Fig. 8.

A: simulated magnitudes of the 3 PCs vs. frequency in the original object manipulation task, including signal-dependent noise, whose standard deviation is proportional to the mean force. The proportionality factor was chosen such that the grasp force magnitude at the task frequency of 0.5 Hz (vertical red dashed line) matches that measured in the original task. However, the noise is not band-limited to the low frequencies, since signal-dependent noise is associated with higher-frequency bands, and thus the grasp force mode magnitude is considerably larger than the measured one at all other frequencies. B: the simulation also exhibits much greater variability in terms of the sum of normal forces, making signal-dependent noise an unlikely source of the grasp force variability. C and D: if, on the other hand, the signal-dependent noise magnitude is scaled to the error observed in the sum of normal forces (D), the resultant grasp force magnitude does not match the experimentally observed one (C).