In movies and tabloids, a ‘sixth sense’ is a magical ability to sense things the rest of us can't. But, in fact, each of us really does have a sixth sense that's so basic we often overlook it. It's called proprioception, and it's what allows us to know the position and orientation of our bodies and limbs, even when we can't see them.
Researchers have studied the various sense organs that contribute to the proprioceptive sense – including stretch receptors located in muscle bodies, force sensors in tendons and nerve fibers in joint capsules – but we know relatively little about how the brain combines all of that information. In particular, how do you know where your hand is? Does the brain track hand position directly, or does it keep track of the elbow and shoulder angles separately?
To examine these questions, Christina Fuentes and Amy Bastian at Johns Hopkins School of Medicine enlisted a group of 10 subjects. Each subject was strapped into an exoskeletal robotic arm that allowed the researchers to measure and control the angle of the subjects' right shoulder and arm and the position of their fingertip. They placed a screen above each subject's arm to block the view of the arm.
Fuentes and Bastian then asked the subjects to do three tasks, each of which required the subjects to estimate their forearm angle. In the first task, the researchers used the robot to move the subjects' forearm to a particular angle, then asked them to use a joystick to rotate a line to match their forearm angle. Second, after the robot moved the arm again, the subjects used the joystick to move a point to the location of their index finger. Since the elbow was fixed, this task also amounted to estimating the forearm angle, but the subjects were thinking about the fingertip position. In the last task, rather than estimating an angle imposed externally by the robot, the subjects actively moved their arms to match the angle of a projected line.
All subjects had the largest errors in the first task, when the robot moved their arm and they were asked about their forearm angle. Interestingly, they performed much better on the second task, even though the only difference between it and the first was that they were asked about fingertip position rather than forearm angle. And they were slightly more precise in the active task, when they had to move their arm to match a displayed angle.
They attribute the differences in precision across the tasks to two effects. In the active task, in which the subjects were most precise, they could use both proprioceptive information and information about how they themselves moved their arms. The accuracy in this task wasn't all that surprising since it's fairly unusual that something else moves our limbs for us.
The differences in the two passive tasks is more informative. It suggests that the brain is better able to gauge hand position than actual joint angles, which makes sense behaviorally. After all, when you reach for a cup of coffee without looking, the thing that really matters isn't your elbow, but your hand position.
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