Left Brain, Right Brain: Facts and Fantasies

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Abstract

Michael Corballis discusses in this essay how the asymmetry of the brain raises questions about genetics, evolution, language, and educational and psychological disabilities; but beware of exaggerated claims of left brain/right brain polarities.

Summary

Handedness and brain asymmetry are widely regarded as unique to humans, and associated with complementary functions such as a left-brain specialization for language and logic and a right-brain specialization for creativity and intuition. In fact, asymmetries are widespread among animals, and support the gradual evolution of asymmetrical functions such as language and tool use. Handedness and brain asymmetry are inborn and under partial genetic control, although the gene or genes responsible are not well established. Cognitive and emotional difficulties are sometimes associated with departures from the “norm” of right-handedness and left-brain language dominance, more often with the absence of these asymmetries than their reversal.

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Most cited references 62

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The functional role of the parieto-frontal mirror circuit: interpretations and misinterpretations.

Giacomo Rizzolatti, Corrado Sinigaglia (2010)

The parieto-frontal cortical circuit that is active during action observation is the circuit with mirror properties that has been most extensively studied. Yet, there remains controversy on its role in social cognition and its contribution to understanding the actions and intentions of other individuals. Recent studies in monkeys and humans have shed light on what the parieto-frontal cortical circuit encodes and its possible functional relevance for cognition. We conclude that, although there are several mechanisms through which one can understand the behaviour of other individuals, the parieto-frontal mechanism is the only one that allows an individual to understand the action of others 'from the inside' and gives the observer a first-person grasp of the motor goals and intentions of other individuals.

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A forkhead-domain gene is mutated in a severe speech and language disorder.

Cecilia S L Lai, Simon E. Fisher, Jane Hurst … (2001)

Individuals affected with developmental disorders of speech and language have substantial difficulty acquiring expressive and/or receptive language in the absence of any profound sensory or neurological impairment and despite adequate intelligence and opportunity. Although studies of twins consistently indicate that a significant genetic component is involved, most families segregating speech and language deficits show complex patterns of inheritance, and a gene that predisposes individuals to such disorders has not been identified. We have studied a unique three-generation pedigree, KE, in which a severe speech and language disorder is transmitted as an autosomal-dominant monogenic trait. Our previous work mapped the locus responsible, SPCH1, to a 5.6-cM interval of region 7q31 on chromosome 7 (ref. 5). We also identified an unrelated individual, CS, in whom speech and language impairment is associated with a chromosomal translocation involving the SPCH1 interval. Here we show that the gene FOXP2, which encodes a putative transcription factor containing a polyglutamine tract and a forkhead DNA-binding domain, is directly disrupted by the translocation breakpoint in CS. In addition, we identify a point mutation in affected members of the KE family that alters an invariant amino-acid residue in the forkhead domain. Our findings suggest that FOXP2 is involved in the developmental process that culminates in speech and language.

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Molecular evolution of FOXP2, a gene involved in speech and language.

Wolfgang Enard, Molly Przeworski, Simon E. Fisher … (2002)

Language is a uniquely human trait likely to have been a prerequisite for the development of human culture. The ability to develop articulate speech relies on capabilities, such as fine control of the larynx and mouth, that are absent in chimpanzees and other great apes. FOXP2 is the first gene relevant to the human ability to develop language. A point mutation in FOXP2 co-segregates with a disorder in a family in which half of the members have severe articulation difficulties accompanied by linguistic and grammatical impairment. This gene is disrupted by translocation in an unrelated individual who has a similar disorder. Thus, two functional copies of FOXP2 seem to be required for acquisition of normal spoken language. We sequenced the complementary DNAs that encode the FOXP2 protein in the chimpanzee, gorilla, orang-utan, rhesus macaque and mouse, and compared them with the human cDNA. We also investigated intraspecific variation of the human FOXP2 gene. Here we show that human FOXP2 contains changes in amino-acid coding and a pattern of nucleotide polymorphism, which strongly suggest that this gene has been the target of selection during recent human evolution.

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Author and article information

Journal

Journal ID (nlm-ta): PLoS Biol

Journal ID (iso-abbrev): PLoS Biol

Journal ID (publisher-id): plos

Journal ID (pmc): plosbiol

Title: PLoS Biology

Publisher: Public Library of Science (San Francisco, USA )

ISSN (Print): 1544-9173

ISSN (Electronic): 1545-7885

Publication date Collection: January 2014

Publication date (Print): January 2014

Publication date (Electronic): 21 January 2014

Volume: 12

Issue: 1

Electronic Location Identifier: e1001767

Affiliations

[1]School of Psychology, University of Auckland, Auckland, New Zealand

Author notes

* E-mail: m.corballis@ 123456auckland.ac.nz

The author has declared that no competing interests exist.

The author(s) have made the following declarations about their contributions: Conceived and written by: MC.

Article

Publisher ID: PBIOLOGY-D-13-03341

DOI: 10.1371/journal.pbio.1001767

PMC ID: 3897366

PubMed ID: 24465175

SO-VID: ad22f23b-1462-4d15-8336-b4016d237336

License:

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Pages: 6

Funding

Some of the research summarised in this article was funded by Contract UOA from the Marsden Fund of the Royal Society of New Zealand. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Left Brain, Right Brain: Facts and Fantasies

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Neuronal Signaling

Most cited references 62

The functional role of the parieto-frontal mirror circuit: interpretations and misinterpretations.

A forkhead-domain gene is mutated in a severe speech and language disorder.

Molecular evolution of FOXP2, a gene involved in speech and language.

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