Investigating Coordinate Transform processes with Electrical Vestibular Stimulation Raymond Reynolds Sports and Exercise Sciences College of Life and Environmental Sciences

Coordinate transform process for balance Same vestibular signal can require different balance response For vestibular reflexes to be effective, must compute head-on-feet transform Can use vestibular stimulation to investigate this process

How does GVS work? Day & Fitzpatrick, J Phys 2005 Head roll around a naso-occiptial axis

Fitzpatrick et al, Current Biology 2006 Effect of GVS on walking trajectory

Can GVS induce vertical torque responses in standing subjects? Can this be used to assess neck proprioception in pitch? Causal link between aberrant transform processes and postural instability? Vertical torque response during stance

Effect of Vestibular Stimulation Upon Vertical Torque

Response to Square Wave Stimuli Small but significant torque and trunk yaw response Affected by head position

Response to Stochastic Stimulation SVS response smoothly modulated by head pitch.

Response to Stochastic Stimulation Correlation Coefficient Time (ms) Head Pitch (degrees)

Variation with Head Pitch Confirms mechanism of GVS: Naso-occiptial rotation

Can now assess changes in response bias & certainty Bias Certainty Are such changes associated with postural instability/fall risk?

Frequency domain reveals two responses Two rotation vectors? Lateral response masquerading as a rotation? Coherence

Otolith and Canal responses to GVS Cathers et al, J Phys 2005

Coupling between lateral sway and torque

Coupling only occurs for M-L sway, not A-P Coherence Biomechanical coupling due to body asymmetry in sagittal plane.

Summary Vestibular stimulation induces vertical torque responses when standing. Provides a new tool to examine links between aberrant coordinate-transform processes and postural instability. Coupling between M-L sway and vertical torque at high frequency. Modulation with head position confirms the dual-action mechanism of GVS: 1)Canal-mediated roll (dominant) 2)Otolith-based linear acceleration.

Effect of High and Low Frequency stimulation 0-5Hz 5-10Hz Yaw Pitch Roll