Super-additive multi-dimensional self-balancing deficit: implication for flight mishaps and falling.

We report super-additive impairment in participants using a joystick to stay upright and avoid crashing while riding in a motion simulator programmed to topple as an inverted pendulum away from an upright direction of balance about two axes (2D) relative to 1D. The 2D test configuration provided participants with full vestibular/somatosensory cues about tilt relative to gravity. The 2D impairment was a) only marginally mitigated by vision relative to blindfolded conditions, and b) worse than a 1D decrement in a 0-g analog condition which precluded gravitational tilt cues by having participants balance in the horizontal roll plane where they were always 90 degrees from the vertical. With practice in the upright 2D balance task over two test days, the deficit declined nearly to the upright 1D level of proficiency. The outsized effect of expanding self-balancing from 1D to 2D and the surprisingly insubstantial visual recovery justify reconsideration of prevailing servo-like models and methods for assessing risk and preventing loss of life from aircraft mishaps, falls, and other tasks involving guidance of an unstable plant. As an alternative explanation, we expand a serial motor decision-making model originally developed for 1D balancing.