Cross-Modal Calibration of Vestibular Afference for Human Balance.

Area of Science:

• Neuroscience
• Vestibular System
• Human Balance Control

Background:

• The vestibular system is crucial for maintaining balance.
• Understanding how the brain interprets vestibular signals is key to explaining balance control mechanisms.

Purpose of the Study:

• To investigate how altered vestibular afferent gain affects balance control.
• To determine the brain's strategy for adapting to erroneous vestibular signals.

Main Methods:

• Manipulated vestibular afferent gain using galvanic vestibular stimulation (GVS) linked to head angular velocity.
• Assessed postural sway in subjects (N=8) under conditions with and without visual/somatosensory cues.
• Applied non-sway-referenced GVS and measured reflex muscle responses to independent vestibular stimuli.

Main Results:

• Increased vestibular afferent gain by 4-fold, significantly increasing postural sway without vision.
• Sway returned to normal after a conditioning period of stable balance with reliable sensory cues.
• Non-sway-referenced GVS was destabilizing, and conditioning did not reduce sway.
• Reflex muscle responses initially decreased but normalized after conditioning, contradicting simple adaptation models.

Conclusions:

• Erroneous vestibular signals profoundly affect balance, especially when not directly related to head motion.
• The central nervous system (CNS) does not immediately ignore destabilizing, erroneous vestibular input.
• Recalibration of a forward sensory model, rather than simple sensory reweighting, best explains adaptation to altered vestibular signals during stable balance.