Abstract
Whenever we grasp and lift an object, our tactile system provides important information on the contact location and the force exerted on our skin. The human brain integrates signals from multiple sites for a coherent representation of object shape, inertia, weight, and other material properties. It is still an open question whether the control of grasp force occurs at the level of individual fingers or whether it is also influenced by the control and the signals from the other fingers of the same hand. In this work, we approached this question by asking participants to lift, transport, and replace a sensorized object, using three- and four-digit grasp. Tactile input was altered by covering participant's fingertips with a rubber thimble, which reduced the reliability of the tactile sensory input. In different experimental conditions, we covered between one and three fingers opposing the thumb. Normal forces at each finger and the thumb were recorded while grasping and holding the object, with and without the thimble. Consistently with previous studies, reducing tactile sensitivity increased the overall grasping force. The gasping force increased in the covered finger, whereas it did not change from baseline in the remaining bare fingers (except the thumb for equilibrium constraints). Digit placement and object tilt were not systematically affected by rubber thimble conditions. Our results suggest that, in each finger opposing thumb, digit normal force is controlled locally in response to the applied tactile perturbation.
Highlights
It is possible to grasp the same object in an infinite number of ways, due to the multiple combinations of digit placement on the object and the level of force exerted by each digit
The aim of this study is to evaluate whether normal forces are adjusted either independently for each digit or whether there is a coupling such that the normal forces of some digits are regulated jointly, in response to the sensory feedback perturbance when covering individual fingers with a rubber thimble
Before analyzing the forces exerted by each finger during threedigit grasping, we evaluated the overall grip force of the grasp and we report on the peak of the grip force
Summary
It is possible to grasp the same object in an infinite number of ways, due to the multiple combinations of digit placement on the object and the level of force exerted by each digit. Grasping and manipulating an object requires that the central nervous system masters this redundancy in degrees of freedom (Bernshtein, 1967; Naceri et al, 2014, 2017). Sensory feedback from proprioception, touch, and other modalities such as vision plays a key role in the control of grasping. Tactile feedback is conveyed by afferent fibers that respond to the deformation of the mechanoreceptors in the skin. Cutaneous mechanoreceptors provide information concerning the timing of the contact, the location of the contact area on the skin, and the direction and amplitude of the contact force (Johansson and Flanagan, 2008). In addition to sensory feedback, finger force control is achieved in an anticipatory fashion during grasping
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