Effortful touch by the hand is essential to engaging with and perceiving properties of objects. The temporal structure of whole-body coordination must reflect the prospective control that provides for both the engagement with and perception of properties of the hefted objects. In the present study, we found signatures of multifractality in the time series of fluctuations in Euclidean displacement in the participants' center of pressure (CoP) as they hefted weighted objects to perceive their heaviness and length. Comparisons of widths of the multifractal spectrums of CoP series with 32 Iterative Amplitude Adjusted Fourier Transform (IAAFT) surrogates provided evidence for multiplicative-cascade dynamics and interactivity across scales, through the continuous t-statistic comparing the original and surrogate widths (tMF). After controlling for the linear properties of CoP series and their interactions with the informational variable (i.e., the moment of inertia of the hefted objects), regression modeling of unsigned error in judgments of heaviness and length revealed that the multifractal evidence for nonlinearity (tMF) significantly influenced unsigned error. The two indicators showed opposite, task-specific effects on accuracy: accuracy in judgments of heaviness and length decreased and increased, respectively, with greater tMF. These findings suggest that multiplicative-cascade dynamics in posture play a role in prospective coordination during the engagement with objects and perception of their properties via effortful touch by the hand. Future work may elucidate how constraint(s) on exploratory kinematics influence the multifractal behavior in such suprapostural perceptual tasks as effortful touch.
Keywords: Biotensegrity; Center of pressure; Dynamic touch; Effortful touch; Haptic perception; Multifractality; Proprioception; Psychophysics; Tensegrity.
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