Innovative strategies for investigating postural control during threshold translations

An everyday activity like standing is automatically carried out by the human postural control system and most often does not require conscious attention. Normal postural sway is imperceptible and might be providing a dither signal for postural control. The innovative strategies presented here yield a better understanding of the mechanism of postural control under the influence of a variety of external stimuli. Adaptive 2-Alternative-Forced-Choice (2AFC) experiments were conducted to determine the psychophysical detection thresholds for short anterior-posterior perturbations of a platform on which the subject stood. However, these psychophysical procedures face a fundamental problem of discriminating guesses from the correct responses. We have implemented a coherent solution to this problem by presenting a novel classification method that compares biomechanical and psychological responses. Our immediate focus was to develop a new biomechanical detection threshold experiment to see whether a change in the Anterior-Posterior Center of Pressure (APCoP) corresponds to a presentation of a stimulus, independent of a psychological response or choice. The new design of a threshold iteration scheme was implemented that could automatically account for guesses without requiring a psychophysical response (e.g., a button press) from the subject, unlike traditional psychophysical techniques.;It was also important to assess the behavior of the postural control system when subjected to sinusoidal translational stimuli near the body's sway frequency and amplitude. In our Sine Lock experiments, an induced sinusoidal platform perturbation locked the subject's natural sway pattern at the frequency of the perturbation. These experiments were implemented to find the psychophysical thresholds to the perception of a sinusoidally induced sway. The aim of this latter study was to use the envelope detection and phase analysis of a biomechanical response (APCoP) to analyze the behavior of the human postural control system. For this purpose, we used a phase-synchronized amplitude demodulation technique. This analysis not only successfully detected the response signal envelopes, but also provided critical phase lag information to characterize the behavior of the postural control system.