Forces

Fingertip forces - sensorimotor prediction and adaptation

When we pick something up – our coffee cup for example – we use our experience of the world to predict how heavy it will be. We are usually pretty successful at doing this, and for good reason – applying too much or not enough force could have serious consequences. Lifting with an inadequate amount of force could render the grip too weak, causing the cup to slip from our grasp. Too much force could be equally disastrous, crushing the cup. In fact, the ability to manipulate objects in a predictive fashion is central to skilled human sensorimotor coordination. When lifting up a familiar object, the gripping and lifting forces that individuals apply are determined well before the lift has been initiated, based on sensorimotor memories. This feed-forward lifting behaviour is even evident when lifting a completely novel object; humans slavishly use visual cues such as the object’s size and material properties guide the initial application of grip and lift forces. Without this feed-forward control, our movements would become slow and awkward. One obvious down side of this predictive control strategy is that objects which have an unexpected weight will invariably be lifted with the incorrect amount of force. Thankfully, we are able to detect any slight overestimations or underestimations that we might have made every time we interact with an object. We implicitly use this remarkable error detection to improve our performance next time; rarely do we make the same mistake twice in a row. Thus, our interest in fingertip force control is twofold: (1) understanding what cues drive the initial predictive application of grip and load forces and (2) determining the mechanisms by which we detect and correct fingertip force errors.