DOI: 10.5061/dryad.q573n5tjh
title: Fast grip force adaptation to friction relies on localized fingerpad strains

description: 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.
description: 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.
description: The data files are separated by participant (one folder per
 participant data). Kinematics and dynamics data are
 included in the files "XX_GL_Y.csv", where XX is the participant
 and Y is the trial number. Friction measurements are
 included in the files "XX_CF_Z.csv", where XX is the participant
 and Z is the normal force level that the participant was told to
 apply. Strains measurements are included in the files
 "strain_array_mvt1.mat". Friction measurement
 are summarized in "index_CF.mat" and "thumb_CF.mat"
 with values "k" and "mu" estimated for the
 "high" and "low" friction condition according to the
 method described in (Barrea et al. 2016).
 "signals_not_sync.mat" contains data summary for each
 subject. The array "strain_array_mvt1" is a
 5-dimension array. <br> The first two dimensions correspond to the
 height and length of the grid on which strains are measured in each
 cell. <br> The third dimension corresponds to the 4 directions in
 which strains are measured (vertical, horizontal, shear and norm of the
 strains).<br> The fourth dimension corresponds to time. The 51st
 element is the time of the first peak of LF and consecutive timestamps are
 separated by 10ms.<br> The fifth dimension corresponds to different
 trials.<br> The correspondence between the trial numbers and the
 index of the corresponding trials are included in the array
 "trialnumbers".<br>