Variability and noise in continuous force production.

In the present 3 experiments, the authors examined the hypothesis, derived from information theory, that increases in the variability of motor responses result from increases in perceptual-motor noise. Three different groups of participants (Ns = 10, 9, and 10, respectively, in Experiments 1, 2, and 3) produced continuous isometric force under either low, intermediate, or high target force levels. When considered together, the results showed that force variability (SD) increased exponentially as a function of force level. However, an index of information transmission (M/SD), as well as measures of noise in both the time (approximate entropy) and the frequency (power spectrum) domains, changed according to an inverted-U-shaped function over the range of force levels. The findings provide further evidence that increased information transmission is related to increases, and not to decreases, in the noisiness of the structure of force output.