Free Energy Principle in Human Postural Control System: Skin Stretch Feedback Reduces the Entropy

Abstract

Human upright standing involves an integration of multiple sensory inputs such as vision, vestibular and somatosensory systems. It has been known that sensory deficits worsen the standing balance. However, how the modulation of sensory information contributes to postural stabilization still remains an open question for researchers. The purpose of this work was to formulate the human standing postural control system in the framework of the free-energy principle, and to investigate the efficacy of the skin stretch feedback in enhancing the human standing balance. Previously, we have shown that sensory augmentation by skin stretch feedback at the fingertip could modulate the standing balance of the people with simulated sensory deficits. In this study, subjects underwent ten 30-second trials of quiet standing balance with and without skin stretch feedback. Visual and vestibular sensory deficits were simulated by having each subject close their eyes and tilt their head back. We found that sensory augmentation by velocity-based skin stretch feedback at the fingertip reduced the entropy of the standing postural sway of the people with simulated sensory deficits. This result aligns with the framework of the free energy principle which states that a self-organizing biological system at its equilibrium state tries to minimize its free energy either by updating the internal state or by correcting body movement with appropriate actions. The velocity-based skin stretch feedback at the fingertip may increase the signal-to-noise ratio of the sensory signals, which in turn enhances the accuracy of the internal states in the central nervous system. With more accurate internal states, the human postural control system can further adjust the standing posture to minimize the entropy, and thus the free energy.

Age-Related Changes in Standing Balance in Preschoolers Using Traditional and Nonlinear Methods

“…In contrast, nonlinear measures suggest that 5-year-old children’s standing balance showed decreased variability, better balance performance, and more intermittency compared with 4-year-old children. These seemingly conflicting results can be explained by the free energy principle ( Friston, 2010 ; Hur et al, 2019 ). The free energy principle is called a minimum entropy principle; a low entropy means that the body posture is relatively predictable ( Hur et al, 2019 ).…”

“…These seemingly conflicting results can be explained by the free energy principle ( Friston, 2010 ; Hur et al, 2019 ). The free energy principle is called a minimum entropy principle; a low entropy means that the body posture is relatively predictable ( Hur et al, 2019 ). In fact, standing still is impossible for humans since it requires excessive efforts; little control efforts are made with flexible postural sway within a specific range.…”

Review of the Upright Balance Assessment Based on the Force Plate

“…Safety during the assessment is very important [ 18 ], and safety systems are implemented to avoid falls caused by loss of balance [ 57 , 58 , 59 ]. It is worth noting that the tactile cues imparted by the seat belt may affect the assessment of postural balance [ 60 ]. Previous research reports claim that when the subject’s body is briefly and lightly touched with objects outside the body, the postural sway can be reduced [ 61 , 62 ].…”

Integrating Tactile Feedback Technologies Into Home-Based Telerehabilitation: Opportunities and Challenges in Light of COVID-19 Pandemic

“…In addition, studies have shown the ability of participants to accurately produce motion according to haptic stimuli provided by a skin stretch device (Bark et al, 2010 ; Stanley and Kuchenbecker, 2012 ; Guinan et al, 2013b ; Norman et al, 2014 ; Chinello et al, 2018 ; Pezent et al, 2019 ; Smith et al, 2020 ), including in gaming applications (Guinan et al, 2012 , 2013a ). Skin stretch feedback encoding the velocity of postural sway along the anterior-posterior direction enhanced standing balance with perturbed sensory systems (removed vision and unreliable vestibular systems) in healthy young adults compared with conditions without skin stretch feedback (Hur et al, 2019 ).…”