Multi-channel EEG recordings during 3,936 grasp and lift trials with varying weight and friction

Abstract

WAY-EEG-GAL is a dataset designed to allow critical tests of techniques to decode sensation, intention, and action from scalp EEG recordings in humans who perform a grasp-and-lift task. Twelve participants performed lifting series in which the object's weight (165, 330, or 660 g), surface friction (sandpaper, suede, or silk surface), or both, were changed unpredictably between trials, thus enforcing changes in fingertip force coordination. In each of a total of 3,936 trials, the participant was cued to reach for the object, grasp it with the thumb and index finger, lift it and hold it for a couple of seconds, put it back on the support surface, release it, and, lastly, to return the hand to a designated rest position. We recorded EEG (32 channels), EMG (five arm and hand muscles), the 3D position of both the hand and object, and force/torque at both contact plates. For each trial we provide 16 event times (e.g., 'object lift-off') and 18 measures that characterize the behaviour (e.g., 'peak grip force').

Figure 1. Methods.

(a) Force and position sensors. F1-F2 correspond to force/torque sensors (ATI Nano), with x corresponding to lift force and z to grip force. P1-P4 correspond to 3D position sensors (Polhemus FASTRAK) attached to the object (P1), the index finger (P2), the thumb (P3) and the wrist (P4). (b) EMG sensor placement: 1-anterior deltoid, 2-brachioradialis, 3-flexor digitorum, 4-common extensor digitorum, 5-first dorsal interosseus. (c) EEG sensor (ActiCap), recording from 32 electrodes. (d) Test object. The object to be grasped was visible on top of the table (cf, panel a) while the rest was hidden from view. The distance between the two contact surfaces (each 35×35 mm) measured 45 mm and they were secured to the object by niobium magnets. The touched surface could easily be replaced. The force applied to each contact plate was measured with mechanically isolated ATI Nano 17 6-axis force/torque sensors. The weight of the object including its magnetic plate was 165 g and could be increased to 330 or 660 g by controlling two electromagnets at the bottom. A set of flexible PVC rods provided low-friction alignment of the object on the table. Above the object was a rectangle in perspex with a centre hole (Ø20 mm). The start of the trial was signalled when the Perspex rectangle was illuminated by a LED. On top of the object was a Polhemus sensor mounted to record the position and orientation of the object.

Figure 2. Sample trial.

(a) Complete trial from just before the onset (indicated by the illumination of the Perspex plate above the object) until that object weighing 165 g was replaced and the hand returned to the starting position. Arrows labelled Digit contact, Load phase onset, etc, mark some of many ‘events’ extracted from all single trials (cf. Table 1). (b) The window marked by a yellow rectangle in (a) shown on an expanded time scale.

Figure 3. Data validation.

(a,b) For channels C4 and Pz (shown in insets) recorded in Participant 3, trials when the object's weight was the same as in the previous trial (Expected weight, n=105; blue lines) and unexpectedly heavy (n=30; red lines) were contrasted using EEGLab. The panels show from top, the power in the alpha and beta bands after sorting the trials by phase at the peak frequency, the average EEG amplitudes, the ERSPs and the ITCs. The colored patches represent 95% confidence intervals. The earliest moment this participant on average could have detected an increased object weight was ~200 ms after object contact (i.e., time zero). (c) All participants adapted their grip force to the object’s weight, i.e., 165, 330 or 660 g in series with sandpaper surfaces. The different weights thus invoked markedly different fingertip forces in all participants. (d) The grip:load force ratio was the same or declined across the three object weights in all participants, i.e., the force coordination was roughly the same irrespective of the object’s weight. (e) In series with the same object weight (330 g) but with contact plates covered with sandpaper, suede or silk, the grip:load force ratio increased with decreasing friction, i.e., in all participants the three contact plates offered different object-fingertip friction and all participants adapted to the prevailing friction. (f,g) When the weight (f) or the contact surfaces (g) was unexpectedly changed between trials, there was a marked change in the load force duration, in the peak grip force and the hold grip force (e.g., all increased when the object had an unexpected increased weight or decreased friction). Data aggregated across all participants. The lines represent the median and the 1st and 3rd quartile, black lines increased weight (f) and increased slipperiness (g) and gray lines decreased weight and slipperiness, respectively, as indicated on the top and bottom axes.