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catallaxy in technical exile

Strong vs weak handedness

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As a (weak) left-hander myself, I’m always fascinated by handedness research. Summarising the latest research reported in this New Scientist article (subscription required)

1) Researchers now think that the more important distinction is between strong vs mixed handedness not left vs right handedness as research on left handedness has produced only a contradictory set of results regarding the various cognitive abilities and neurological differences associated with handedness. They estimate that strong left handers may comprise only 2-3% of the population though left handed people generally comprise about 10% of the population. Incidentally if strong left handerd are only 2-3% of the population this would mean that mixed handeds make up 70-80% of the left handed population, in which case there would be a strong correlation between the characteristics previously attributable to left handedness but now really found to be associated with mixed handedness, assuming there isn’t as high a percentage of mixed handeds among the right handed population (which would be a plausible assumption if these people are likely to be picked up in the research as ambidextrous rather than right handed).

2) The new distinction makes sense on the grounds that mixed handed people would tend to have a larger corpus calloseum, the bundle of nerve fibres that links the two hemispheres of the brain ( in the past this has been assumed, wrongly, to be associated with left handedness though there is a correlation – it won’t be true of strong left handers). This will manifest itself in performance on various tasks.

3) Reevaluating a study of musicians made researchers arrive at this new distinction. There was no strong correlation between left and right handedness on the one hand and preference for musical instruments in particular. But mixed handers tend to prefer string instruments and strong handers tend to prefer keyboards and drums – because ‘people playing drums or piano, with the left and right hands often following independent lines of music, ought to find it beneficial to have a smaller corpus callosum, as this would minimise interference between the two hemispheres’. Conversely string players needed tighter communication between the two hemispheres.

4) The new distinction predicts other cognitive differences (see extract below), though it’s still unclear whether strong left handers might still be neurologically different from strong right handers in other ways:

Christman’s findings suggest that strong and mixed-handers should show differences in areas such as belief in improbable events, memory and accident-proneness. “The common thread is that for cognitive tasks requiring activity in both hemispheres, mixed-handers perform better,” Christman says. The same should be
true when strong-handers perform tasks requiring more shielding between the hemispheres.

Take, for example, the well-known “Stroop task”, in which subjects are asked to name the colour of lettering on a card. The task is made less than straightforward by making the letters spell out colour names: the word “yellow” written in green, for example …mixed-handed subjects scored lower on these tests, presumably because they found it harder to prevent the automatic reading action of the left hemisphere from interfering with the right hemisphere’s effort to identify the colour.

Similar differences between strong and mixed-handers should be found in memory, because memory also works in a cross-hemispheric manner. Encoding of memories takes place in the left hemisphere, while recall happens in the right. Christman had subjects keep a daily journal for two weeks. When asked afterwards to recollect -without looking at their diaries – what they had done on each day, mixed-handers performed better. Likewise, he found that mixed-handers’ childhood memories date back further than those of strong-handers.

Walter Daniel and Ronald Yeo at the University of New Mexico in Albuquerque went back to a study that originally suggested that left-handers were more prone to have car accidents. After reassessing the numbers to reflect handedness degree instead of direction, they found that is is mixed-handers who are statistically more at risk …

… one of Christman’s protégés, Chris Niebauer, who now teaches at Slippery Rock University of Pennsylvania, claims that handedness is correlated in a more general way to people’s attitudes. Niebauer has collected results that he says suggest that mixed-handers are less likely to believe in creationism or be homophobic and more likely to be hypochondriac. He explains this by drawing on renowned neurologist V.S. Ramachandran’s view that the left hemisphere controls rational thought, while the right is the world-view challenger, updating beliefs when contradictory information becomes overwhelming. Niebauer argues that in
mixed-handers, a larger corpus callosum helps the right hemisphere revise its beliefs more frequently, updating the person’s overall belief system and washing away entrenched ideas: in the case of hypochondriacs, for instance, reacting to every little bump or scrape as if it were a first sign of catastrophic injury.


Written by Admin

April 12, 2006 at 9:08 am

Posted in Uncategorized

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