Hack68.Test Your Handedness


Hack 68. Test Your Handedness

We all have a hand preference when undertaking manual tasks. But why is this so? And do you always prefer the same hand, or does it vary with what you are doing? Does the way people vary their hand preference differ between right- and left-handers?

The world is a right-handed one, as will be obvious to left-handers. Most tools are made for right-handed people. Implements such as scissors, knives, coffee pots, and so on are all constructed for the right-handed majority. In consequence, the accident rate for left-handers is higher than for rightand not just in tool use; the rate of traffic fatalities among left-handers is also greater than for right.1

The word "sinister," which now means "ill-omened," originally meant "left-handed." The corresponding word for "right-handed" is "dexter," from which we get the word "dexterous."

T.S.

Nine out of 10 people are right-handed.2 The proportion appears to have been stable over thousands of years and across all cultures in which handedness has been examined. Anthropologists have been able to determine the incidence of handedness in ancient cultures by examining artifacts, such as the shape of flint axes. Based on evidence like this and other evidence such as writing about handedness in antiquity, our species appears always to have been a predominantly right-handed one.

But even right-handers vary in just how right-handed they are, and this variation may have a link to how you use the different sides of your brain [Hack #69] .

6.8.1. In Action

Have a go at the following tests to determine which is your dominant hand and just how dominant it is. Do each test twiceonce with each handand record your score, in seconds, both times. You don't have to do all of them; just see which you can do given the equipment you have on hand.


Darts

Throw three darts at a dartboard. (Be very careful when doing this with your off-hand!) Add up the distances from the bull's-eye.


Handwriting

Measure the time that it takes to write the alphabet as one word, six times. Start with the hand you normally write with and rest for 1 minute before starting with the other hand.


Drawing

Measure the time that it takes to draw a line between two of the lines of some lined paper. Add a penalty of 2 seconds for each time your line touches one of the ruled lines.


Picking up objects with tweezers

Using tweezers, measure the time that it takes to pick up and transfer 12 pieces of wire from one container to another.


Stoppering bottles

Measure the time, in seconds, it takes to put the lids on five jars, the corks back in five wine bottles, or the cap back on five beer bottles.

Here's how to calculate your handedness quotient:

(Left-hand score - Right-hand score) / (Right-hand score + Left-hand score) x 100

You can now see how the score differs for the different tasks and take an average to see your average dominance. Negative numbers mean right-handedness, positive numbers mean left-handedness. Bigger numbers mean greater dominance by one hand.

6.8.2. How It Works

By doing the previous tests, you can see that you can still use your off-hand for some things and that it is easier to use your off-hand for some things than for other things. Most people have some things for which they use their dominant hand, some things they may use both for, and some for which they use their off-hand.

So, in a sense, describing people as left-handed or right-handed is limiting because it puts them into only one category and ignores the extent to which they may be in that categoryor in between the two. This is why, of course, we used behavioral measures to work out the handedness quotient, rather than just asking people.3

Handedness is only weakly genetic. The child of two left-handers has a 45-50% chance of being left-handed, and thus handedness must partly be to do with how the child is brought up as well, so we know that there is a large nongenetic influence on whether you turn out to be a left-hander. Evidence also suggests that left-handedness may be associated with neurological insult in the womb or during delivery.4

If you try the test out on a few people, you will see that left-handed people more easily use their right hand than right-handed people use their left hand. In part, this is probably because our right-handed world forces left-handers to learn to use their right more, and it could also be for deeper reasons to do with brain lateralization as well.

Nine out of 10 people use their right hand predominantly, and at least 9 out of 10 people have their major functions on their left side.5 This includes around two-thirds of left-handers. Everyone else, a significant minority, either uses the right hemisphere for speech or uses both hemispheres.6

One test of which half of the brain is dominant for language is the Wada test. This involves a short-acting anesthetic (e.g., sodium amytal) being injected into the carotid artery. This transiently anesthetizes the left hemisphere, thus testing the functional capabilities of the affected half of the brain. People for whom the left hemisphere is indeed dominant for language (i.e., most of us) will temporarily become aphasic, losing the ability to comprehend or produce language. If counting at the time, you'll stop being able to do so for a few beats when injected with the anesthetic.


The reason most people are still left-brainers for language may be due to how our brain functions became lateralized [Hack #69] before the evolution of language, the brain lateralizing separately from the use of our hands.

It has been suggested that the speech areas of the brain developed near the motor cortex because hand gestures were the principal form of communication before speech.7 Studies show that, when a participant observes hand and mouth gestures, parts of the motor cortex (F5) and Broca's area (found in the left frontal lobe, specifically involved in the production of language) are stimulated. It is argued that before speech our ancestors used gestures to communicate, much as monkeys and apes do now (i.e., lip smacks). And so the human speech circuit is a consequence of the precursor of Broca's area, which was endowed (before speech) with mechanisms to recognize action made by others, from which speech developed.

It is plausible that only the one hand (the right) was used for a more efficient and simple way of communicating. This would explain why language and hand dominance are on the same side (remember, the left side of the brain controls the right side of the body, so left-language dominance and right-hand dominance are both due to the left side of the brain).

If this were the norm during evolution, it may help to explain why most left-handers still have speech areas in the left hemisphere. However, this still doesn't answer the question of why the right hand was dominant in the beginning. At present, this can be only speculation; the important point is that right- and left-handedness are distributed differentlythey are not mirror images of each other, which has implications for the genetics of handedness and the laterality of other functions.

It has been argued that the original hand preferences evolved from a postural position preference of the right hand and consequently a left preference for reaching in arboreal (tree-living) species.8 So, with postural demands becoming less pronounced in ground-dwelling species, the left hand remained the dominant one for highly stereotyped tasks like simple reaching, whereas the right became the preferred one for more manipulative tasks or tasks requiring some skill. In other words, we would hang on with the left hand and pick fruit with the right.

Although this is an interesting theory for why the majority of the population is right-handed, it does not give any indication as to why some people are left-handed. Are left-handed people highly skilled in reaching? Are left-handed people as skilled in manipulative tasks as their right-handed counterparts? Regretfully, these questions have to wait for further research.

6.8.3. End Notes

  1. Salive, M. E., Guralink, J. M., & Glynn, R. J. (1993). Left-handedness and mortality. American Journal of Public Health, 83, 265-267.

  2. Annet, M. (1972). The distribution of manual asymmetry. The British Journal of Psychology, 63, 343-358.

  3. Hartlage, L. C., & Gage, R. (1997). Unimanual performance as a measure of laterality. Neuropsyhological Review, 7(3), 143-156.

  4. Bakan, P. (1971). Handedness and birth order. Nature, 229, 195.

  5. Davidson, R. J., & Hugdahl, K (eds.) (1995). Brain Asymmetry. Cambridge, MA: MIT Press.

  6. Rasmussen, T., & Milner, B. (1977). The role of early left-brain injury in determining lateralization of cerebral speech functions. Annuls of the New York Academy of Sciences, 299, 355-369.

  7. Rizzolatti, G., & Arbib, A. (1998). Language within our grasp. Trends in Neurosciences, 21, 188-194.

  8. MacNeilage, P. E. (1990). The "Postural Origins" theory of primate neurobiological asymmetries. In N. A. Krasneger et al. (eds.), Biological and Behavioural Determinants of Language Development, 165-168, Hillsdale, NJ: Erlbaum.

6.8.4. See Also

  • Laska, M. (1996). Manual laterality in spider monkeys (Ateles geoffroyi) solving visually and tactually guided food-reaching tasks. Cortex, 32(4), 717-726.

Karen Bunday



    Mind Hacks. Tips and Tools for Using Your Brain
    Mind Hacks. Tips and Tools for Using Your Brain
    ISBN: 596007795
    EAN: N/A
    Year: 2004
    Pages: 159

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