Hold your arm out underneath a table, without touching it, so you cannot see it. Now, without moving your arm under the table, move a coin around on top of the table until you think it is above your index finger. How do you know where your finger is? You are not using any of the traditional five senses, but a different sense entirely - proprioception.
Proprioception is the sense of where your limbs are in relation to the rest of your body. Whenever you cannot see any part of your body, you still know where it is thanks to proprioception. It is a vital sense, allowing you to move without having to keep an eye on what you are moving.
But how good is our proprioception? Christina Fuentes and Amy Bastian, scientists at the Kennedy Krieger Institute, are particularly interested in our proprioception across joints. They have recently conducted three experiments to test the accuracy and precision of human proprioception of the elbow, discovering that the more your elbow is extended or flexed, the more inaccurate your proprioception of it.
"We had the subjects in the experiments put their right arm into the KINARM, an exoskeletal robotic arm that can be used to change the angle and position of their arm," Fuentes said. "We used a screen to block their view of their arm, and projected images that appeared to be in the same plane as their arm."
In each of the experiments, Fuentes tested the subjects' ability to match the actual location of their arm and elbow to a projection on the screen, which consisted of a line representing the upper arm, a dot at the end of that line representing the elbow, and either another line for the forearm or a dot for the index finger.
In the first experiment, the subjects used a joystick in their left hand to adjust the position of the line representing their forearm until the angle of the projected lines matched that of the elbow. After 12 seconds, the robotic arm adjusted the angle of the shoulder or elbow, and the process was repeated. While changing the shoulder angle had no impact on the subjects' estimates, changing the elbow angle up or down from an ideal of 60 degrees worsened the subjects' accuracy.
In the second experiment, subjects used the joystick to adjust the position of the dot representing their index finger until the dot was atop the finger, according to their proprioception.
"There is evidence suggesting that the central nervous system primarily represents limb position in terms of endpoint position - that is, our proprioception works by positioning our fingers and toes relative to the body, then using their positions to work out where the arms and legs are. Therefore, we hypothesized that the subjects would produce a more precise group of elbow angle estimates if they were estimating the position of their fingertips, rather than the angle of their elbows," Fuentes said.
In the final experiment, subjects moved their own forearm to match the position of the line representing it on the screen. The goal was to compare whether actively moving one's arm onto the line would be more accurate (producing closer estimates) and more precise (producing estimates off by nearer to the same amount) than moving the line onto the arm, or moving a dot onto the fingertip.
As it turned out, in all three experiments, for all the different elbow and shoulder angles, the average accuracy was about two degrees. Further, the pattern found in the first experiment held true: Subjects overestimated the elbow angle by more and more as the actual elbow angle increased above 75 degrees, and underestimated by larger amounts as the actual elbow angle fell below 45 degrees.
However, the precision improved in the second and third experiments. In the first experiment, the estimates were spread out by almost 6 degrees, while in the other two experiments, the spread was around 4 degrees.
