Fig. 1: Task set-up and recording regions for two non-human primates (Monkey J and Monkey C).

a Both subjects performed a 3D naturalistic reach-and-grasp task in a 50 cm × 50 cm × 50 cm workspace. An object located on a wand was always present in the 3D workspace and visible to the monkey. The wand was continuously moved by the experimenter to random locations spanning a large spatial area in front of the subjects. Subjects naturalistically reached to the object (without a go cue), grasped it, and then returned to the resting position. They performed the reach-and-grasp continuously in time for the whole course of the recording session (without a trial structure). There was no go cue, no instructions as to when to reach, and no requirements on reach, hold and grasp durations as the subjects performed the task. Subjects performed reaches to the objects, decided on the hold durations, reach durations, and grasp durations naturalistically. We used motion capture technology using retroreflective markers to track 27 (Monkey J) or 25 (Monkey C) joint angles on the right shoulder, elbow, wrist, and fingers at each point in time. The picture is recreated from marker trajectories of a sample recording session for Monkey J. b Recording regions covered primary motor cortex (M1), dorsal premotor cortex (PMd), ventral premotor cortex (PMv), and prefrontal cortex (PFC) for Monkey J and covered PMd and PMv for Monkey C. Recording electrodes are shown as black circles. Brain sulci (PS principal sulcus, AS accurate sulcus, CS central sulcus, PD precentral dimple) around the recording regions are shown by grey lines. The inter-electrode distance was 1.5 mm.