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WIREs Cogn Sci
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Right hand, left brain: genetic and evolutionary bases of cerebral asymmetries for language and manual action

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Abstract Most people are right‐handed and left‐cerebrally dominant for language. This pattern of asymmetry, as well as departures from it, have been reasonably accommodated in terms of a postulated gene with two alleles, one disposing to this common pattern and the other leaving the direction of handedness and language asymmetry to chance. There are some leads as to the location of the gene or genes concerned, but no clear resolution; one possibility is that the chance factor is achieved by epigenetic cancelling of the lateralizing gene rather than through a chance allele. Neurological evidence suggests that the neural basis of manual praxis, including pantomime and tool use, is more closely associated with cerebral asymmetry for language than with handedness, and is homologous with the so‐called “mirror system” in the primate brain, which is specialized for manual grasping. The evidence reviewed supports the theory that language itself evolved within the praxic system, and became lateralized in humans, and perhaps to a lesser extent in our common ancestry with the great apes. WIREs Cogn Sci 2012, 3:1–17. doi: 10.1002/wcs.158 This article is categorized under: Psychology > Brain Function and Dysfunction

Hypothesized distributions of hemispheric asymmetry for language (top panel) and handedness (bottom panel) as a function of genotype. (After Annett, Ref 5)

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Hypothetical distribution of hemispheric asymmetry for language (top panel) and handedness (bottom panel) according to a two‐gene theory in which one gene, L1, influences hemispheric asymmetry and handedness, and a second gene, L2, combines additively with L1 to influences both hemispheric asymmetry and handedness.

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Common areas of activation in processing symbolic gestures and spoken speech, showing marked left hemispheric bias. (Reprinted with permission from Ref 70. Copyright 2009 National Academy of Sciences)

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Activation in left and right hemispheres induced by covert word generation in affected and unaffected members of the KE family. Broca's area is activated only in the unaffected members. (Reprinted with permission from Ref 106. Copyright 2003 Nature Publishing Group)

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Scatterplot of laterality indices (LIs) and fractional anisotropy (FA) scores. Line A is the regression line. Line B shows how the plot may be divided (left) into those with LIs > 0 (with one exception) and (right) those with LIs scattered above and below zero. The former might be regarded as made up predominantly of individuals inheriting at least one copy of the RS+ allele, and the latter of individuals with the RS—genotype. (Reprinted with permission from Ref 126 Copyright 2011 Elsevier Ltd)

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