Date Published: May 31, 2018
Publisher: Public Library of Science
Author(s): Yang Sun Park, Kyung Koh, Hyun Joon Kwon, Okjin Lee, Jae Kun Shim, Rod K. Dishman.
Human central nervous system (CNS) undergoes neurological changes during the aging process, leading to declines in hand and finger functions. Previous studies have shown that the CNS can independently process multi-finger force control and moment of force control. However, if both force and moment control are simultaneously imposed by motor task constraints, the CNS needs to resolve competing interests of generating negative and positive covariances between fingers, respectively, which causes “conflict of interest or COI”. Here, we investigated how aging affects the CNS’s abilities to solve COI through a new experimental paradigm. Both elderly and young subjects performed a constant force production task using index and middle fingers under two conditions, multi-finger pressing with no COI and with COI. We found that the elderly increased variance of a virtual finger (VF: an imagined finger producing the same mechanical effect as both fingers together) in time-to-time basis (i.e. online control), while increasing covariance between individual fingers (IF) forces in trial-to-trial basis (i.e. offline control) with COI than no COI. Aging affects the CNS’s abilities to solve COI by deteriorating VF actions in online control and IF actions in offline control.
Our hands are one of the most versatile parts of the human body, and we use them to perform a variety of day-to-day activities such as eating, writing, driving, etc. During aging, the hands undergo many neurological and biomechanical changes, negatively impacting hand dexterity and consequentially the quality of life in the elderly [1, 2]. Previous studies have shown that aging is associated with the declines in strength , muscle mass , and finger dexterity  as well as degeneration of the central nervous system . However, the effect of aging on the CNS’s control mechanisms of hand function is still poorly understood, specifically regarding the association between aging and external task constraints that are imposed by motor tasks .
We analyzed multi-finger actions in a hierarchical manner as VF actions at the higher level and IF actions at the lower level (Fig 2). Several dependent variables were quantified from VF and IF actions using the HVD model (Fig 3).
Our study investigated aging-related changes in the hierarchical organization of multi-finger force control during two-finger pressing tasks, which induced COI problem to the CNS. We employed the HVD model which quantified several distinct aspects of hierarchically organized multi-finger actions of VF actions at the higher level and IF actions at the lower level. We hypothesized that the aging would be associated with the deterioration in VF and IF actions when an additional constraint as the COI problem was introduced to the CNS in our experiment. The hypothesis was largely confirmed by the experimental results. However, interestingly, aging affected VF and IF actions differently for online and offline controls. At the VF level, the elderly group showed deterioration of online VF control when an additional task constraint causing a COI problem was introduced, while the young group did not change their performance in online control of VF. Interestingly, at the IF level, the elderly group used smaller offline workspace (i.e. VarOFF) and smaller offline synergy (i.e. inverse of CovOFF)  when the additional constraint was introduced. However, the young group showed the unchanged workspace or synergy regardless of the task constraints.
During multi-finger actions, individual fingers show phenomena of mutual dependence due to the enslaving effects [40, 41]. Previously, the hypothetical CNS commands to individual fingers (i.e. finger force modes) have been calculated  from estimation of couplings between individual finger forces (i.e. finger enslaving, ). The analysis of our experimental data in the finger mode space might have provided additional insights into the actions fingers and interactions between them. However, application of the mode analysis to our study might have been challenging because our study employed two different tasks for NC and AC, and AC task is associated with a different set of task constraints and the finger force mode analysis depends on task constraints. A moment-stabilizing task requires certain levels of finger forces that are required for keeping the resultant moment of force as compared to the force-stabilizing task. In addition, a previous study has shown that the dynamic process of finger force production may be associated with the changes in the enslaving between fingers . Due to these analytical challenges, the analysis of our study was limited in the finger force space. However, if one assumes that the enslaving between fingers is constant in our study, the main findings of our study should still stay hold, specifically those differences observed between NC and AC tasks.