Get a grip: The neuroscience of how we pick things up
Countless times a day you pick something up: keys from the counter, the newspaper from the lawn, a paper clip from a desk. You complete these tasks thoughtlessly, but your brain takes them seriously, making precise calculations about how hard to grab each object so that it doesn’t slip from your hand. Neuroscientists have long tried to understand just what goes into those calculations, and new research out of Harvard finds they’re even more sophisticated than experts had thought.
The previous view was that the brain estimates the appropriate amount of “grip force” based on expectations from previous experience about how much force is needed to pick up different kinds of objects. You’ve brushed your teeth tens of thousands of times, and somewhere along the line you developed a pretty good sense of how hard you need to hold the toothbrush so that it remains in your hand as you move it across your teeth. Prior studies have found that your brain takes that learned force, and then adds an extra 20 percent or more — a built-in safety margin applied in all gripping situations.
But in research published in June in the Journal of Neuroscience, Maurice Smith and Alkis Hadjiosif discovered there’s more to the story.
Smith, a bioengineering professor, and Hadjiosif, a postdoctoral fellow, developed an experiment in which subjects put their hands on a grip-strength sensor attached to a robotic arm used manipulate an object. The elaborate setup allowed the researchers to do two things: They could measure how hard people were gripping and also exert a force of their own that in essence tried to pull the object out of peoples’ hands. In some rounds of the experiment that pulling force was applied steadily and in others it was more random, so that people couldn’t anticipate when a sudden increase in grip strength would be needed to prevent losing control of the object. Importantly, the average pulling force was the same in both the steady and the random conditions.
The experiment roughly simulated what it’s like to try and hold onto an object in gusty conditions and it yielded unexpected results. If prevailing theories had held, the researchers would have seen all people in the experiment exert the same amount of grip force — the expected amount required to grip the object, plus that extra safety margin. Instead they found that in situations where the pulling force was steady, the safety margin nearly disappeared. And in situations where the pulling force was highly variable, the safety margin went up a lot.
“The safety margin is not just based on how much force you’re going to produce,” Smith says. “It’s instead controlled moment-by-moment by your level of certainty [about how much force is needed].”
The results provide strong evidence that when it comes to picking things up, our brains focus less on expectations about the amount of force needed to grip a particular object, and more on assessing the uncertainty in a given situation — your confidence, for example, that you know the weight of the contents of a bag, or whether there’s a slick of cooking oil on the handles of a pot.
“It is surprising that the safety margin is based on a direct estimate of uncertainty, and is not based on our experience, which has developed during childhood,” writes Jeroen B.J. Smeets, a professor in the department of human movement sciences at VU University Amsterdam in an e-mail. He adds, “This is a clear example of progress in understanding human motor control.”
Overall, the approach to grip-strength Smith and Hadjiosif uncovered is efficient. Rather than applying cautionary grip force in mundane situations, we can save that energy for circumstances that are harder to predict. It can also leave us vulnerable. The other day I was carrying a glass across the kitchen. I thought I knew what was going on, then I bumped against a cabinet and the glass fell from my hand as though I’d barely been holding onto it at all. It was one situation where I would have been happy if a little extra grip force had come standard.
Kevin Hartnett is a writer in South Carolina. He can be reached at firstname.lastname@example.org.
Via: Google Alert for Neuroscience