, to the size of regions such as the planum temporale, to—at the largest scale—the torsion or "wind" in the human brain, reflected shape of the skull, which reflects a backward (posterior) protrusion of the left occipital bone and a forward (anterior) protrusion of the right frontal bone.[1] In addition to gross size differences, both neurochemical and structural differences have been found between the hemispheres. Asymmetries appear in the spacing of cortical columns, as well as dendritic structure and complexity. Larger cell sizes are also found in layer III of Broca's area.

The human brain has an overall leftward posterior and rightward anterior asymmetry (or brain torque). There are particularly large asymmetries in the frontal, temporal and occipital lobes, which increase in asymmetry in the antero-posterior direction beginning at the central region. Leftward asymmetry can be seen in the Heschl gyrus, parietal operculum, Silvian fissure, left cingulate gyrus, temporo-parietal region and planum temporale. Rightward asymmetry can be seen in the right central sulcus (potentially suggesting increased connectivity between motor and somatosensory cortices in the left side of the brain), lateral ventricle, entorhinal cortex, amygdala and temporo-parieto-occipital area. Sex-dependent brain asymmetries are also common. For example, human male brains are more asymmetrically lateralized than that of females. However, gene expression studies done by Hawrylycz and colleagues and Pletikos and colleagues, were not able to detect asymmetry between the hemispheres on the population level.[2][3]

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  1. ^ Marjorie LeMay (June 1977). "Asymmetries of the skull and handedness. Phrenology revisited". Journal of the Neurological Sciences. 32 (2): 243–253. PMID 874523. doi:10.1016/0022-510X(77)90239-8. 
  2. ^ Hawrylycz, Michael J. (20 September 2012). "An anatomically comprehensive atlas of the adult human brain transcriptome". Nature. 489: 391–399. PMC 4243026Freely accessible. PMID 22996553. doi:10.1038/nature11405. 
  3. ^ Pletikos, Mihovil; Sousa, Andre MM; et al. (22 January 2014). "Temporal Specification and Bilaterality of Human Neocortical Topographic Gene Expression". Neuron. 81 (2): 321–332. PMC 3931000Freely accessible. PMID 24373884. doi:10.1016/j.neuron.2013.11.018.