Somatosensory control of precision grip during unpredictable pulling loads

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Abstract
During manipulation involving restraint of ‘active’ (mechanically unpredictable) objects, it is presumed that the control of the grip and other reaction forces more regularly relies on somatosensory input than during manipulation of ‘passive’ (mechanically predictable) objects. In companion studies we have shown that grip forces are automatically adjusted to the amplitude and the rate of distal pulling loads imposed through an ‘active’ object held in a precision grip. In this study anesthesia of either one or both digits holding the manipulandum was used to examine whether the grip force regulation was dependent on afferent signals from the digits. Five types of trapezoidal load force profiles of various rate and amplitude combinations were given in an unpredictable sequence while the subject was prevented from seeing the hand. Grip forces, load forces and position of the manipulandum in the pulling direction were recorded. With both digits anesthetized the load amplitude changes yielded considerably less grip force modulation and in many trials obvious grip force responses were absent. Moreover, the latencies between the onset of the load changes and the observed grip force responses were much prolonged. However, there was pronounced inter-individual variation. Subjects exhibiting a lower stiffness in the pulling direction, probably due to more flexed fingers when holding the manipulandum, showed a higher force modulation, higher responsiveness to the load ramps and shorter latencies. Hence, under certain conditions afferent input from receptors proximal to the digits could be utilized to provide some grip regulation. The evoked grip force responses showed an initial response similar to the normally occurring ‘catch-up’ response, but it was not graded by the load force rate. Also, there was no ‘tracking’ response, suggesting that the latter was contingent upon a momentto-moment control using afferent input from the digits. With only one digit anesthetized (thumb) the handicap was less severe. Thus, the grip force regulation was impaired under any condition of digital anesthesia, i.e., afferent input from both index finger and thumb was required for the adequate operation of the grip force regulation.