Large Postural Sways Prevent Foot Tactile Information From Fading: Neurophysiological Evidence

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.

Cortical facilitation of somatosensory inputs using gravity-related tactile information in patients with bilateral vestibular loss

“…In particular, the body sways can be regulated within a small area by reducing the amplitude of the centre of pressure displacement (not recorded in the present study) and increasing its frequency. Such tactile exploration, by changing the pressure onto the ground with the feet, likely contributes to improve the signal-to-noise ratio of sensory cues and to determine the limit of stability (Fabre et al 2021; Latash et al, 2003; Murnaghan et al, 2011). The increase in plantar sole afferents through this strategy could be particularly important in the absence of vestibular information.…”

“…Electroencephalographic (EEG) activity was recorded continuously from 64 Ag/AgCl surface electrodes embedded in an elastic cap (10-20 system, BioSemi ActiveTwo system: BioSemi, Netherlands). Specific to the BioSemi system, "ground" electrodes were replaced by As in previous studies (Duysens et al, 1995, Fabre et al, 2021Staines et al 2000), the largest responses to foot stimulations were observed at electrode Cz (vertex). The SEPs were recorded at this electrode which overlays the sensorimotor feet regions located on the inner surface of the longitudinal fissure.…”

Overpressure on fingertips prevents state estimation of the pen grip force and movement accuracy

“…For instance, slow adapting afferent fibers (Merkel and Ruffini) are known to decrease their instantaneous firing frequency throughout a sustained stimulus (e.g., normal force exerted on the skin surface) while the rapidly adapting type I and II (Meissner and Pacinian corpuscle) afferents only respond to brisk mechanical transients (Knibestöl, 1975;Vedel and Roll, 1982;Vallbo et al, 1995;Trulsson, 2001;Kennedy and Inglis, 2002). Combining brain imaging and behavioral data, a decreased somatosensory transmission was observed, from the periphery to the cortical areas, this was provoked by skin compression on the foot sole when standing, supporting a heavy weight (Handrigan et al, 2012;Lhomond et al, 2016) or when pressure is increased within a small surface area (Fabre et al, 2021).…”

“…Assuming that this manipulation is effective, we expected drawing accuracy to be altered similarly in both overpressures conditions as compared to the baseline condition (i.e., natural pen grip). The decrease in the somatosensory evoked potentials (P50N80 SEPs) in response to electric stimulation of the tactile afferents is considered as the neural signature of an attenuated sensory processing (Lhomond et al, 2016: Fabre et al, 2021. According to the seminal study of Desmedt and Robertson (1977), the SEP in response to finger stimulation originates, for the early component (<60 ms), from the primary somatosensory cortex (SI) and is highly representative of the stimulus characteristics (e.g., intensity, frequency).…”

Partial Unweighting in Obese Persons Enhances Tactile Transmission From the Periphery to Cortical Areas: Impact on Postural Adjustments

“…The interaction between the plantar sole and the support is essential for determining body orientation in space or more particularly with respect to the support on which the feet are resting (e.g., Morasso and Schieppati, 1999 ; Carriot et al, 2004 ; Mouchnino and Blouin, 2013 ). Tactile receptors embedded within the skin of the foot are able to detect very small changes in force relative to the person's weight when standing motionless (e.g., Morasso and Schieppati, 1999 ; Fabre et al, 2021 ). This allows the brain to identify our balance state prior to gait initiation, for example, even though no other information from visual, vestibular, or proprioceptive origin is present (Mouchnino and Blouin, 2013 ).…”