ABSTRACT
How do we Scratch an Itch?
Imagine a simple situation: you have an itch on your left arm, and you reach with your right arm to scratch it. Scratching an itch at first seems like a simple, automatic task, but a little more reflection shows that it can actually be quite complicated. We do not need to see; rather, we can accomplish the task knowing only where the itch happens to be on our body, and how far our limbs are flexed or extended: we need only somatosensory information. We move the reaching arm and the itching arm together – they are coordinated. This bimanual coordination allows us to reach arbitrary parts of our body: we are able to scratch anywhere on either arm. The task cannot always be accomplished just by moving the reaching hand in a straight line; sometimes other parts of our body – or the itching arm itself – become obstacles which must be avoided or moved. How are the movements of two arms coordinated to avoid collisions? How is the body-as-a-target represented? How do we reach for a target that may change location in space? How is the brain able to create a motor trajectory that will coordinate the body's limbs, avoid self-collisions, and still achieve the goal of reaching the itch? A framework for this problem is pictured in Figure 1, and some example configurations of the model framework are shown in Figure 2.
