Ask a Neuroscientist: Motor Skills and Handedness

Eric (age 18) asks: How different are the fine motor skills in your dominant hand rather than in your non-dominant hand? Say, if I have used a computer mouse for my entire life with my right hand, but am left-handed, would my computer mouse accuracy improve if I now switched to using the mouse with my left hand? How long would it take to catch up to my right-handed computer mouse skills?

My response:

Hi Eric,

As a neuroscientist interested in motor skill acquisition, I think this is a fascinating question. Let me try to break your question down a little bit into some sub-questions:

1. Are you genetically programmed to have a dominant hand? 2. Can training change or compensate for your natural tendencies? 3. Is the rate of motor skill learning in your dominant hand faster than in your non-dominant hand?

Ok, now let’s work through these. First up: Are we genetically programmed to have a dominant hand?

As I’m sure you know, people vary in which hand they consider dominant (right-handed vs. left-handed) as well as in how ambidextrous they are (how able they are to use both their right and their left hands). If you’re wondering how left-handed you are, check out this questionnaire. Ok, so some people are right-handed and some people are left-handed, but is this trait due to nature or nurture? Most likely, it’s a combination. One the one hand (ha!), handedness runs in families. On the other hand, identical twins can often have different dominant hands.

Natural tendencies towards left- or right-handedness have their roots in brain lateralization, which is just the idea there is asymmetry in our brain structure. The classic example of brain lateralization is language processing. Almost everyone uses only their left hemisphere for language function. Interestingly though, some lefties but very few righties use their right hemisphere instead.

The question of how and why handedness originates in the brain is far from solved, but here are a few more interesting facts for you before I move on:

1.  There are higher incidences of language-related disorders like dyslexia and stuttering among lefties, perhaps because of underlying differences in brain lateralization during development 2. Around 10% of humans are left-handed, an incidence that seems to be mostly independent of culture and has remained relatively stable in recent history. 3. Humans are not the only species that display handedness. For example, many monkeys will also display handedness in laboratory settings though it is unclear and controversial whether non-human handedness is homologous to human handedness.

All right, then, given that you may have some inborn tendency to use one hand over the other, how can training change or compensate for that tendency?

The answer to this question gets back again to the nature vs. nurture debate but focuses specifically on whether directed training is effective. From personal experience, I’d have to say that training can go a long way. Like you, I’m a lefty living in a righty’s world and have been trained to use a right-handed mouse. Now, even when I’m using the trackpad on my laptop, which is centered and not handed at all, I use my right hand. I was also initially taught to play lacrosse right-handed, so now I’ve ended up right-hand dominant in that one random skill. Otherwise, I’m a pure lefty. It seems pretty readily apparent, even without turning to scientific studies, that you can train yourself to use either hand to do most things. By the way, you and I are not the only lefties that have had to learn to do things right-handed. In the past, lefties were often forced to become righties because being left-handed was considered wrong, evil or a bad omen. That outdated sentiment is even built into our language: the Latin root of the English word “sinister” is sinistra, meaning “on the left,” while the root of “dexterity” is dexter, meaning “on the right.”

If you can learn to do things with either hand though, why do we have handedness? Is the rate of motor skill learning in your dominant hand faster than in your non-dominant hand? In other words, if you switched to using the computer mouse with your left hand, would your skill quickly overtake that of your right hand? I like this question a lot because I think it poses a pretty interesting hypothesis: a natural tendency toward faster motor skill learning with one hand would interact with environmental factors (i.e. practice) to determine your handedness. I managed to find a couple of studies that bear on this question.

Pursuit Rotor Task Screen Shot

In the first study, 24 healthy adult right-handed subjects were asked to do two tasks. First, they had to use a stylus to follow a point rotating around in a circle on a screen at 30 rpm. Their score was the amount of time their stylus tip was in contact with the point over the course of a 20 second trial (this task is called the Pursuit Rotor Task). Second, they had to tap the fingers of each of their hands as many times as they could in 10 seconds. At baseline, the subjects could tap more times with their right hand than their left (134 on average with the right hand vs. 120 with the left) and could do the Pursuit Rotor Task a little bit better (1.72 sec vs. 1.07 sec). So far, not super surprising that righties are better with their right hands. What was more interesting, though, was that the subjects also improved more with their right hand than their left with practice. After 10 trials, the subjects scored 4.82 sec on average with their right hand but just 3.08 sec on average with their left hand in the Pursuit Rotor task. This data would suggest that the rate of learning was indeed faster in the right hands of righties. However, this study is not without its flaws. First of all, no lefties were tested. Boo! But more importantly, the Pursuit Rotor Task the scientists used involved the use of a stylus, which is basically like a pen. Everyone already has tons of practice using a pen with their dominant hand, but not with their non-dominant hand. Thus, previous experience could have played a big role in confounding their results. I would prefer to see a study where subjects had to learn a motor skill that was more completely unfamiliar. Ideally, the subjects, regardless of handedness, would start out at about the same baseline skill so that you could really assess learning. It also would have been interesting if the scientists had asked how many extra trials with the left hand would be required before their subjects reach the same level of skill as they had with their right hand after 10 trials (unfortunately, no such luck).

Exp2Setup

The second study I’ll tell you about was a little more complicated. In this study, they used 93 healthy adult right-handed subjects and trained them on a Sequential Visual Isometric Pinch Task. In this task, the subjects had to squeeze a force transducer between their thumb and forefinger to control a cursor on a computer screen. They had to hit five different targets on the screen, each of which required applying a different amount of force to hit. Subjects were given a score on this task based both on their accuracy and speed. Half the subjects were trained to do the task with their right hand and half were trained to do it with their left hand.

Here’s the complicated part: the scientists actually manipulated the subjects’ brain activity while they were learning to see if that had an effect. To do this manipulation non-invasively, they used a technique called transcranial direct current stimulation (tDCS). Basically, some low current travels between two electrodes, which can be placed such that the current passes through the skull and influences brain activity. In this case, the scientists placed one electrode over M1, the primary motor cortex, and the other electrode on the shoulder of the same side of the body. Running current through this type of setup had been shown previously to increase M1 excitability (simplification: it enhances the function of that part of the brain).

All the subjects in this study got electrodes placed over M1 on both sides, but they received different types of stimulation. A third got their right M1 stimulated, a third got their left M1 stimulated and a third got no stimulation. Importantly, the subjects could not tell if they were getting stimulation or not (the current used is very low, so you wouldn’t be able to feel it on your skin). In the end, there were 6 groups total: left-hand learners, split into 3 stimulation groups and right-hand learners, split into 3 stimulation groups. The logic was that if stimulation (i.e. enhancement of function) of either side of the brain enhanced skill learning, then that side of the brain was being used for learning, whereas if it had no effect, that’s because that part of the brain wasn’t involved. It’s not the cleanest logic because we can’t be totally sure whether the stimulation delivered is perfectly enhancing normal function without introducing some other artifact, but bear with it for the moment.

The question being asked in this study, then, was not just “Are righties better at learning with their right hand?” It was also “Which hemisphere of the brain do righties use when learning with their right and left hands? Is there any brain lateralization for motor skill learning?”

The Results: First, these scientists observed no differences in any of the groups’ skills at baseline (that satisfies the problem I was complaining about in Study 1). Second, there were no differences in learning rate by hand! This finding contradicts Study 1, but I’m slightly more tempted to believe this one because they solved the baseline effect problem. Third, there was an effect of left M1 brain stimulation on learning. Let’s talk more about that.

Left M1 brain stimulation causes improved skill learning

The figure above shows the data for the right-hand learners and the left-hand learners combined. Only left M1 stimulation had a statistically significant effect on final skill. This is a pretty cool finding as it suggests that motor skill learning may indeed be lateralized to the left side. If we break the data down by which hand the subjects were trying to learn with, though, the picture gets a little more complex.

Results of left and right M1 brain stimulation during learning broken down by hand

Now, no there are no significant effects (probably due to insufficient statistical power), but there are some interesting trends. Both left and right M1 brain stimulation might help with left-hand learning, but only left M1 brain stimulation seems to help with right-hand learning. That difference between the hands might be explained by the fact that the right M1 actually controls the left hand and the left M1 controls the right hand, and it would still be consistent with lateralized learning (a separate learning circuit might control a more direct motor control circuit).

Of course, the authors would have to follow up on this study to confirm such a hypothesis. At the very least, they would have to run more subjects through their task to see if the trends would hold up and become statistically significant. Another important part of a future study would also be to test lefties. It is unclear from the data shown here whether motor skill learning is always lateralized to the left, or if that effect would be reversed in lefties. Finally, another cool thing to try in the future would be to ask the subjects to do their learning in a functional magnetic resonance imaging (fMRI) scanner. That would allow scientists to look at their subjects’ brain activity while they were learning with one hand or the other to see if there are differences in how the two sides of the brain are activated. For example, maybe even if learning can occur at the same rate in both hands, it would take less concentration or be more metabolically efficient for righties to learn with their right hand. Learning in a scanner could be difficult – the subjects would have to hold their heads very still – but you can imagine a task where your fingers are moving but your head is motionless.

subjectinscanner

In fact, there are studies that have looked at motor skill learning in an fMRI scanner and identified various brain areas involved in motor learning including M1 as well as supplementary motor cortex, striatum, and cerebellum. However, without a study to look explicitly at handedness-specific learning, it’s hard to say what would happen. I also think it’s worth noting that fMRI studies as rule only include right-handed subjects. Data from left-handed subjects tends to be more variable and so lefties are often excluded unless the study is specifically trying to ask a question about left-handedness. In a way, this exclusion is understandable, but it does severely limit our knowledge of how lefties’ brains work!

On that note, one final study’s findings that could influence your thinking about forced handedness switching: in a study of lefties forced to become righties in school, scientists found that such converts had a smaller striatum (one of those brain areas above identified as important for motor learning) than either lefties who stayed lefties or righties who stayed righties. So forcing yourself, from childhood, to go against your natural tendencies might do especially strange things, independent of which hand is dominant.

In conclusion, it is not at all clear that you would gain any advantage by switching the hand you use a mouse with. As long as you are well practiced, it may not make much of a difference. If you do want to switch to using a mouse with your left hand, you’ll learn eventually, but it may take you a while to get as good as you are with your right hand, since you’ve probably already had quite a lot of practice. That said, the studies I’ve described are far from the last word on the matter. In the end, the only way to know if switching to left-handed mouse usage is better for you is to try it out. To know for the general population, you’d have to design a scientific study addressing some of the caveats I’ve mentioned here. I apologize if this is not the definitive (or concise) response you were hoping for, but, if nothing else, I hope this long-winded “I don’t know” answer to your seemingly simple question illustrates to you the unexpected journeys that scientific research can often take you on. Perhaps I can convince you to follow this white rabbit down its hole and pursue a career in research?