Light Touch Alters Vestibular-Evoked Balance Responses: Insights into Dynamics of Sensorimotor Reweighting
How Light Touch Alters Vestibular-Evoked Balance Responses
Background
Balance control is a crucial mechanism for maintaining posture and movement in the human body, relying on the integration of multiple sensory inputs, including visual, vestibular, and tactile information. The vestibular system provides essential balance information by sensing head movement and positional changes. However, when these sensory inputs conflict, the central nervous system (CNS) must adjust its reliance on different sensory modalities through “sensorimotor reweighting” to maintain balance.
Light touch refers to minimal fingertip contact with a stable surface (typically less than 2 newtons of force), which can significantly reduce body sway (center of pressure, COP). However, electrical vestibular stimulation (EVS) introduces erroneous vestibular information, increasing body sway. The conflict between these two sensory inputs provides a unique opportunity to study the dynamic mechanisms of sensorimotor reweighting.
This study was conducted by Megan H. Goar, Michael Barnett-Cowan, and Brian C. Horslen from the Department of Kinesiology and Health Sciences at the University of Waterloo, Canada. It was published on December 3, 2024, in the Journal of Neurophysiology, aiming to explore how light touch affects vestibular-evoked balance responses and reveal the dynamic processes of sensorimotor reweighting.
Research Process and Experimental Design
Participants and Ethical Approval
The study recruited 16 healthy young adults (aged 18-35) for Experiment 1 and 10 for Experiment 2. All participants had no neurological or orthopedic disorders and provided informed consent. The study was approved by the University of Waterloo Research Ethics Board (REB #44217).
Experimental Setup and Equipment
Participants stood barefoot on two force plates (AMTI OR6-5) with their feet spaced apart at a distance equal to their foot length. Their heads were fixed at a specific angle (18°) to ensure effective vestibular stimulation. EVS was applied via electrodes placed on the mastoid processes, delivering a stochastic signal ranging from 0 to 25 Hz with a peak current of 4.5 mA.
Experimental Conditions
The experiment included two conditions: no-touch and light touch. In the light touch condition, participants lightly touched a load cell with their index finger, applying 1-2 newtons of force. In the no-touch condition, participants hovered their finger without contacting any surface.
Experiment 1: Single-State Experiment
Experiment 1 consisted of two trial types: baseline trials and single-state trials. Baseline trials lasted 60 seconds, during which participants performed no-touch or light touch without EVS. Single-state trials lasted 200 seconds, with participants performing no-touch or light touch while receiving EVS. Linear systems analysis (e.g., coherence and gain) was used to quantify the relationship between EVS and COP.
Experiment 2: Switching Experiment
Experiment 2 introduced switching trials, lasting 300 seconds, where participants alternated between no-touch and light touch every 6-15 seconds. The transitions were controlled by the experimenter using a lever system to study the dynamic changes in the EVS-COP relationship when light touch was introduced or removed.
Data Analysis
COP data were recorded using force plates and analyzed offline using MATLAB. Coherence and gain were used to quantify the relationship between EVS and COP. Coherence reflects the proportion of COP variations explained by EVS, while gain reflects the amplitude of COP responses to EVS.
Key Findings
Light Touch Reduces COP Displacement
The results showed that light touch significantly reduced COP displacement. In baseline trials, COP displacement decreased by 49% (Experiment 1) and 52% (Experiment 2) under light touch. In single-state trials, COP displacement decreased by 45% (Experiment 1) and 50% (Experiment 2). This indicates that light touch effectively reduces body sway, even under the interference of EVS.
Light Touch Increases High-Frequency Vestibular Contributions
Although light touch reduced overall COP displacement, it increased coherence between EVS and COP in the high-frequency range (12-30 Hz). Both Experiment 1 and Experiment 2 showed significant increases in high-frequency coherence under light touch, particularly in the 12-28.5 Hz range. This suggests that light touch not only reduces vestibular-evoked sway but also enhances the contribution of high-frequency vestibular information to balance control.
Changes in Gain
In contrast to coherence, light touch significantly reduced the gain between EVS and COP. In Experiment 1, gain decreased by 58% under light touch, and in Experiment 2, it decreased by 68%. This indicates that light touch reduces the amplitude of COP responses to EVS.
Dynamic Changes in Switching Experiments
The switching experiments revealed that changes in coherence and gain occurred before or shortly after the transition from light touch to no-touch. However, when transitioning from no-touch to light touch, changes in coherence and gain were delayed. This suggests that the CNS adjusts vestibular-motor control more rapidly when losing light touch information but requires more time to evaluate and adapt when gaining light touch information.
Conclusions and Significance
Scientific Value
This study reveals how light touch modulates vestibular-evoked balance responses through sensorimotor reweighting. Light touch not only reduces vestibular-evoked sway but also increases the contribution of high-frequency vestibular information to balance control. This finding expands our understanding of multisensory integration, particularly in situations of sensory conflict.
Practical Applications
The results have potential applications in designing therapeutic interventions and assistive technologies to enhance postural stability. For example, light touch could serve as a non-invasive balance training method to improve postural control in elderly individuals or patients with balance disorders.
Research Highlights
- Discovery of High-Frequency Vestibular Contributions: Light touch increased coherence between EVS and COP in the high-frequency range, a phenomenon not previously reported.
- Dynamic Sensorimotor Reweighting: The switching experiments revealed different adaptation mechanisms in the CNS when losing or gaining light touch information.
- Distinction Between Mechanical and Sensorimotor Stabilization: Through quartile analysis of load cell data, the study confirmed that the effects of light touch primarily rely on sensorimotor stabilization rather than mechanical stabilization.
Additional Valuable Information
The study also explored the impact of light touch force magnitude on the EVS-COP relationship, finding that small variations in light touch force (below 2 newtons) had minimal effects on coherence and gain. This suggests that the stabilizing effect of light touch depends more on its availability than on the specific force applied.
Through innovative experimental design and in-depth data analysis, this study highlights the significant role of light touch in vestibular-motor control, providing new directions for future research on balance control.