Pinching Force Changes by Modulating the Interaction Gain Over the Fingertip

Motor control between the thumb and index finger is critical for holding and interacting precisely with an object. In case of an improper pinching force, especially when it stems from sensory problems, modulating the interaction gain by changing the contact surface properties is a promising solution because of its simplicity and minimal risk. In this pilot study, three healthy human subjects participated in the pinching experiment with two different surface areas to test the effect of interaction gain on motor control (88 mm² and 176 mm² as $1\times $ and $2\times $ surface areas). Subjects were asked to match their pinching force to the reference fingertip pressure applied to the other hand. The results demonstrated that subjects applied less force to the thumb when the contact surface area was smaller, perhaps to compensate for the increase in pressure, while the contact surface area did not change the force applied to the index finger. The ratio of the force applied to the $2\times $ surface area to the force applied to the $1\times $ surface area was 1.19±0.02 (Mean±STE) and 1.00±0.02 for the thumb and index finger, respectively. The pinching force was also affected by the tactile memory. When subjects pinched the $2\times $ surface area after pinching the $1\times $ surface area, the pinching force was higher than the one at pinching the $2\times $ surface area from the beginning. These results suggest that environmental intervention is strong enough to bias the motor output of the sensorimotor system.