Visual information gain and the regulation of constant force levels.

Visual information is essential in human motor control, and especially in the continuous modulation of isometric force. The gain of visual feedback, that is, the amount of space used to represent change in force, has been shown to affect both the magnitude and time-dependent properties of variability in the force output. However, little is known regarding the interacting effects of visual gain and target force level on force variability and whether the effects of force level can be mediated by a gain that is adjusted to force level. We examined the effect of different types and levels of visual feedback gain and target force level (1, 2, 4, 8, and 12 N) on the magnitude (standard deviation, SD) and regularity (approximate entropy, ApEn) of isometric force variability. Young adults performed an isometric force task with high and low levels of constant (same gain level for all forces) and scaled (proportional to force level) gain. The magnitude of force variability increased exponentially as a function of force level once the SD was corrected for the limits of the display area. The time-dependent properties of force variability remained constant across force levels when gain was adjusted to force level. These findings suggest that the time-dependent properties of force variability are the result an interaction between visual feedback and task force level demands, while the increases in SD over force levels are primarily due to the invariant properties of human muscle and the motor system.