Proceedings of The Physiological Society
Cardiff University (2009) Proc Physiol Soc 17, PC03
Craniocentric vertical torque responses to vestibular stimulation
R. F. Reynolds1
1. School of Sports and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom.
Figure 1. Modulation of SVS-Torque Cross Correlation by Head Pitch Randomly varying current (SVS) was applied between the mastoid processes continuously for 80s. Vertical torque was measured concurrently. Subjects were asked to adopt a specific head pitch during each trial (+/- 45 deg). Head pitch was measured as the angle of Reid’s plane versus horizontal. The relationship between SVS and torque was then assessed by cross correlation.
Electrical stimulation of the vestibular nerve causes walking subjects to turn left or right, depending upon head pitch and stimulus polarity (1). Such responses require the stance leg to generate torque around a vertical axis. Whether this also occurs with both feet on the ground is open to question. Here I determine the effect of vestibular stimulation upon vertical torque (free Z moments) when standing quietly, and see whether the response is modulated by head position. Square-wave current stimuli (2mA, 3s) were applied to the mastoid processes in subjects standing on a force plate with eyes closed and feet together. With the head facing 60 degrees down and the anode electrode behind the right ear, small but significant torque oscillations were observed. These occurred with a latency of 100ms, reaching a peak of 0.05 Nm at 190ms, lasting around 1s. This response was accompanied by clockwise trunk rotation (0.3 deg/s peak). Reversing stimulus polarity caused an equal response in the opposite direction. With the head facing forwards the stimulus had no significant effect upon torque. Stochastic Vestibular Stimulation (SVS; 0-5Hz, 1.5mA RMS) was used to investigate the effect of head position in greater detail, allowing the response to be characterised in time and frequency using cross-correlations and cross-spectra, respectively (2). The cross-correlation was smoothly modulated by head pitch, being maximal with the head up and down, reaching a minimum with the head approximately level (-8 deg; see Fig 1). However, coherence analysis revealed two distinct frequency responses at 3 and 7 Hz. The former was heavily modulated by head pitch, whereas the latter was not. These data provide evidence of craniocentric vertical torque responses to vestibular stimulation when standing. The observed modulation of the response with head pitch is consistent with the CNS interpreting the stimulus as head rotation around a naso-occipital axis, due to canal activation (3). Previous research also shows the presence of an additional shorter latency response to vestibular stimulation due to otolith activation (4), resulting in lateral force responses. In the current experiment, mechanical coupling between lateral force and torque would explain the presence of the high frequency torque response unaffected by head position. In support of this, cross correlations between force and torque revealed significant peaks at positive lags. Hence, low frequency stimulation (0-5Hz) is best suited to assessing the influence of neck pitch upon the vestibular-torque response, since it primarily activates the canal response. The technique described here constitutes a new experimental tool with which to assess neck proprioception in the pitch axis. This could be used to quantify alterations in neck sensation caused by pathology or sensory illusions (5).
Where applicable, experiments conform with Society ethical requirements