Haptic perceptual intent in quiet standing affects multifractal scaling of postural fluctuations

Research on dynamic touch has shown that when a rod strapped to the shoulders is wielded via axial rotations, flexions-extensions, and lateral bending of the trunk, participants can selectively perceive whole rod length and partial rod length (e.g., a leftward segment) with precision comparable to wielding by hand (Palatinus, Carello & Turvey, 2011). The present research addressed whether this haptic ability is preserved in quiet standing, when postural control is limited to center of pressure (COP) fluctuations at the mm/ms scale, and, if so, whether the intentions ("perceive partial," "perceive whole") are distinguishable within the fluctuations. Given standard manipulations of rod length and attached mass, participants provided significantly distinct, appropriately scaled, whole and partial estimates of rod length. COP displacement time series were subjected to multifractal, detrended fluctuation analysis. The resultant spectrum of fractal scaling exponents for gradually different-sized fluctuations revealed that "perceive partial" was manifest as larger exponents for progressively smaller fluctuations than "perceive whole." Our results indicate (a) that the significant mechanical variables for haptically perceiving object extent are available in the small scale of normal body sway, and (b) that these seemingly "passive" movements reflect the intention of the perceiver.