Proprioception & Vestibular Input
For the Proprioception and the Vestibular Input, we plan to use vibrotactile transducers to stimulate mechanoreceptors placed on the muscles and otolith organs of the vestibular system. These transducers offer the advantage to turn sound waves into mechanical vibrations that can be transferred to the body to induce the perception of limbs movement and alter the sense of balance. Indeed, it well known from muscle vibration experiments that the senses of position and movement of the limbs (the kinesthesia) can be altered.
Muscles: Cutaneous receptors located in the skin around finger, elbow, ankle, and knee joints provide exteroceptive and proprioceptive information. Similar to muscle spindles, these receptors both encode movement kinematics and show directional sensitivity (Lee et al, 2013). When a vibration of approximately 70–100 Hz is applied to a tendon of the biceps or triceps muscle of a physically immobile limb that is obstructed from view, a sensation of arm displacement is generated (Naito et al. 1999). Notably, increasing the vibration frequency increases the velocity of the perceived illusory movement (Roll and Vedel, 1982). When the vibratory stimulation is interrupted the spindle’s discharge decreases, inducing the perception that the limb is returning toward its original position.
Otolith organs: The otoliths (the utricular and saccular maculae) are the gravity sensing organs of the inner ears. Air-conducted sounds and bone-conducted vibration have been proposed as two effective methods to evoke vestibular myogenic potentials originating from selective activation of the otolithic end organs (Manzari et al, 2010): bone-conduced vibration at frequency of 500 Hz produces consistent craniocentric whole-body responses in standing subjects (Curthoys & Grant, 2015; Welgampola & Day, 2006). The characteristics of the response are compatible with it being mediated by vestibular input, although the sway direction is different to that evoked by galvanic vestibular stimulation. This suggests that different patterns of input are produced by the two types of stimulation, possibly involving different proportions of afferents from the otoliths and semicircular canals. If so, bone-conducted sound, used either in isolation or in combination with GVS, may enable investigation of hitherto unexplored aspects of vestibular function in intact freely behaving human subjects.