What the "broken escalator" phenomenon teaches us about balance.

Gait adaptation is crucial for coping with varying terrain and biological needs. It is also important that any acquired adaptation is expressed only in the appropriate context. Here we review a recent series of experiments that demonstrate inappropriate expression of gait adaptation. We show that a brief period of walking onto a platform previously experienced as moving results in a large forward sway aftereffect, despite full awareness of the changing context. The adaptation mechanisms involved in this paradigm are extremely fast, just 1-2 discrete exposures to the moving platform result in the motor aftereffect. This aftereffect occurs even if subjects deliberately attempt to suppress it. However, it disappears when the location or method of gait is altered, indicating that aftereffect expression is context dependent. Conversely, making gait self-initiated increases sway during the aftereffect. This aftereffect demonstrates a profound dissociation between knowledge and action. The absence of generalization suggests a relatively simple form of motor learning, albeit involving high-level processing by cortical and cerebellar structures.