Is there a timing synergy during multi-finger production of quick force pulses?

We studied whether characteristics of individual finger force profiles covaried across repetitions of a quick force pulse production task to stabilize the required magnitude and timing of the peak force. Subjects produced series of quick force pulses by pressing with all four fingers of the right hand on force sensors under the instruction to keep the magnitude of the peak of total force at 15 N and reach the force peaks at prescribed times. Individual finger force pulses were then reshuffled across trials to create a surrogate data set. The surrogate data set showed a lower average peak force with a larger dispersion. This finding has been interpreted as pointing at predominantly negative covariation among finger force pulses in the actual data that stabilized the required magnitude of the total force, a force synergy. The difference between the actual and surrogate data sets was significant early into the pulse time, starting about 40 ms after the pulse initiation. This finding points at a central nature of the negative covariation without a major role played by visual or proprioceptive feedback. In contrast, the surrogate data set showed smaller dispersion of the timing of the total peak force, suggesting positive covariation of the timings of individual finger force pulses in the actual data interpreted as the lack of a timing synergy. These results have been confirmed with principal component (PC) analysis. The first PC for the timing of the individual finger peak forces accounted for over 90% of the total variance for the actual data set and for under 40% of the total variance for the surrogate data set. The fourth PC for the magnitudes of the finger forces accounted for under 4% of the total variance for the actual data set and for over 15% of the variance for the surrogate data set. The data are interpreted within the uncontrolled manifold hypothesis; they support the hierarchical control scheme suggested by Schöner.