| When humans perform closed loop control tasks like in upright standing or while balancing a stick, their behavior exhibits non-Gaussian fluctuations with long-tailed distributions. The origin of these fluctuations is not known. We investigate, if they are caused by self-organized critical noise amplification which emerges in control systems when an unstable dynamics becomes stabilized by an adaptive controller that has finite memory. More precisely, optimal on-line adaptation of a controller using finite memory generically leads to a critical situation which entails power-law-fluctuations. Starting from this theory, we formulate a realistic model of adaptive closed loop control by including constraints on memory and delays. To test this model, we performed psychophysical experiments where humans balanced an unstable target on a screen. It turned out, that the model reproduces the long tails of the distributions together with other characteristic features of the human control dynamics. Fine tuning the model to match the experimental dynamics identifies parameters characterizing a subject˘s control system which can be independently tested. Our results suggest, that the nervous system involved in closed loop motor control nearly optimally estimates system parameters on-line from very short epochs of past observations. The underlying principle also might have some potential for explaining non-Gaussian behavior of other systems with similar features in their dynamics. |
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