Effect of motor planning on use of motor abundance

Abstract

This study examined the hypothesis that the degree to which motor redundancy is used to coordinate joint motions for reaching is influenced by motor planning and enhanced when the task requires greater movement flexibility. Subjects reached at arm's length to the same centrally placed target under conditions where the target location was either certain or uncertain, using a double-step paradigm. The hypothesis was evaluated by partitioning the across-trials variance of the joint configuration at each percent of the reach into a component corresponding to the use of different joint angle combinations to achieve an equivalent hand position (GEV) and a component leading to a variable hand position (NGEV). Pointer-tip movement variability along the path and variable targeting error did not differ between conditions. Larger overall joint variance was found for the uncertain target condition. Most of this increase was GEV, which was significantly higher in the uncertain condition for control of both movement extent and movement direction. In contrast, NGEV differed between the two conditions only for the control of movement extent early in the reach, suggesting that target uncertainty led to inter-trial timing variability along the movement path. The results suggest that more flexible patterns of joint coordination are used when the nervous system must plan reaching movements to an uncertain target direction.

Fig.1

(A) Experimental Set-up, (B) target position at the beginning of the trial and after the subject started to move (front panels) and (C) 3-D movement trajectories under both certain (left-hand side) and uncertain (right-hand side) conditions to the center target location (gray circles).

Fig.2

Averaged across-trials pointer-tip velocity (A) and GEV (thick lines) and NGEV (thin lines) components of joint configuration variance, normalized per DOF, related to control of movement extent (B) and movement direction (C). Data are from one representative participant performing reaching to the central target under both uncertain (dashed line) and certain (solid line) target conditions. Vertical lines demarcate four phases during which the measures were averaged for uncertain (dashed) and certain (solid) conditions.

Fig.3

Components of joint configuration variance, averaged across subjects and combined across phases τ_accand τ_vel(early), and τ_decand τ_end (late) of Fig. 2. Results related to (A) control of movement extent and (B) control of movement direction for uncertain (GEV: open bar; NGEV: light gray fill) and certain (GEV: dark gray fill; NGEV: black fill) target conditions. Error bars depict standard errors. ‡ indicates significant differences.