Having a close left-handed relative makes right-handers better at remembering events than those from exclusively right-handed families, new research suggests. There is a downside, however, as members of these ambidextrous families may be relatively impaired in their ability to recall facts.
According to the study, having a left-handed sibling or parent means the organisation of your brain is intermediate between a pure 鈥榣efty鈥� and a pure 鈥榬ighty鈥�.
Specifically, Stephen Christman and Ruth Propper at the University of Toledo, Ohio claim that people with 鈥榣efties鈥� in the family have a larger corpus callosum 鈥� the connection between the brain hemispheres. This makes you better at certain memory tasks, but worse at others, they believe.
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Two types of memories are involved. Episodic memories are those with a context that is separate from the information itself 鈥� for example, where you parked your car or where you left your keys. Semantic memories on the other hand are things 鈥榶ou just know鈥�, such as the dates of the First World War or the recipe for apple pie.
Filling the gaps
The researchers showed 180 right-handed subjects lists of words. Some of this group was asked to recall as many of the words as possible once the list had been taken away. This tests episodic memory because the subjects have to remember the words they were taught.
Others from the group were given fragments of words with a letter missing and asked to fill in the gaps. This semantic test simply relies on knowing how the correct word should be spelt. Subjects with close left-handed relatives did better at the first 鈥榬emember鈥� task, but worse at the second 鈥榢now鈥� task.
鈥淭he key difference is not whether you are right handed, but whether you are strongly or weakly handed,鈥� explains Christman.
Making the connection
A definitive explanation for the results is still some way off, says Christman. But he suspects that it might involve the roles that different brain hemispheres play in memory. He believes the information itself tends to be stored in the left hemisphere, while the place and time context resides in the right.
Both these components will be useful in episodic memories, so he suspects that people with a large corpus callosum linking their hemispheres 鈥� such as those from more ambidextrous families 鈥� will do better at these tasks. Semantic memory requires only one hemisphere, so it may be that those with fewer connections between the hemispheres have less interference and perform better.
Chris McManus, an expert in handedness at University College London, agrees that people with left-handed relatives have a brain that is 鈥渟lightly more like that of a left-hander鈥�.
But he is sceptical about Christman鈥檚 explanation. The link between a weak-handedness and a large corpus callosum is 鈥渄istinctly controversial鈥�, he says.
Journal reference: Neuropsychology (vol 15 (4))