On the origin of finger enslaving: control with referent coordinates and effects of visual feedback

Abstract

When a person tries to press with a finger, other fingers of the hand produce force unintentionally. We explored this phenomenon of enslaving during unintentional force drifts in the course of continuous force production by pairs of fingers of a hand. Healthy subjects performed accurate force production tasks by finger pairs Index-Middle, Middle-Ring, and Ring-Little with continuous visual feedback on the combined force of the instructed (master) fingers or of the non-instructed (enslaved) fingers. The feedback scale was adjusted to ensure that the subjects did not know the difference between these two, randomly presented, conditions. Across all finger pairs, enslaved force showed a drift upward under feedback on the master finger force, and master force showed a drift downward under feedback on the enslaved finger force. The subjects were unaware of the force drifts, which could reach over 50% of the initial force magnitude over 15 s. Across all conditions, the index of enslaving increased by about 50% over the trial duration. The initial moment of force magnitude in pronation-supination was not a consistent predictor of the force drift magnitude. These results suggest that during continuous force production enslaving increases with time, possibly due to spread of excitation over cortical finger representations or other mechanisms such as increased synchronization of firing of alpha-motoneurons innervating different compartments of extrinsic flexors. These changes in enslaving, interpreted at the level of control with referent coordinates for the fingers, can contribute to a variety of phenomena including unintentional force drifts.

The Nature of Finger Enslaving: New Results and Their Implications

“…The drifts in the other force variable could be up or down, but across all conditions, the relative amount of enslaved force increased. This increase in enslaving was large in magnitude (on the order of 25% over 10 s) as illustrated in Figure 6 (Abolins et al, 2020b;Hirose et al, 2020). Similar drifts in enslaving were observed during tasks performed by different finger pairs of the hand, Index + Ring, Middle + Ring, Index + Middle, and Ring + Little.…”

“…In stark contrast to this assumption, a series of recent studies have demonstrated unintentional (and unperceived!) drifts in enslaving that happen over very brief time intervals (Abolins et al, 2020b;Hirose et al, 2020;Ricotta et al, 2021). These observations have documented a large increase in indices of enslaving highly consistent across conditions and subjects.…”

“…When RC E < X 0 , neural factors start to contribute to the enslaved force production leading to relatively higher indices of enslaving. Usually, enslaving phenomena are studied within the latter range; that is why, many studies provide data in favor of the predominance of neural factors such as overlap of cortical projections and spread of cortical excitation to nearby areas (Abolins et al, 2020b;Hirose et al, 2020;Latash et al, 2002b;Schieber & Santello, 2004).…”

“…Such drifts can reach very large magnitude, up to 40% of the initial force level, with the subject being unaware of the force change. In recent studies (Abolins et al, 2020b;Cuadra et al, 2021;Hirose et al, 2020;Ricotta et al, 2021) with force production by a subset of fingers of a hand, showing visual feedback on the force produced by the instructed fingers (master fingers) led to a drift in the force produced by the enslaved fingers toward higher magnitudes. In contrast, when visual feedback on the enslaved finger force was provided (the subjects were unaware of the feedback manipulations due to adjustments in the feedback gain), the master finger force drifted to lower magnitudes.…”

“…I = index; M = middle; R = ring; L = little finger. Reproduced by permission fromAbolins et al (2020b).…”