investigating coordinate transform processes with electrical vestibular stimulation raymond reynolds...
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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
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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
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How does GVS work?
Day & Fitzpatrick, J Phys 2005
Head roll around a naso-occiptial axis
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Fitzpatrick et al, Current Biology 2006
Effect of GVS on walking trajectory
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Effect of GVS on walking trajectory
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• 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
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Effect of Vestibular Stimulation Upon Vertical Torque
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Response to Square Wave Stimuli
Small but significant torque and trunk yaw response
Affected by head position
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Response to Stochastic Stimulation
SVS response smoothly modulated by head pitch.
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Response to Stochastic Stimulation
CorrelationCoefficient
Time (ms)Head P
itch (d
egrees)
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Variation with Head Pitch
Confirms mechanism of GVS:Naso-occiptial rotation
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Can now assess changes in response bias & certainty
Bias Certainty
Are such changes associated with postural instability/fall risk?
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Frequency domain reveals two responses
Two rotation vectors?Lateral response masquerading as a rotation?
Coh
eren
ce
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Otolith and Canal responses to GVS
Cathers et al, J Phys 2005
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Coupling between lateral sway and torque
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Coupling only occurs for M-L sway, not A-PC
oher
ence
Biomechanical coupling due to body asymmetry in sagittal plane.
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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.
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Effect of High and Low Frequency stimulation
0-5Hz 5-10Hz
Yaw
Pitch
Roll