Stretching the skin of the fingertip creates a perceptual and motor illusion of touching a harder spring
We investigated how artificial tactile feedback in the form of a skin-stretch affects perception of stiffness and grip force adjustment. During interactions with objects, information from kinesthetic and tactile sensors is used to estimate the forces acting on the limbs. These enable the perception of the mechanical properties of objects to form, and the creation of internal models to predict the consequences of interactions with these objects, such as feedforward grip-force adjustments to prevent slippage. Previous studies showed that an artificial stretch of the skin of the fingertips can produce a linear additive effect on stiffness perception, but it remains unclear how such stretch affects the control of grip force. Here, we used a robotic device and a custom-built skin-stretch device to manipulate kinesthetic and tactile information. Using a stiffness discrimination task, we found that adding artificial tactile feedback to a kinesthetic force can create the illusion of touching a harder spring which affects both perception and action. The magnitude of the illusion is linearly related to the amplitude of the applied stretch. We also isolated the contribution of tactile stimulation to the predictive and reactive components of grip force adjustment, and found that unlike in other cases of perceptual illusions, the predictive grip force is modulated consistently with the perceptual tactile-induced illusion. These results have major implications for the design of tactile interfaces across a variety of touch applications such as wearable haptic devices, teleoperations, robot-assisted surgery, and prosthetics.\n\nSignificance StatementSensing forces, using kinesthetic and tactile modalities, is important for assessing the mechanical properties of objects, and for acting the objects while stabilizing grasp against slippage. A major challenge in understanding the internal representations that allow for a predictive grip force control during contact with objects is to dissociate the contribution of tactile and kinesthetic stimuli. To date, this contribution was investigated only in impaired cases either through local anesthesia or in patients with sensory impairment. Our study demonstrates using a programmable mechatronic device that artificially applied skin-stretch creates an illusion of a greater load force that affects grip force control and stiffness perception. These results are applicable in tactile technologies for wearable haptic devices, teleoperation, robot-assisted surgery, and prosthetics.