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 unp redictable changes in loading forces) with a precision grip of the dig its, we maintain a stable grasp by modulating our grip force using som atosensory 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 t he grip force in parallel with the load force (''tracking''). The size s 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 sen sorimotor transformation based on two additional factors: (1) the fric tion at the digit-object interface and (2) the grip force present at t he start of the load increase. The present experiments sought to deter mine whether such scaling occurs and to characterize its control. Subj ects restrained a manipulandum held between the tips of the thumb and index finger. At unpredictable times a pulling force appeared, directe d away from the subject's hand. Each pull had a trapezoidal load profi le 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. Th e grip force at the start of the load ramp (intertrial grip force), an d the amplitudes of the catch-up and secondary grip responses scaled i n proportion to the inverse friction. We interpret these results to in dicate a uniform scaling of the transformations controlling the intert rial grip force, the catch-up response, and the secondary response. In itial-state information from tactile cues available upon object contac t appeared to update the frictional scaling value. This conclusion is based on observations of immediate changes in the intertrial grip forc e upon contact with a new surface, and because differences in force-ra te profiles appeared virtually by the onset of the catch-up response. Similarly, the intertrial grip force also constituted initial-state in formation. The size of the catch-up and secondary grip force responses varied inversely with the size of the intertrial grip force. These sc alings of the load-to-grip-force sensorimotor transformation for frict ion and intertrial grip force level appear to be functionally adaptive , because they contribute to a stable grasp (prevent object slips) whi le avoiding exceedingly large safety margins.