Psychopharmacology (2004) 177: 217–223 DOI 10.1007/s00213-004-1933-4 ORIGINAL INVESTIGATION Mark L. Latash . Jae Kun Shim . Vladimir M. Zatsiorsky Is there a timing synergy during multi-finger production of quick force pulses? Received: 11 March 2004 / Accepted: 5 May 2004 / Published online: 22 July 2004 # Springer-Verlag 2004 Abstract 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. Keywords Finger . Force . Human . Synergy . Timing Introduction The famous problem of motor redundancy (Bernstein 1967; Turvey 1990a, b) has been studied using a variety of tasks and multi-effector systems including force produc- tion by fingers of the human hand acting in parallel (Li et al. 1998; Latash et al. 2002a). When several fingers produce a time profile of the total force, F TOT (t), that needs to reach a particular magnitude at a particular time, individual finger force profiles, F i (t), may show different covariation across several trials at the same task. Imagine that in one trial a finger produces a higher force as compared with its typical average contribution to the total force. By doing this, it introduces an error into F TOT (t). Other fingers may continue producing their typical average forces, or they may also produce higher forces that would amplify the original error in F TOT (t), or they may produce lower forces to compensate for the error introduced by the first finger. In earlier studies, the term synergy has been used only with respect to the last case, i.e., when individual fingers show error compensation with respect to a particular performance variable such as F TOT (t) (Latash et al. 2001, 2002a; Scholz et al. 2002). Imagine now that a finger in one trial produces a force profile, which differs from its typical contribution not in magnitude but in timing, i.e., it happens a bit earlier or a bit later. This timing error may lead to errors in the timing of F TOT (t). Similarly to the earlier example, one may ask the question whether other fingers adjust the timing of their force pulses to (partly) compensate for the effects of the original error on the timing of F TOT (t). In other words, are there timing synergies among fingers that stabilize a certain timing feature of F TOT (t), for example the time of its peak value, in addition to synergies in the magnitudes of finger forces? A number of studies have shown that fingers demon- strate negative force covariation across trials, i.e., syner- gies with respect to the magnitude of total force, M. L. Latash (*) . J. K. Shim . V. M. Zatsiorsky Department of Kinesiology, The Pennsylvania State University, University Park, PA , 16802, USA e-mail: mll11@psu.edu Fax: +1-814-8634424