The Importance of Being in Touch

Abstract

This paper describes a series of studies resulting from the finding that when free floating in weightless conditions with eyes closed, all sense of one's spatial orientation with respect to the aircraft can be lost. But, a touch of the hand to the enclosure restores the sense of spatial anchoring within the environment. This observation led to the exploration of how light touch of the hand can stabilize postural control on Earth even in individuals lacking vestibular function, and can override the effect of otherwise destabilizing tonic vibration reflexes in leg muscles. Such haptic stabilization appears to represent a long loop cortical reflex with contact cues at the hand phase leading EMG activity in leg muscles, which change the center of pressure at the feet to counteract body sway. Experiments on dynamic control of balance in a device programmed to exhibit inverted pendulum behavior about different axes and planes of rotation revealed that the direction of gravity not the direction of balance influences the perceived upright. Active control does not improve the accuracy of indicating the upright vs. passive exposure. In the absence of position dependent gravity shear forces on the otolith organs and body surface, drifting and loss of control soon result and subjects are unaware of their ongoing spatial position. There is a failure of dynamic path integration of the semicircular canal signals, such as occurs in weightless conditions.

Specific Posture-Stabilising Effects of Vision and Touch Are Revealed by Distinct Changes of Body Oscillation Frequencies

“…A light touch is a potent stabilising stimulus, able to replace vision in subjects with impaired vestibular system ( 132 – 134 ). In the present study, like in several others [( 7 ), see Lackner ( 47 ) for a recent review], the haptic information arose from the index fingertip lightly touching the force pad and from the muscles active in this task. In many studies, the vertical force of the fingertip on the force pad representing the earth-fixed reference was generally well-below 1 N and was considered to be inadequate for mechanical stabilisation ( 60 , 135 , 136 ).…”

“…Further, reweighting of the proprioceptive information normally occurs, as attested by the reduction in the amplitude of the soleus muscle H-reflex during unperturbed stance ( 44 ). Moreover, the reflex excitability of the motor neurons of the leg muscles is decreased when the stance is stabilised by holding onto a solid frame ( 45 , 46 ) or by lightly touching fixed support ( 47 ). The role of proprioception can be more important when the balance is challenged ( 48 ) without vision.…”

Effects of postural threat on perceptions of lower leg somatosensory stimuli during standing

“…Participants stood barefoot while strapped to a rigid structure (see Figure 1C ) limiting the amount of anteroposterior (AP) sway ( Lackner, 2021 ). In addition, reducing the amount of AP sway allowed for a consistent stimulus across height conditions as postural threat can change sway amplitude and leaning ( Adkin and Carpenter, 2018 ) which could otherwise influence the amount of ankle rotation.…”

Postural instability in HIV infection: relation to central and peripheral nervous system markers

“…The apparently simple task of standing still on a balance platform requires multiple sensory inputs to maintain upright balance involving adjustment to perturbations and ignoring distractions competing with sensory information that can enhance stability (for review, [21,22]). Each of these influential factors is potentially subserved by different brain regions, and when acting together, they support the multifaceted processes contributing to upright stance [17,21].…”