The role of tactile sensation in online and offline hierarchical control of multi-finger force synergy

Abstract

The hand, one of the most versatile but mechanically redundant parts of the human body, must overcome imperfect motor commands and inherent noise in both the sensory and motor systems in order to produce desired motor actions. For example, it is nearly impossible to produce a perfectly consistent note during a single violin stroke or to produce the exact same note over multiple strokes, which we denote online and offline control, respectively. To overcome these challenges, the central nervous system synergistically integrates multiple sensory modalities and coordinates multiple motor effectors. Among these sensory modalities, tactile sensation plays an important role in manual motor tasks by providing hand-object contact information. The purpose of this study was to investigate the role of tactile feedback in individual finger actions and multi-finger interactions during constant force production tasks. We developed analytical techniques for the linear decomposition of the overall variance in the motor system in both online and offline control. We removed tactile feedback from the fingers and demonstrated that tactile sensors played a critical role in the online control of synergistic interactions between fingers. In contrast, the same sensors did not contribute to offline control. We also demonstrated that when tactile feedback was removed from the fingers, the combined motor output of individual fingers did not change while individual finger behaviors did. This finding supports the idea of hierarchical control where individual fingers at the lower level work together to stabilize the performance of combined motor output at the higher level.

Deficits in motor abilities for multi-finger force control in hemiparetic stroke survivors

“…The theory of "motor abundance" (Latash 2012), as compared to a "motor redundancy", is a key concept for the flexibility and multiformity. The fundamental assumption here is that the behaviors of the human motor system are structured so that abundant movement repertoires can be realized through seemly redundant motor effectors (Latash 2010) while providing flexibility and multiformity to the abundant motor system (Koh et al 2015). Greater flexibility represents the ability of the CNS to produce the same motor performance within a trial using more synergistic interactions between individual effectors, while greater multiformity quantifies the CNS' ability to generate the same motor performance across trials with greater interactions between individual effectors.…”

“…To assess these motor abilities, various submaximal force-matching tasks and computational methods have been developed (Lodha et al 2010(Lodha et al , 2012Naik et al 2011;Lindberg et al 2012;Kang and Cauraugh 2014). Recently, a hierarchical variability decomposition (HVD) model was developed for the purpose of quantifying specific motor abilities that contribute to the motor behaviors of a redundant motor system (Koh et al 2015). A fundamental concept of the HVD model is that the overall motor task error quantified as mean square error can be decomposed into three main contributors: systematic error, within-trial variability, and between-trial variability, which respectively quantify accuracy, steadiness, and reproducibility in the model.…”

“…The steadiness of the force within a trial can be estimated by calculating the variability of the force such as variance over 5 s, while the reproducibility across trials can be evaluated through the variability of the force over 10 trials. These three quantities are considered task-relevant because each of them can directly influence the overall motor performance (Todorov and Jordan 2002;Latash 2012;Koh et al 2015).…”

Age-related Changes in Multi-finger Synergy during Constant Force Production with and without Additional Mechanical Constraint

“…Moreover, within-task analysis showed significant differences in the elderly based on whether a constraint element was added or not, which can be attributed to the elderly having aging-related decreases in flexibility and the ability for the two fingers to cooperate in real-time (Koh et al, 2015). Moreover, appearances of interactions between the elderly and young for between-trial can be interpreted as the elderly using multiformity, namely, producing more diverse and complex forms of combinations with the two fingers, to a greater degree than the young under the NAC condition and having less reproducibility for repeating the motor performance.…”

“…Motor synergy is analyzed at two levels, within-trial and betweentrial, to show diverse aspects of control (Koh et al, 2015;Scholz, Kang, Patterson, & Latash, 2003). Within-trial analysis was used to study the moment-to-moment interactions of basic elements for successful task performance (Tuller, Turvey, & Fitch, 1982), while between-trial analysis was used to study various interactions of basic elements during repeat performances of the same task.…”