Effects of unilateral stroke on multi-finger synergies and their feed-forward adjustments

Abstract

We explored the changes in multi-finger synergies in patients after a single cortical stroke with mild motor impairments. We hypothesized that both synergy indices and anticipatory synergy adjustments prior to the initiation of a self-paced quick action would be diminished in the patients compared to age-matched controls. The patients with history of cortical stroke, and age-matched controls (n=12 in each group) performed one-finger and multi-finger accurate force production tasks involving both steady-state and quick force pulse production. Finger interdependence (enslaving) and multi-finger synergies stabilizing total force were quantified. The stroke patients showed lower maximal finger forces, in particular in the contralesional hand, which also showed increased enslaving indices. Multi-finger synergies during steady-state force production were, however, unchanged after stroke. In contrast, a drop in the synergy index prior to the force pulse generation was significantly delayed in the stroke patients. Our results show that mild cortical stroke leads to no significant changes in multifinger synergies, but there is impairment in feed-forward adjustments of the synergies prior to a quick action, a drop in the maximal force production, and an increase in enslaving. We conclude that studies of synergies reveal two aspects of synergic control differentially affected by cortical stroke.

Keywords: anticipatory synergy adjustments; feed-forward control; hand; stroke; synergy.

Figure 1

A multi-slice representation of the damaged areas manually traced by a trained technician on T1 or T2 weighted brain images.

Figure 2

An illustration of the experimental setup (A) and typical feedback provided to the subjects for the single-finger ramp task (B) and for the accurate force pulse task (C).

Figure 3

The enslaving indices (EN) for the index (I), middle (M), ring (R), and little (L) fingers of the affected (filled bars) and less-affected (open bars) hands for the stroke patients. Average values are presented with standard error bars. Note the larger enslaving in the affected (contralesional) hand.

Figure 4

Time profiles of the averaged total force with SE shades computed across trials for typical subjects of each group during the quick force pulse task. The trials were aligned by the initiation of the force pulse (t₀). The dashed line shows the data for the left hand (affected side) of a stroke subject (T_peak = 0.22 s) and the solid line shows the data for the left hand of a CS (T_peak = 0.14 s).

Figure 5

The average across subjects time profiles of the synergy index (z-transformed, ΔV_Z) with standard error shades. The data for the stroke group are shown with the dashed line and light shade; the data for the control group (CS) are shown with the solid line and darker shade. The data for the contralesional hand of the stroke patients and the average data across both hands of CS are presented. Note the very similar steady-state values and the much earlier drop in ΔV_Z in preparation to the force pulse.