Vertical torque responses to vestibular stimulation in standing humans

Abstract

Non-technical summary Galvanic vestibular stimulation (GVS) is a method for activating the human vestibular nerve with electricity. It induces sensations of head movement which cause sway and eye movements, and affect navigation. GVS is used here to demonstrate a novel vestibular reflex. Stimulation of standing subjects caused them to generate torque around a vertical axis, resulting in trunk rotation. Response magnitude and direction were systematically altered by head orientation in a manner consistent with GVS causing a sensation of head roll. This is relevant for balance control because vestibular information is only useful for fall prevention when interpreted in the context of head orientation. These findings therefore provide a method for investigating this neural transformation process. This can be used to diagnose deficiencies in the vestibular control of balance caused by ageing and/or neurological disease.Abstract The effects of electrical vestibular stimulation upon movement and perception suggest two evoked sensations: head roll and inter-aural linear acceleration. The head roll vector causes walking subjects to turn in a direction dependent on head pitch, requiring generation of torque around a vertical axis. Here the effect of vestibular stimulation upon vertical torque (T z ) was investigated during quiet stance. With the head tilted forward, square-wave stimuli applied to the mastoid processes evoked a polarity-specific T z response accompanied by trunk yaw. Stochastic vestibular stimulation (SVS) was used to investigate the effect of head pitch with greater precision; the SVS-T z cross-correlation displayed a modulation pattern consistent with the head roll vector and this was also reflected by changes in coherence at 2-3 Hz. However, a separate response at 7-8 Hz was unaffected by head pitch. Head translation (rather than rotation) had no effect upon this high frequency response either, suggesting it is not caused by a sense of body rotation induced by an inter-aural acceleration vector offset from the body. Instead, high coherence between medio-lateral shear force and T z at the same frequency range suggests it is caused by mechanical coupling to evoked medio-lateral sway. Consistent with this explanation, the 7-8 Hz response was attenuated by 90 deg head roll or yaw, both of which uncouple the inter-aural axis from the medio-lateral sway axis. These results demonstrate two vertical torque responses to electrical vestibular stimulation in standing subjects. The high frequency response can be attributed to mechanical coupling to evoked medio-lateral sway. The low frequency response is consistent with a reaction to a sensation of head roll, and provides a novel method for investigating proprioceptive-vestibular interactions during stance.

The effect of fear of falling on vestibular feedback control of balance

“…Extant published data in which posture was altered support the general hypothesis. For example, Reynolds () altered head pitch systematically and showed craniocentric modulation of ground reaction torque, including reversal of sign, at short and medium latency. Horslen et al.…”

“…With the head forward configuration (as used in our study) the coherence bandwidth of electrically evoked force plate responses linked to head orientation lies at 2–3 Hz (Dakin et al. ; Reynolds ). This bandwidth includes those responses modulated at short and medium latency by head pitch (Reynolds ).…”

“…For example, while the short‐latency response occurs only in muscles required for balance, it is currently unclear whether the lower extremity short‐ and medium‐latency responses are independent or comprise a coordinated balance response (Fitzpatrick and Day ; Mian et al. ; Reynolds ).…”

A dominant role for mechanical resonance in physiological finger tremor revealed by selective minimization of voluntary drive and movement

“…In our second approach to resolving the debate, we minimize central input by studying artificially evoked finger tremor using electrical stimulation of the relaxed extensor muscle. The technique is conceptually similar to the use of broad-band "stochastic" stimulation to probe the response of the vestibular system (Dakin et al 2007;Reynolds 2011;Scinicariello et al 2002). We use random broad-band noise stimulation to ensure that what we observe is not the response to an idiosyncratic component of the input signal.…”

Postural reorientation does not cause the locomotor after-effect following rotary locomotion

“…Yaw (i.e., rotation about the vertical axis) of each sensor was calculated by multiplying the recorded Euler angles by a rotational matrix according to the Tait-Bryan sequence (Reynolds 2011). Yaw data were low-pass filtered (0.2 Hz, 4th order, zero-phase butterworth) to remove step-by-step oscillations in angular signals, thus providing mean orientation of segments.…”