Fast grip force adaptation to friction relies on localized fingerpad strains

Humans can quickly adjust their grip force to a change in friction at the object-skin interface during dexterous manipulation in a precision grip. To perform this adjustment, they rely on the feedback of the mechanoreceptive afferents innervating the fingertip skin. Because these tactile afferents encode information related to skin deformation, the nature of the feedback signaling a change in friction must somehow originate from a difference in the way the skin deforms when manipulating objects of different frictions. To better characterize the origin of the underlying sensory events, we asked human participants to perform a grip-lifting task with a manipulandum equipped with an optical imaging system. This system enabled to monitor fingertip skin strains through transparent plates of glass that had different levels of friction. We observed that, following an unexpected change in friction, participants adapted their grip force within 370ms after contact with the surface. By comparing the deformation patterns when unexpectedly switching from a high to a low friction condition, we found a significant increase in skin deformation inside the contact area arising over 100ms before the motor response, during the loading phase, suggesting that local and partial deformation patterns prior to lift-off are used in the nervous system to adjust the grip force as a function of the frictional condition.

Methods
Participants stood in front of a table on which the device was positioned. After an auditory cue, they were instructed to grip and lift the device to a height of about 20cm within 0.8s, and then hold it still for 1.5s. They then performed three fast point-to-point movements (0.8s) with pauses (1.5s) in-between. Auditory cues were used to pace each movement. The participants were requested (and often reminded during the experiment) to use a minimal amount of GF. The glass plates were cleaned with alcohol after each trial. This served the purpose of getting images as clean as possible. Also, this procedure removed sweat that could alter the topography of the glass plates at a microscopic level and thus the level of friction. After each block of five trials, participants were instructed to take a break on the other side of the room, from a location where they could not see the experimenter manipulating the device. During that break, the experimenter interchanged the plates such that the friction was changed from high to low or from low to high. This procedure was quick and took a maximum of 2 minutes. Half of the participants started with the high friction condition and the other half with the low friction. The coefficient of friction was measured for each material at the end of the experiment. In total, the experiment lasted between one and a half and two hours for each participant.