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Neuroimaging’s Bias Against Left-handers

Neuroskeptic iconNeuroskepticBy NeuroskepticAugust 17, 2019 3:03 PM
left right brain


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Left-handed people are under-represented as volunteers in human neuroimaging studies, according to a new paper from Lyam M. Bailey, Laura E. McMillan, and Aaron J. Newman of Dalhousie University.

Bailey et al. analyzed a sample of 1,031 papers published in 2017, finding that just 3.2% of participants were non-right-handed, even though this group makes up about 10-13% of the general population.

These findings are hardly unexpected. The exclusion of non-right-handed people from neuroimaging (especially fMRI) studies is standard practice in the field. If anything, I was surprised by how high the 3.2% figure was.

The reason usually given for the right-handers-only policy is that non-right-handed people are more likely to have atypical brain lateralization of language.

In most people, language functions are found in the left hemisphere of the brain. About 4% of right-handers show right hemisphere or mixed-hemisphere lateralization of language, while in non-right-handers, the rate is about 30%.

Atypical language lateralization isn’t harmful, but it could create difficulties for neuroscientists studying language, by adding variability to the results. So the focus on right-handers makes sense in this context.

But as Bailey et al. point out, the taboo against adextral (non-right-handed) participants has spread far beyond the neuroscience of language:

Researchers studying language-related processes arguably have a principled justification to exclude adextral subjects from their investigations…[but] in recent decades this cautionary practice has apparently spread to most other domains of cognitive neuroscience.

In fact, Bailey et al. found that non-language studies are just as exclusionary as language-related ones. The only difference was that language studies were more likely to report the handedness of their participants.

It is far from clear that a blanket exclusion of non-right-handers is needed for studies of, say, memory, emotion, or decision-making, in which there is no language component.

Bailey et al. conclude with 4 reasons why neuroscientists should end the ban on lefties, at least for non-language studies. The authors argue that including more representative samples would be better for science, but they also raise an ethical case for inclusion.

Speaking as someone whose experience as a student research volunteer was important in shaping my early career, I found the following point especially compelling:

Participation in neuroimaging research may be considered as a rich learning experience in and of itself – particularly for students who wish to pursue a research career in a related field.

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