Processes underlying unintentional finger-force changes in the absence of visual feedback

Abstract

Previous reports show that the forces produced by the fingers of one hand drop exponentially over time in the absence of visual feedback on the forces. We study the force production by the index fingers of both hands with no visual feedback. Subjects produced a specified total force with a specific contribution from each finger by pressing on force sensors. We observed that in the absence of visual feedback: (1) The finger forces dropped with time by an amount proportional to the magnitude of the initial force. For low initial force values (<7 % of MVC of individual finger force), the finger forces showed an increase; (2) the total force (sum of finger forces) evolution showed similar features; (3) finger forces changed in a way that facilitated more equitable force production by the two fingers; (4) all the force-time changes resemble exponential functions with similar time constants (~15 s). We propose that two processes interact to produce these patterns. (1) RC back-coupling: The central nervous system defines referent coordinates (RCs) for the digit tips, and the difference between the referent and actual coordinates leads to force production. If actual coordinates are not allowed to move to referent ones, referent coordinates show a slow drift toward the actual ones, leading to a force drop. (2) Sensory adaptation: This process, possibly related to sensory receptor characteristics, leads to an increase in finger force. RC back-coupling provides a common account of this and other reported phenomena of hand force or position changes across transient, external perturbations.

On the nature of unintentional action: a study of force/moment drifts during multifinger tasks

“…The exploration of the magnitude of the ME and nME drifts (drifts within the UCM and ORT subspaces) has suggested no consistent differences in the magnitude of the ME and nME deviations. This result has important implications for an earlier hypothesis on the origin of the unintentional force drift (Ambike et al 2015(Ambike et al , 2016a) discussed later in this section. …”

“…Within the aforementioned hypothesis that slow drift originates in the UCM space (Ambike et al 2015), the observations of about equal ME and nME deviations suggest a very strong coupling between relaxation processes within the UCM and ORT spaces. This conclusion seems to be at odds with the basic assumptions of the UCM hypothesis and numerous observations of much larger variance within UCM compared with variance within ORT (reviewed in Latash et al 2007;.…”

“…For example, when a person is asked to walk toward an obstacle and step over it, repeating the task leads to a slow drop in the clearance between the foot and the obstacle, particularly pronounced for the trailing foot, and sometimes the trailing foot hits the obstacle (Heijnen et al 2012(Heijnen et al , 2014. Force production in isometric conditions leads to a slow drop in force when the subject is deprived of visual feedback; this is observed for moderate force levels that are not expected to lead to fatigue (Ambike et al 2015;Slifkin et al 2000;Vaillancourt and Russell 2002). These spontaneous changes in performance have been explored in only a handful of studies despite their potential importance for understanding the system for movement production demonstrated, for example, by studies that showed a significantly faster performance drift in patients with Parkinson's disease (Jo et al 2015;Vaillancourt et al 2001).…”

“…Note that the UCM-based framework is applicable even to apparently nonredundant tasks, such as force production by a single finger because all natural tasks are abundant at other, hidden, levels of analysis (e.g., at the level of muscle involvement). The interpretation is based on the idea that different stability within the UCM and ORT subspaces (Schöner 1995) is associated with different typical times of natural relaxation processes within these subspaces, fast in the more-stable ORT and slow in the less-stable UCM (Ambike et al 2015). Spontaneous slow drifts are expected to happen primarily within the UCM while perturbation-induced fast drifts are expected to originate in ORT.…”

“…A conceptually different explanation has been suggested (Ambike et al 2015;Jo et al 2015) based on an approach to human motor actions as reflections of natural motion of a physical system (including both neural and muscle elements and the external forces) toward its most stable state corresponding to the minimum of potential energy . According to this view, force production by a finger in isometric conditions is associated with setting a referent coordinate (RC) for the fingertip and a magnitude of apparent stiffness (k, which is also a reflection of shifts in spatial RCs for the participating muscles) (Ambike et al 2016b;Feldman 2015;Pilon et al 2007).…”