The sources of two components of variance: an example of multifinger cyclic force production tasks at different frequencies

In a multifinger cyclic force production task, the finger force variance measured across trials can be decomposed into two components, one that affects the combined force output (bad variance) and one that does not (good variance). Previous studies have found similar time patterns of bad variance and force rate leading to an approximately linear relationship between them. Based on this finding and a recently developed model of multifinger force production, we expected the bad variance during cyclic force production to increase monotonically with the rate of force change, both within a cycle and across trials at different frequencies. Alternatively, bad variance could show a dependence on task frequency, not on actual force derivative values. Healthy subjects were required to produce cyclic force patterns to prescribed targets by pressing on unidimensional force sensors, at a frequency set by a metronome. The task was performed with only the index finger, and with all four fingers. In the task with all four fingers, the good variance increased approximately linearly with an increase in the force magnitude. The bad variance showed within-a-cycle modulation similar to that of the force rate. However, an increase in the frequency did not lead to an increase in the bad variance that could be expected based on the natural relationships between action frequency and the rate of force change modulation. The results have been interpreted in the framework of an earlier model of multifinger force production where bad variance is a result of variance of the timing parameter. The unexpected lack of modulation of the bad variance with frequency suggests a drop in variance of the timing parameter with increased frequency. This mechanism may serve to maintain a constant acceptable level of variance under different conditions.