Every time you reach for something, there’s a squabbling match in your brain

By Ed Yong | September 27, 2010 3:00 pm


Even the simplest of actions can arise from hidden competition. Imagine reaching for a cup. There are many ways of doing this, depending on where the cup is, whether you’re right- or left-handed, whether the handle is turned towards you, and so on.  Your brain works through all of these possibilities at the same time, and they compete against one another until one wins out. Only after this neural battle does your arm start to move.

A new study suggests that these competitive processes affect even simple decisions, like which hand we use to pick up an object. Even though this is one of the most common decisions we ever make, it has rarely been studied. To remedy that, Flavio Oliveira from the University of California, Berkeley filmed the movements of volunteers as they reached for targets that had been projected onto a table in front of them.

In some trials, the volunteers (all right-handed) were told to only use their left or right hands. On others, they could choose for themselves. In the latter trials, they always used the nearest hand to pick up targets at the far edges of the table, but they used either hand for those near the middle. Their reaction times were slower when they had to choose which hand to use, and particularly if the target was near the centre of the table.

This much is expected, but it supports the idea that the brain is choosing between possible movements associated with each hand. At the centre of the table, when the choice is least clear, it takes longer to come down on one hand or the other.

Think of it as a roomful of people, all with different ideas about what to do, and all shouting out their personal suggestions. Their tenacity and volume are based on what they see. If the target is on the right of the table, those who favour a “reach-with-the-right-hand” plan start shouting more loudly, and drown out the “left-hand” crowd. If the table is in the middle, everyone on the room shouts equally loudly, and it’s harder to work out which group has the loudest voice.

Next, Oliveira performed the same experiment while using a magnetic pulse to temporarily disrupt specific parts of the volunteers’ brains. He focused on an area called the posterior parietal cortex or PPC, which plays an important role in planning our movements. If it’s damaged, people often find it difficult to reach for and grab at objects.

When Oliveira directed the pulse at the PPC on the left half of the brain (which largely influences the right hand), the volunteers were more likely to use their left hand to clutch at the targets. They were still strongly right-handed, but they used their left hand more than they normally would, especially for targets near the centre of the table. However, aiming the pulse at the right PPC had no effect, and neither did pulses at other parts of the brain.

By disrupting the left PPC, Oliveira also disrupted the movement plans associated with the right hand. In our imaginary room, the half of the crowd who were advocating for the right-hand strategy suddenly fell silent. With them out of play, the left-hand crew were all that remained and were more likely to win their shouting match.

It’s not clear why disrupting the right PPC (which largely influences the left hand) had no effect. But Oliveira thinks that for right-handed people, the preference for the right hand is so strong anyway that there’s little room for inducing any further bias over the left hand. It would, of course, be interesting to repeat the experiment with left-handed people to see if the reverse is true.

Oliveira says that our decisions and actions “can be viewed as a dynamic process in which many possible motor responses are competing at any one time, with the accumulation of evidence in favour of each candidate… continuously changing.”

Reference: PNAS http://dx.doi.org/10.1073/pnas.1006223107

Image from David Ip

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Comments (8)

  1. Now Ed, should I actually hear the voices yelling at each other, because my Dr feels we should try to control that with medication?

  2. Whoa whoa whoa. You can shut off somebody’s brain with a magnetic field? Where do I get that in handheld form? Life could be far more slapstick…

  3. Brian Too

    I thought this was known already? Haven’t psychologists already established that any time action is considered, the brain produces an array of possibilities and then inhibits most of them? I thought it was known as dampening or something similar.

    Even radical and anti-social possibilities are produced. For instance, to extend the example given in the article, you might consider tipping the entire table over in order to get the cup. Then you discard that possibility, thinking “no, that might break the cup”, or any of a thousand other reasons not to do that. It all happens very quickly too and can operate under the threshold of perception.

  4. Yep, this sort of competitive decision-making is known (although more recently that you might imagine). The “new” bit is the relevance of hand choice.

    But I don’t like news stories that only focus on the latest new bit. It’s not how science works. New discoveries sit on the shoulders of giants. In this case, yes, there’s a news hook (hand choice) but it also gave me a chance to talk about this aspect of decision-making that’s not well known to a general audience.

  5. Chris M.

    The left-handed suggestion is interesting; you’d think people who were ambidextrous (or some approximation of it) would be better. Much harder to find as research subjects, but in this case the effort is probably worth it given the difficulty of proving the findings. Of course, if you do get the same sidedness result for the more easily-found left handers, it strongly supports lateral specialization in the brain. Does anyone know whether that’s been shown for motor tasks, outside handedness? I can’t recall myself.

  6. Aurora

    I’m about to reach for my cup of tea, and there’s now a 3rd set of voices in my head, whispering: “You’ve triggered the shouting-match again…”

  7. Ha! The price of knowledge!

  8. I feel obliged to point out that a lot of the ‘decision making’ has already taken place by the time a given reach is executed. For instance, the affordances of the object have already ruled out many (technically possible) reaches and constrained the decision space. Biases such as recent movement history then further constrain the space (several researchers have gotten interested in this recently, we cite some in Kent et al). On a longer time scale ‘use dependant learning’ creates reaching-to-grasp perception/action systems that do what you typically do, meaning many (technically possible) reaches aren’t ever actually options. The brain is only part of the system that ‘decides’ and it’s not at all clear that ‘decision’ is the correct description of what is actually a complex, time extended dynamic process.

    Brian Too, the net result is that the space of possibilities, while still large for any given reach-to-grasp action, is not infinite or even likely to contain really unusual options, unless there’s genuine malfunction in the system. Just because a patient with frontal lobe damage might generate and fail to inhibit tipping the table doesn’t mean normal function requires inhibiting such an option.

    Kent, SW; Wilson, AD; Plumb, MS; Williams, JH; Mon-Williams, M Immediate movement history influences reach-to-grasp action selection in children and adults. J Mot Behav, 41, 10-15, 2009


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