Functional stabilization of unstable fixed points: human pole balancing using time-to-balance information.

Humans are often faced with tasks that require stabilizing inherently unstable situations. The authors explored the dynamics of human functional stabilization by having participants continually balance a pole until a minimum time criterion was reached. Conditions were manipulated with respect to geometry, mass, and characteristic "fall time" of the pole. Distributions of timing between pole and hand velocities showed strong action-perception coupling. When actions demonstrated a potential for catastrophic failure, the period of hand oscillation correlated well with the perceptual quantity "time to balance" (tau(bal) = theta/theta), but not other quantities such as theta and theta alone. This suggests that participants were using available tau(bal) information during critical conditions, although they may not have been attending to this type of perceptual information during typical, noncritical motions of successful performance. In a model analysis and simulation, the authors showed how discrete tau(bal) information may be used to adjust the parameters of a controller to perform this task.