Detecting thresholds and interactions among displacement, acceleration, and velocity, for young adults on a horizontally translated platform

We are interested in determining the lower limits of detectability of the control system for standing posture and balance. Our Sliding Linear Investigative Platform For Analyzing Lower Limb Stability (SLIP-FALLS) allows us to study the psychophysical thresholds for movement detection in standing without the interference of vibratory cues. Three neurologically intact young adults were recruited to determine their initial detection thresholds to five displacements using our optimized two alternative forced choice, parameter estimation by sequential testing, protocol that varied acceleration according to the previous responses of the subject. A negative log-log relationship was seen between the acceleration thresholds and displacement length. A positive log-log relationship was seen between the measured peak velocities and displacement length. When plotted on the same graph, acceleration and velocity thresholds intersect just above the mean quiet standing center-of-pressure values for length and velocity, and just below mean frequency. It appears that for displacement lengths less than normal sway, acceleration is used for detection; but above normal sway, velocity is used