Modular Control of Pointing beyond Arm's Length

Abstract

Hand reaching and bipedal equilibrium are two important functions of the human motor behavior. However, how the brain plans goal-oriented actions combining target reaching with equilibrium regulation is not yet clearly understood. An important question is whether postural control and reaching are integrated in one single module or controlled separately. Here, we show that postural control and reaching motor commands are processed by means of a modular and flexible organization. Principal component and correlation analyses between pairs of angles were used to extract global and local coupling during a whole-body pointing beyond arm's length. A low-dimensional organization of the redundant kinematic chain allowing simultaneous target reaching and regulation of the center of mass (CoM) displacement in extrinsic space emerged from the first analysis. In follow-up experiments, both the CoM and finger trajectories were constrained by asking participants to reach from a reduced base of support with or without knee flexion, or by moving the endpoint along a predefined trajectory (straight or semicircular trajectories). Whereas joint covaried during free conditions and under equilibrium restrictions, it was decomposed in two task-dependent and task-independent modules, corresponding to a dissociation of arm versus legs, trunk, and head coordination, respectively, under imposed finger path conditions. A numerical simulation supported the idea that both postural and focal subtasks are basically integrated into the same motor command and that the CNS is able to combine or to separate the movement into autonomous functional synergies according to the task requirements.

Space-by-time modular decomposition effectively describes whole-body muscle activity during upright reaching in various directions

“…http://dx.doi.org/10.1101/155085 doi: bioRxiv preprint first posted online Jun. 24, 2017; shaped across subjects and conditions, as previously observed in whole-body reaching 1 movement (Thomas et al, 2005;Berret et al, 2009). Targets were attained with an overall 2 mean spatial error of 10mm ±2mm (from 8 to 15 mm for T6-T3 and T9-T5 respectively).…”

Variant and invariant features characterizing natural and reverse whole-body pointing movements

“…The absolute value of the jerk associated with the shoulder's marker was computed as the second derivative of its velocity absolute value, and it was used to quantify the smoothness of the movements. Each elevation angle was defined as the angle between the corresponding body joint with respect to the vertical direction (Berret et al 2009). The five elevation angles taken into consideration in this study were those relative to the shank (Sh), thigh (Th), trunk (Tr), humerus (Hu) and forearm (Fo).…”

“…This choice was equivalent to operating an amplitude normalization of the joint displacements, so as to avoid biases in the analysis introduced by the joint displacements with the largest excursions. The number of PCs considered suitable to obtain a good compromise between the goodness of signal reconstruction and the level of physical interpretation of the solution was set according to an objective criterion: after examining the data reconstruction performance (expressed in percentage of variance accounted for) in function of increasing numbers of PCs, only the minimum number of PCs that could explain together at least the 90% of the total variation associated with the original data set (Berret et al 2009;Thomas et al 2005) was considered. The loadings relative to such PCs were analysed to understand how variability of the angular displacements was captured by each PC.…”

“…Specific motor patterns were found characterizing, for instance, point-to-point arm movements (Morasso 1981;Soechting and Laquaniti 1981), handwriting tasks (Lacquaniti 1989;Polyakov et al 2009) and locomotion (Ivanenko et al 2004). Other investigations (Berret et al 2009; Thomas et al 2005) have shown that whole-body pointing (WBP) movements to reach a target from a standing position are organized according to a single coordinative structure in the space of the elevation angles, to assure simultaneous forward displacement of the global centre of mass (CoM) and hand trajectory formation.…”

“…During the returning movement instead, the task is mainly postural, as there is not target to point anymore and one has only to assume an upright standing position. Therefore, whereas strict coordination was found during downward movements (Berret et al 2009), the subsequent return to the upright standing position might thus be obtained for instance by means of separate co-variation of the two groups of joints to make the relative body links (respectively, those of the arms and those of the trunk plus lower body) assume a vertical orientation in two consecutive moments.…”