Stretching the skin immediately enhances perceived stiffness and gradually enhances the predictive control of grip force

Abstract

When manipulating objects, we use kinesthetic and tactile information to form an internal representation of their mechanical properties for cognitive perception and for preventing their slippage using predictive control of grip force. A major challenge in understanding the dissociable contributions of tactile and kinesthetic information to perception and action is the natural coupling between them. Unlike previous studies that addressed this question either by focusing on impaired sensory processing in patients or using local anesthesia, we used a behavioral study with a programmable mechatronic device that stretches the skin of the fingertips to address this issue in the intact sensorimotor system. We found that artificial skin-stretch increases the predictive grip force modulation in anticipation of the load force. Moreover, the stretch causes an immediate illusion of touching a harder object that does not depend on the gradual development of the predictive modulation of grip force.

The Effect of Between-Probe Variability in Haptic Feedback on Stiffness Perception and Grip Force Adjustment

“…We will note that the black capsule is the lower part of a device that can be used to create artificial skin stretch via moving tactors. This device was used in other studies with the same setup to study the dissociable role of the two haptic modalities (tactile and kinesthetic) [27]. In this work, participants grasped the device directly above the force sensor and not above the tactors, and the skin stretch stimulation was never actuated.…”

“…We examined the GF applied by the participants and the trajectories of the vertical position of the hand (y axis) for every trial in each of the variability conditions. Classic analyses in the study of grip force control are: (i) to examine the grip force in relation to the load force by analyzing the ratio between the peak GF and peak load force (LF) [10,27,28]:…”

“…Comparing (23) and (27) highlights that the intercept of the regression in (23) represents a grip force baseline that is unaffected by the planned interaction with the elastic force field. On the other hand, the slope, which represents the grip force modulation in anticipation of the load force, is affected by the estimates of both the elastic force field stiffness and the contact dynamics.…”

“…During object manipulation, we apply grip force to prevent the object from slipping due to load forces (e.g., gravity forces [15][16][17][18][19], environment interaction forces [20][21][22][23], and inertial forces [17,24]) which act on the object. The grip force is mostly predictive [25,26] and comprised of two components: (1) a baseline and (2) a modulation of the grip force in anticipation of the load force [22,27,28]. The baseline component is the grip force applied when there is no load force acting.…”

“…The modulation component combines the internal representation of the object with that of the environmental dynamics to predict the load forces that may act on a manipulated object. This internal representation is formed by the nervous system [25,[29][30][31] and is updated during repeated interactions with an object [27,28,32].…”