Investigating Coordinate Transform processes with Investigating Coordinate Transform processes with Electrical Vestibular StimulationElectrical Vestibular Stimulation

Raymond ReynoldsRaymond ReynoldsSports and Exercise SciencesSports and Exercise Sciences

College of Life and Environmental SciencesCollege of Life and Environmental Sciences

Coordinate transform process for balance

Same vestibular signal can requiredifferent 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

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

CorrelationCoefficient

Time (ms)Head P

itch (d

egrees)

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?

Coh

eren

ce

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-PC

oher

ence

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