Prediction of object contact during grasping.

The maximum grip aperture (MGA) during prehension is linearly related to the size of objects to be grasped and is adapted to the haptically sensed object size when there is a discrepancy between visual and haptic information. We have investigated what information is used to drive this adaptation process and how the onset of fingertip forces on the object is triggered. Subjects performed a reach-to-grasp task, where the object seen and the object grasped physically never were the same. We measured the movements of the index finger and the thumb and the contact forces between each fingertip and the object. The subjects' adaptation of the MGA was unrelated both to different fingertip velocities at the moment of object contact, or the fingertip forces. Instead, the 'timing' of contact between the fingers and the object was most consistently influenced by introducing a size discrepancy. Specifically, if the object was larger than expected, the moment of contact occurred earlier, and if the object was decreased in size, then the contact occurred later. During adaptation, these timing differences were markedly reduced. Also, the motor command for applying forces on the object seemed to be released in anticipation of the predicted moment of contact. We therefore conclude that the CNS dynamically predicts when contact between the fingertips and objects occur and that aperture adaptation is primarily driven by timing prediction errors.