Interacting Noise Sources Shape Patterns of Arm Movement Variability in Three-Dimensional Space

Abstract

Reaching movements are subject to noise in both the planning and execution phases of movement production. The interaction of these noise sources during natural movements is not well understood, despite its importance for understanding movement variability in neurologically intact and impaired individuals. Here we examined the interaction of planning and execution related noise during the production of unconstrained reaching movements. Subjects performed sequences of two movements to targets arranged in three vertical planes separated in depth. The starting position for each sequence was also varied in depth with the target plane; thus required movement sequences were largely contained within the vertical plane of the targets. Each final target in a sequence was approached from two different directions, and these movements were made with or without visual feedback of the moving hand. These combined aspects of the design allowed us to probe the interaction of execution and planning related noise with respect to reach endpoint variability. In agreement with previous studies, we found that reach endpoint distributions were highly anisotropic. The principal axes of movement variability were largely aligned with the depth axis, i.e., the axis along which visual planning related noise would be expected to dominate, and were not generally well aligned with the direction of the movement vector. Our results suggest that visual planning-related noise plays a dominant role in determining anisotropic patterns of endpoint variability in three-dimensional space, with execution noise adding to this variability in a movement direction-dependent manner.

Contribution of execution noise to arm movement variability in three-dimensional space

“…The fact that the clockwise and counterclockwise sequences typically had their largest components of movement variability along the depth axis might seem surprising as by design these sequences did not require significant movement components in depth. However, this observation is consistent with the findings of Apker et al (2010), which were obtained under similar conditions. Moreover, these investigators showed that this trend was not related to the orientation of the terminal components of the average, executed movements in the frontal plane, which were largely orthogonal to the depth axis in that study and the present one (see Fig.…”

“…As with the single subject shown in Fig. 3, the average proportion of variance accounted for by the first eigenvector was generally large and very consistent across subjects and sequence types (mean: 77 Ϯ 4%), consistent with other studies of endpoint variability in 3-D space (Apker et al 2010;McIntyre et al 1997McIntyre et al , 1998. In contrast to the single subject data, there was little difference between the average amount of variance accounted for by the eigenvectors for the clockwise/counterclockwise sequences and the eigenvectors for the inward/outward sequences (78 and 76%, respectively).…”

“…As a result of this target arrangement, the second movement in a sequence was either largely contained with the same frontal plane as the first movement (in the case of clockwise/counterclockwise movements) or had a large component parallel to the depth axis (for inward/outward movements). This allowed a more comprehensive examination of the interactions between execution noise and planning noise than was previously attempted (Apker et al 2010 Individual trials began with the illumination of the starting position, which cued the subjects to visually align their fingertip with this position (visual feedback was always present during this epoch). After holding for 350 ms within a 4-cm window centered on the starting position, a T1 would be illuminated, cueing the first movement.…”

“…In support of this idea, sensory and execution noise have been shown to interact "near-optimally" in the temporal domain to determine overall levels of behavioral variability (Faisal and Wolpert 2009). In a recent study of unconstrained reaching movements to targets in the frontal plane, we (Apker et al 2010) argued that this may also be the case in the spatial domain. In particular, we showed that visually related planning noise played a dominant role in determining patterns of endpoint variability in three-dimensional (3-D) space, with execution noise contributing to this variability in a direction-dependent manner (i.e.…”

“…As in Apker et al (2010), planning and execution noise were accentuated by randomizing target positions from trial to trial and by switching the final target position during movement, which required rapid, online changes in movement planning and execution. The switching of targets was performed in such a way that the resulting sequences of two reaches were either both chiefly contained within a frontal plane ("frontal sequences") or involved an initial reach within the frontal plane and a second that was directed toward or away from the subject ("depth sequences").…”