Finger force changes in the absence of visual feedback in patients with Parkinson’s disease

Abstract

Objectives We investigated the unintentional drift in total force and in sharing of the force between fingers in two-finger accurate force production tasks performed without visual feedback by patients with Parkinson’s disease (PD) and healthy controls. In particular, we were testing a hypothesis that adaptation to the documented loss of action stability could lead to faster force drop in PD. Methods PD patients and healthy controls performed accurate constant force production tasks without visual feedback by different finger pairs, starting with different force levels and different sharing patterns of force between the two fingers. Results Both groups showed an exponential force drop with time and a drift of the sharing pattern towards 50:50. The PD group showed a significantly faster force drop without a change in speed of the sharing drift. These results were consistent across initial force levels, sharing patterns, and finger pairs. A pilot test of four subjects, two PD and two controls, showed no consistent effects of memory on the force drop. Conclusions We interpret the force drop as a consequence of back-coupling between the actual and referent finger coordinates that draws the referent coordinate towards the actual one. The faster force drop in the PD group is interpreted as adaptive to the loss of action stability in PD. The lack of group differences in the sharing drift suggests two potentially independent physiological mechanisms contributing to the force and sharing drifts. Significance The hypothesis on adaptive changes in PD with the purpose to ensure stability of steady states may have important implications for treatment of PD. The speed of force drop may turn into a useful tool to quantify such adaptive changes.

The synergic control of multi-finger force production: stability of explicit and implicit task components

“…Links between this phenomenon and the stability of produced mechanical variables have been hypothesized earlier (Ambike et al 2015, 2016) and supported by observations in patients with Parkinson’s disease (Vaillancourt et al 2001; Jo et al 2016). In earlier studies, this phenomenon was observed only for the explicitly instructed normal force component in pressing tasks.…”

“…Their existence has been supported by the phenomenon of anticipatory synergy adjustment (ASA; Olafsdottir et al 2005), because ASAs occur when performance – and therefore NV1 – remains unchanged, so another control process should be responsible for the ASAs, possibly associated with NV2. Recent observations in neurological patients have also shown dissociation between synergy indices and ASAs: while patients with subcortical disorders show both reduced synergy indices and smaller ASAs (Park et al 2012, 2013; Jo et al 2015), patients with mild cortical stroke show unchanged synergy indices and significantly smaller ASAs (Jo et al 2016). …”

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

“…However, within our scheme, the drift in performance is not due to deterioration of memory on RC but to its natural drift during continuous task performance without salient sensory feedback. In fact, a pilot study has shown that resting for a comparable time does not lead to a force drop (Jo et al 2016) suggesting that the subjects do not show force drift because they forget the RC (or the corresponding force magnitude).…”

The nature of constant and cyclic force production: unintentional force-drift characteristics

“…For various quantities (means and amplitudes for the cyclic F-task, and mean force for the continuous F-task), exponential functions of the form F(t) = a × e −t/τ + c were fit to the data, where t is time, and τ is the time constant (seconds) of the exponential change in F. This particular functional form was motivated by earlier studies (e.g., Ambike et al 2015;Jo et al 2015) as well as by the typical exponential forms of relaxation processes in physical systems. Note that at the time of feedback removal (t = 0), the variable value is given by F(0) = a + c, and at the steady state (t = ∞), it is F(∞) = c. Thus, a estimates the net drop in F. These fits were performed for data averaged across subjects.…”

“…Indeed, after the original publication, only a few studies explored this phenomenon (Vaillancourt et al 2001, Vaillancourt and Russell 2002, Shapkova et al 2008, and it was also invoked in studies of "slacking", a phenomenon observed in stroke survivors who show a drop in effort when helped by an external device (Reinkensmeyer et al 2009). The original interpretation that the phenomenon reflected a limitation in the working memory was indirectly supported by a study of patients with Parkinson's disease (Vaillancourt et al 2001) and has not been challenged until recently Jo et al 2015).…”