Abstract
Behavioral stability partially depends on the variability of net outcomes by means of the co-varied adjustment of individual elements such as multi-finger forces. The properties of cyclic actions affect stability and variability of the performance as well as the activation of the prefrontal cortex that is an origin of subcortical structure for the coordinative actions. Little research has been done on the issue of the relationship between stability and neuronal response. The purpose of the study was to investigate the changes in the neural response, particularly at the prefrontal cortex, to the frequencies of isometric cyclic finger force production. The main experimental task was to produce finger forces while matching the produced force to sine-wave templates as accurately as possible. Also, the hemodynamics responses of the prefrontal cortex, including oxy-hemoglobin concentration (ΔHbO) and the functional connectivity, were measured using functional near-infrared spectroscopy. The frequency conditions comprised 0.1, 1, and 2 Hz. The uncontrolled manifold (UCM) approach was applied to compute synergy indices in time-series. The relative phase (RP), the coefficient of variation (CV) of the peak and trough force values were computed as the indices of performance accuracy. The statistical parametric mapping (SPM) was implemented to compare the synergy indices of three frequency conditions in time-series. A less accurate performance in the high-frequency condition was caused not by the RP, but mainly by the inconsistent peak force values (CV; p < 0.01, = 0.90). The SPM analysis revealed that the synergy indices were larger in the low-frequency than in high-frequency conditions. Further, the ΔHbO remained unchanged under all frequency conditions, while the functional connectivity decreased with an increase in the frequency of cyclic force production. The current results suggested that the concurrent activation of the prefrontal region mainly depends on the frequency of cyclic force production, which was associated with the strength of stability indices and performance errors. The current study is the first work to uncover the effect of frequency on the multi-finger synergies as to the hemodynamic response in the prefrontal cortex, which possibly provides a clue of the neural mechanism of synergy formation and its changes.
Highlights
Exploring behavioral stability is a common performance goal of many daily life activities such as holding and moving a glass of water while preventing spilling over, as well as experimental tasks performed in a laboratory
If most of the force variance of individual effectors is confined within the uncontrolled manifold (UCM) (i.e., VUCM) and if the variance observed in the ORT space (i.e., VORT) is relatively small, we may conclude that the net force is stabilized by the co-varied adjustment of individual forces by the effectors
Two-way repeatedmeasured analysis of variance (ANOVA) performed with the factors of frequency and phase showed no significant main effects and factor interaction effects on the relative phase (RP)
Summary
Exploring behavioral stability is a common performance goal of many daily life activities such as holding and moving a glass of water while preventing spilling over, as well as experimental tasks performed in a laboratory. If most of the force variance of individual effectors is confined within the UCM (i.e., VUCM) and if the variance observed in the ORT space (i.e., VORT) is relatively small, we may conclude that the net force is stabilized by the co-varied adjustment of individual forces by the effectors This phenomenon was further confirmed by studies using external perturbation (Scholz et al, 2007; Krishnan et al, 2011, 2012), where the understanding of experimental observation is accompanied by the definition of stability in classical mechanics (Taga, 1995; Hasan, 2005; Bruijn et al, 2013). Stable performance, to some extent, is the basic premise of accurate and precise performance; there are diverse approaches and standards for the quantification of stability and accuracy (Hasan, 2005; Winter, 2009; Kim et al, 2018)
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