Abstract
Cutaneous foot receptors are important for balance control, and their activation during quiet standing depends on the speed and the amplitude of postural oscillations. We hypothesized that the transmission of cutaneous input to the cortex is reduced during prolonged small postural sways due to receptor adaptation during continued skin compression. Central mechanisms would trigger large sways to reactivate the receptors. We compared the amplitude of positive and negative post-stimulation peaks (P50N90) somatosensory cortical potentials evoked by the electrical stimulation of the foot sole during small and large sways in 16 young adults standing still with their eyes closed. We observed greater P50N90 amplitudes during large sways compared with small sways consistent with increased cutaneous transmission during large sways. Postural oscillations computed 200 ms before large sways had smaller amplitudes than those before small sways, providing sustained compression within a small foot sole area. Cortical source analyses revealed that during this interval, the activity of the somatosensory areas decreased, whereas the activity of cortical areas engaged in motor planning (supplementary motor area, dorsolateral prefrontal cortex) increased. We concluded that large sways during quiet standing represent self-generated functional behavior aiming at releasing skin compression to reactivate mechanoreceptors. Such balance motor commands create sensory reafference that help control postural sway.
Highlights
Sensory perception and motor behavior are closely interrelated
To assess whether the postural sway amplitude altered the transmission of plantart sole cutaneous inputs to the cortex, we compared the amplitude of the P50N90 between the Large and Small sways
The amplitude of the P50N90 was greater during Large sways than Small sways (z = 3.10, P = 0.0019, Wilcoxon test, Fig. 2B)
Summary
Sensory perception and motor behavior are closely interrelated This has been well demonstrated in the seminal study by Hellebrandt (1938), showing that, even when standing still, the foot sole undergoes pressure variations due to postural sways that stimulate cutaneous receptors (Morasso and Schieppati 1999). Large sways would represent a functional response of the postural system to reactivate sensory inputs that inform the body about an equilibrium state. This hypothesis is consistent with Carpenter et al.’s (2010) findings, showing that the displacement of the center of pressure (CP) mainly increases when the body center of mass (CM) is prevented from moving freely (see Murnaghan et al 2011). In Carpenter et al (2010), the increase in CP displacement, while preventing movement of the body, suggests that the goal of the CP displacements was mainly to stimulate plantar sole cutaneous receptors in the absence of sensory afference from other sensory systems (e.g., vestibular, visual, and proprioceptive)
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