Somatosensory control of precision grip during unpredictable pulling loads

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Abstract
In manipulating ‘passive’ objects, for which the physical properties are stable and therefore predictable, information essential for the adaptation of the motor output to the properties of the current object is principally based on ‘anticipatory parameter control’ using sensorimotor memories, i.e., an internal representation of the object's properties based on previous manipulative experiences. Somatosensory afferent signals only intervene intermittently according to an ‘event driven’ control policy. The present study is the first in a series concerning the control of precision grip when manipulating ‘active’ objects that exert unpredictable forces which cannot be adequately represented in a sensorimotor memory. Consequently, the manipulation may be more reliant on a moment-to-moment sensory control. Subjects who were prevented from seeing the hand used the precision grip to restrain a manipulandum with two parallel grip surfaces attached to a force motor which produced distally directed (pulling) loads tangential to the finger tips. The trapezoidal load profiles consisted of a loading phase (4 N/s), plateau phase and an unloading phase (4 N/s) returning the load force to zero. Three force amplitudes were delivered in an unpredictable sequence; 1 N, 2 N and 4 N. In addition, trials with higher load rate (32 N/s) at a low amplitude (0.7 N), were superimposed on various background loads. The movement of the manipulandum, the load forces and grip forces (normal to the grip surfaces) were recorded at each finger. The grip force automatically changed with the load force during the loading and unloading phases. However, the grip responses were initiated after a brief delay. The response to the loading phase was characterized by an initial fast force increase termed the ‘catch-up’ response, which apparently compensated for the response delay — the grip force adequately matched the current load demands by the end of the catch-up response. In ramps with longer lasting loading phases (amplitude ≥ 2 N) the catch-up response was followed by a ‘tracking’ response, during which the grip force increased in parallel with load force and maintained an approximately constant force ratio that prevented frictional slips. The grip force during the hold phase was linearly related to the load force, with an intercept close to the grip force used prior to the loading. Likewise, the grip force responses evoked by the fast loadings superimposed on existing loads followed the same linear relationship. The grip force response to the unloading phase showed a rather smooth, nearly bell shaped, rate profile that suggested it was programmed for the inter-trial grip force. The stiffness in the loading direction increased with the load force in a manner suggesting that the two forces were subjected to similar coordinative constraints as in the manipulation of ‘passive’ objects.