Handedness Strength:
Stroop Task and cross-brain communication

Mixed blessings

How do you define a left-hander?  Pretty much everyone would agree that if you write with your left hand, then you’re left-handed. But what if you play racquet sports with your right hand, can use scissors equally well in either hand, or have a dominant right eye?

According to psychologist Stephen Christman at the University of Toledo in Ohio, we have been looking at handedness in the wrong way.  He says it is not being right or left-handed that matters, but the strength of your preference for one hand over the other.  According to his controversial idea, people are not either left-handed or right-handed but “strong-handed” or “mixed-handed”.

Increasingly, researchers are looking at the variations in the functions of the brain to try and solve the puzzle of why 10 – 13 per cent of humans are left-handed, and in doing so, the interest has shifted from the mere “left or right?” question, to “how left or how right biased are you?”.   Christman believes that the population is quite evenly split between strong and mixed-handers, although strong right-handers still predominate.  “We’ve been carving up the handedness pie completely wrong” he says.  According to this “degree not direction” model, people who exclusively use one hand for tasks like writing, eating, brushing teeth and throwing a ball make up one distinct group – the strong-handers.  Mixed-handed people, in contrast, may favour one hand for primary functions like writing and eating, but demonstrate mixed preferences for other tasks.

Much research has been done to explore the theory that left-handed people have a larger corpus callosum – the cable like bundle of nerves that links the two hemispheres of the brain – than right-handers, and it has been suggested that handedness is simply a signal as to the size of the corpus callosum. Christman took this one stage further, by testing musicians to see if they were predisposed to play certain musical instruments because of their handedness.  His initial expectation was that left-handers would be over represented among string players – who need tight communication between their left and right hands – because a larger corpus callosum ought to mean improved communication between the two hemispheres.  Conversely, people playing drums or piano with the left and right hands often following independent lines of music, and should find it easier to separate the actions of the two hemispheres if they had a smaller corpus collosum connecting them – i.e right-handers. 

What Christman actually found from his study was that the differences he expected happened among mixed and strong handers as opposed to left and right handers.  String instruments being favoured by mixed-handers and keyboard and drums for strong-handers.  Christman now believes that handedness can indeed be viewed as a sign of the size of corpus callosum, but only if you assess the mixed versus strong handedness, rather than left versus right.

Christman’s findings suggest that mixed handers do better at tasks that require interplay between the two sides of the brain, whilst strong-handers are good at tasks requiring more shielding between the hemispheres – i.e letting one side of the brain undertake its activity without interference from the other.

But there is a snag in Christman’s theory.  It predicts that strong left-handers’ brains and cognition patterns should be similar to those of strong right-handers.  In practice, the few strong left-handers in the study showed differences not only from mixed-handers but from strong right-handers as well.  Christman feels there may, therefore be a third unique group, and is continuing his studies.  “Deep down”, he says”, I still think direction matters”.

How well do the two sides of your brain communicate?

Below is an example of a widely used “Stroop Task” in which subjects are asked to name the colour of lettering on a card.  The task is complicated by making the letters spell out colour names:  the word “white” written in green, for example.  Reading is generally considered to be a left-hemisphere function, whereas colour recognition is a right-hemisphere task.  If you consider yourself mixed handed, according to Christman’s theory you should score lower on these tests, since you should find it harder to prevent the automatic reading action of the left hemisphere from interfering with the right.hemisphere’s efforts to identify the colour.

Try to work across and down this graphic saying out lod the COLOUR of each word (NOT what the word actually says)

To find out more about the Stroop Task, and take the test live online, visit: