Predictive feed-forward sensory control during grasping and manipulation in man

Abstract

During dexterous manipulation the basal relationships expressed in the employed fundamental muscle synergies are tuned precisely not only to the manipulative intent, but also to the physical properties of the object. Recent findings indicate that the sensorimotor mechanisms involved depend largely on predictive rather than servocontrol mechanisms: The CNS monitors specific, more-or-less expected, peripheral sensory events and use these to directly apply control signals that are appropriate for the current task and its phase. On a fast time scale, discrete mechanical events encoded in populations of somatosensory afferents trigger compensatory actions to task perturbations, and allow task progress to be monitored for timing the release of motor commands related to the serial manipulative phases. This type of predictive feed-forward sensory control is termed 'sensory discrete-event driven control'. On an extended time scale, previous experience with the object at hand or similar objects is used to adjust the motor commands parametrically in advance of the movement, e.g. for the object's weight and surface friction. Through vision, for instance, common objects can be identified in terms of the grip and lifting forces necessary for a successful lift. This ability to directly parameterize the default motor commands is termed 'anticipatory parameter control'.