Friction at the digit-object interface scales the sensorimotor transformation for grip responses to pulling loads

When restraining a mechanically "active" object (one that exerts unpredictable changes in loading forces) with a precision grip of the digits, we maintain a stable grasp by modulating our grip force using somatosensory information related to the loading forces. The response to ramp load increases consists of an initial fast rise in grip force ("catch-up") followed by a secondary response that steadily increases the grip force in parallel with the load force ("tracking"). The sizes of these response components scale in proportion to the loading rate. However, maintaining a stable grasp without employing an exceedingly large grip force may require further scaling of this load-to-grip sensorimotor transformation based on two additional factors: (1) the friction at the digit-object interface and (2) the grip force present at the start of the load increase. The present experiments sought to determine whether such scaling occurs and to characterize its control. Subjects restrained a manipulandum held between the tips of the thumb and index finger. At unpredictable times a pulling force appeared, directed away from the subject's hand. Each pull had a trapezoidal load profile beginning and ending at 0 N with 4-N/s ramps; each ramp was 1 s in duration. The texture of the gripped surfaces varied among sandpaper, suede, and rayon, which represented increasingly slippery surfaces. The grip force at the start of the load ramp (intertrial grip force), and the amplitudes of the catch-up and secondary grip responses scaled in proportion to the inverse friction. We interpret these results to indicate a uniform scaling of the transformations controlling the intertrial grip force, the catch-up response, and the secondary response. Initial-state information from tactile cues available upon object contact appeared to update the frictional scaling value. This conclusion is based on observations of immediate changes in the intertrial grip force upon contact with a new surface, and because differences in force-rate profiles appeared virtually by the onset of the catch-up response. Similarly, the intertrial grip force also constituted initial-state information. The size of the catch-up and secondary grip force responses varied inversely with the size of the intertrial grip force. These scalings of the load-to-grip-force sensorimotor transformation for friction and intertrial grip force level appear to be functionally adaptive, because they contribute to a stable grasp (prevent object slips) while avoiding exceedingly large safety margins.