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What's in a Grasp?

Simple acts of picking up a water glass or turning a handle are the product of multilayered cognitive plans and sophisticated neural computations.

David A. Rosenbaum, Oliver Herbort, Robrecht van der Wel, Daniel J. Weiss

2014-09RosenbaumF1.jpgClick to Enlarge ImageA peculiar detail in the famous painting The Night Watch shows that Rembrandt van Rijn was not only a great painter but also an astute observer of human behavior. Off to the left of center, half in shadow, stands a red-uniformed guard who holds the ramrod for his musket in an uncomfortable-looking, thumb-down position. Why did Rembrandt pose the guard this way? A thumb-up posture would have probably allowed the guard to have greater power and more control. But there is another consideration: a thumb-down posture would put the guard in a stronger or more resolute-looking pose when he withdrew the rod from the barrel.

We suspect Rembrandt wanted to convey that the guard, along with other members of the night watch, would be in a strong position to defend the Netherlands. The artist may have understood that the way one grasps an object depends not just on the properties of the object at the time of the grasp, but also on what one plans to do with the object. Rembrandt may have sensed that what’s in a grasp is in large part what’s in the mind of the grasper.

As cognitive psychologists, we are interested in the mind. We analyze behavior to gain insight into the intelligence behind it. In particular, we study how people and animals grasp objects. We do this to learn about action planning and control, and we pursue this topic not just for theoretical reasons but also for practical ones. Roboticists may find our work useful for informing the design of more adept artificial agents. Healthcare professionals may find our work relevant for informing medical diagnosis and rehabilitation. Regulators, managers, and engineers may benefit from our work insofar as it may inform the design of skill-training systems and safer and more efficient setups for work, transport, and play.

We believe that a useful way to study action planning is to observe changes in behavior as a function of the behavior that follows. If an action differs depending on the subsequent action, the anticipatory effect can be said to reflect planning. Anticipatory changes of this sort have been studied in depth in speech production—for example, in rounding the lips before pronouncing the “t” in “tulip” (as opposed to the “t“ in “tailpipe,“ for example)—a phenomenon called coarticulation. We have been among the first to focus on the manual analog of such changes, which we call comanipulation.

2014-09RosenbaumF2.jpgClick to Enlarge ImageTwo naturalistic observations form the basis of our research. One concerns the orientation of the hand as it reaches out to grasp something; the other concerns the height of the grasp. The first observation came from a common scene: a waiter in a restaurant filling glasses with water. To begin with, the glasses were upside down on a table. The waiter took hold of each glass not with his thumb up, which is the usual way to grasp a glass, but with his thumb down. (Throughout this article, we use the terms “thumb-up” or “thumb-down” to refer to the base of the thumb.) After grasping each glass, the waiter turned it upright, held it with his thumb up while pouring, and then set the filled glass back down on the table, as demonstrated by our student volunteer in the photographs at right. Presumably, the waiter used a thumb-down grasp to allow him to complete the task using postures that were comfortable and easy to control.

According to this interpretation, the waiter’s grasp was shaped not only by the visual appearance of the glass (an instance of what we call first-order planning), but also by what he planned to do with the glass subsequently (an instance of what we call second-order planning). Like the guard in The Night Watch, the waiter apparently changed his grasp according to what he planned to do next.

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